# Sewage Sludge Biochar Improves Water Use Efficiency and Bean Yield in a Small-Scale Field Experiment with Different Doses on Sandy Soil Under Semiarid Conditions

Raví Emanoel De Melo, Vanilson Pedro Da Silva, Diogo Paes Da Costa, Maria Fernanda de A. Tenório Alves, Márcio Henrique Leal Lopes, Eline Dias Barbosa, José Henrique de Souza Júnior, Argemiro Pereira Martins Filho, Gustavo Pereira Duda, Antonio Celso Dantas Antonino, et al.

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Raví Emanoel De Melo, Vanilson Pedro Da Silva, Diogo Paes Da Costa, Maria Fernanda de A. Tenório Alves, Márcio Henrique Leal Lopes, et al. Sewage Sludge Biochar Improves Water Use Efficiency and Bean Yield in a Small-Scale Field Experiment with Different Doses on Sandy Soil Under Semiarid Conditions. AgriEngineering, 2025, 7 (7), pp.227. 10.3390/agriengineering7070227. hal-05166272

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Article

# Sewage Sludge Biochar Improves Water Use Efficiency and Bean Yield in a Small-Scale Field Experiment with Different Doses on Sandy Soil Under Semiarid Conditions

Raví Emanoel de Melo <fnref n="1" /> ID, Vanilson Pedro da Silva <fnref n="1" /> ID, Diogo Paes da Costa <fnref n="1" />,
Maria Fernanda de A. Tenório Alves <fnref n="1" /> ID, Márcio Henrique Leal Lopes <fnref n="1" /> ID, Eline Dias Barbosa <fnref n="1" />,
José Henrique de Souza Júnior <fnref n="1" />, Argemiro Pereira Martins Filho <fnref n="1" />, Gustavo Pereira Duda <fnref n="1" /> ID,
Antonio Celso Dantas Antonino <fnref n="2" /> ID, Maria Camila de Barros Silva <fnref n="1" />, Claude Hammecker <fnref n="3" /> ID,
José Romualdo de Sousa Lima <fnref n="1" /> ID and Érika Valente de Medeiros <fnref n="1" />,* ID

<figure><img src="image_2.png" /></figure>

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updates

Academic Editor: Francesco Marinello

Received: 23 May 2025

Revised: 18 June 2025

Accepted: 30 June 2025

Published: 9 July 2025

**Citation:** Melo, R.E.d.; Silva, V.P.d.; Costa, D.P.d.; Alves, M.F.d.A.T.; Lopes, M.H.L.; Barbosa, E.D.; Júnior, J.H.d.S.; Filho, A.P.M.; Duda, G.P.; Antonino, A.C.D.; et al. Sewage Sludge Biochar Improves Water Use Efficiency and Bean Yield in a Small-Scale Field Experiment with Different Doses on Sandy Soil Under Semiarid Conditions.
*AgriEngineering* **2025**, *7*, 227.
https://doi.org/10.3390/agriengineering7070227

**Copyright:** © 2025 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
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(https://creativecommons.org/licenses/by/4.0/).

## Abstract

Soil degradation and water scarcity pose major challenges to sustainable agriculture in semiarid regions, requiring innovative strategies to enhance water use efficiency (WUE) and soil fertility. This study assessed the effects of sewage sludge biochar (SSB) on soil properties, WUE, and common bean yield through a small-scale controlled field experiment under rainfed conditions in Northeast Brazil. Four SSB application rates (5, 10, 20, and 40 t ha⁻<fnref n="1" />) were compared with conventional NPK fertilization, treated sewage sludge (SS), and chicken manure (CM). The application of 20 t ha⁻<fnref n="1" /> (B20) significantly improved soil organic carbon, nitrogen content, water retention, and microbial biomass. B20 also increased WUE by 148% and grain yield by 146% relative to NPK, while maintaining safe levels of potentially toxic elements (PTE) in bean grains. Although 40 t ha⁻<fnref n="1" /> (B40) enhanced soil fertility further, it posed a risk of PTE accumulation, reinforcing the advantage of B20 as an optimal and safe dose. These results highlight the potential of SSB to replace or complement conventional fertilizers, especially in sandy soils with limited water retention. The study supports SSB application as a sustainable soil management practice that aligns with circular economy principles, offering a viable solution for improving productivity and environmental resilience in semiarid agriculture.

**Keywords:** soil fertility; water management; carbon sequestration; food security; sustainable agriculture

## 1. Introduction

The UN projects that the global population will reach 9.7 billion by 2050 [1]. This has had an effect on agricultural systems in semiarid regions, putting more pressure on the

<footnote n="1">¹ Postgraduate Program in Agricultural Production (PPGPA), Federal University of Agreste of Pernambuco (UFAPE), Garanhuns 55292-270, Pernambuco (PE), Brazil; ravi.melo@ufrpe.br (R.E.d.M.); vanilsonpedrosilva236@gmail.com (V.P.d.S.); diogocosta1@yahoo.com.br (D.P.d.C.); mftenorioalves@gmail.com (M.F.d.A.T.A.); mhll9785@gmail.com (M.H.L.L.); elinediasbarbosa@gmail.com (E.D.B.); juniorhhhhenrique@gmail.com (J.H.d.S.J.); argemiro.ufra@gmail.com (A.P.M.F.); gpduda@gmail.com (G.P.D.); camilabarross@gmail.com (M.C.d.B.S.); romualdo.lima@ufape.edu.br (J.R.d.S.L.)</footnote>

<footnote n="2">² Department of Nuclear Engineering (DEN), Federal University of Pernambuco (UFPE), Recife 50670-901, Pernambuco (PE), Brazil; antonio.antonino@ufpe.br</footnote>

<footnote n="3">³ Institute de Recherche Pour le Développement (IRD), Place Pierre Viala, 34060 Montpellier, France; claudé.hammecker@ird.fr</footnote>

<footnote>* Correspondence: evmbio@gmail.com or erika.valente@ufape.edu.br</footnote>
reduction of soil and water resources, which are essential for agricultural production [2,3]. On the other hand, with the advancement of industrialization and urbanization combined with climate change, industrial water use and the urban population have increased exponentially, causing an overload of agricultural water and increasing competition [4,5]. In addition, the increase in droughts in the last decade has limited crop yields and made them dependent on water availability [5]. Another factor is the indiscriminate use of chemical fertilizers, which aggravate soil degradation through inadequate management, resulting in low fertility and water retention capacity, which are fundamental factors for achieving sustainable agriculture with food security [6,7]. Ensuring food security in agricultural production has been one of the drivers for the application of biochar in global agriculture [8]. These factors make it essential to develop strategies to reduce water consumption by crops without affecting yield—that is, to increase water use efficiency (WUE)—to ensure water availability and meet global food production demands [2].

The use of sewage sludge, an abundant residue generated by wastewater treatment plants, is a promising solution for improving agricultural productivity in semiarid regions. Although its composition varies depending on its source and the type of treatment applied, sewage sludge may contain contaminants that require proper management. The thermochemical transformation of this material into biochar, a carbon-rich product obtained through the pyrolysis of organic biomass, has gained prominence as a sustainable agricultural reuse practice [9,10]. Pyrolysis of sewage sludge at high temperatures (>500 °C) not only eliminates pathogens and degrades persistent organic pollutants, including pharmaceuticals and PFAS, but also stabilizes potentially toxic elements (PTE) by transforming them into less mobile forms (oxidizable/residual fractions) [11]. Additionally, the resulting biochar possesses high aromaticity, surface area, porosity, and alkalinity, which further enhance its capacity to adsorb contaminants and retain nutrients, making it a safer and more effective soil amendment than raw sludge [12]. However, few studies have been conducted in dry tropical regions, and they are even rarer in the extensive semiarid area of Brazil. This region, like many other dry tropical areas, is characterized by wide variations in annual precipitation and periodically experiences drought episodes [13]. Globally, extensive data are available on the influence of biochar on crop yields; however, these are pot or microclimate experiments that limit the provision of more robust results [14,15].

In this context, sewage sludge biochar (SSB), derived from pyrolyzed wastewater treatment residues, has emerged as a strategic amendment for semiarid soils. SSB combines the benefits of biochar, such as high porosity, carbon stability, and nutrient retention, with a sustainable solution for managing an abundant and potentially polluting waste. In Brazil, sewage sludge production exceeds 370,000 tons annually [10]. When processed via pyrolysis, the resulting biochar reduces pathogenic risks and heavy metal mobility [16]. Agronomically, SSB improves soil structure, increases water retention and nutrient availability, and contributes to higher crop yields, especially in sandy soils with low fertility [9,17]. Biochar properties contribute to improving soil structure, increasing water and nutrient retention capacity, and contributing to carbon sequestration, mitigating greenhouse gas emissions [18–20]. Despite progress in understanding the effects of biochar on improving agricultural production, several critical knowledge gaps remain, such as the appropriate application rate and the mechanisms of action used.

Biochar application can alter the soil environment [21], depending on the application rate. The influence of biochar on soil physical-hydric, chemical, and biological properties is significantly influenced by the biochar application rate [22,23]. The determination of the correct application rate is essential for improving soil quality and crop yield [24]. Lima et al. [25] reported that, compared with the control, biochar application at application rates of 20 and 40 t ha⁻¹ increased common bean productivity by up to 150%.
Figueiredo et al. [26] reported that biochar from biosolids produced at 300 °C at an application rate of 15 t ha⁻¹ in a Dystrophic Red Latosol offered greater corn crop productivity after four harvests. Obia et al. [27] reported higher corn and soybean crop yields with the application of biochar produced from pigeon pea at an application rate of 4 t ha⁻¹. In a study that used doses of coffee husk biochar to treat bean crops, Lima et al. [28] reported reduced evapotranspiration and greater WUE at doses of 11 and 12.1 t ha⁻¹, respectively. However, studies have shown that high application rates lead to increased alkalinity and reduced crop yield [29]. In addition, the presence of PTE at high application rates raises concerns about the safety and environmental impacts of biochar use in agroecosystems [30].

By converting sewage sludge into biochar, this study contributes to the discussion of the reuse of problematic waste at a global level in agricultural production, aligning with the principles of a circular economy and sustainability. The expected results have the potential to influence the applicability of public policies focused on waste management and the adoption of sustainable practices in semiarid regions, with limited water resources and a growing demand for increased crop productivity. Thus, the integration of biochar into agricultural practices represents a significant step toward achieving sustainable agriculture and facing global crises [2,31,32], providing insights that will assist in the adoption of more sustainable agricultural practices in regions with structural barriers to agricultural development, in addition to adaptive solutions for regions with similar edaphoclimatic conditions, where the sustainability of the agricultural system is an emergency priority. This study aimed to: (i) evaluate the effects of SSB on soil properties and water efficiency under semiarid conditions; and (ii) identify the optimal application rate that balances productivity gains with environmental safety. Therefore, determining the most appropriate dose is essential for the safe and effective use of SSB in dryland agriculture. In this context, two hypotheses were formulated: (i) the application of SSB improves the soil's physical-hydraulic and chemical properties, increasing water retention and microbial biomass; and (ii) application rates above 20 t ha⁻¹ lead to the accumulation of PTE in common bean grains beyond safe thresholds.

**2. Materials and Methods**

**2.1. Soil Sampling**

Soil sampling was conducted in layers ranging from 0–10 to 10–20 cm in depth prior to the installation of the experiments to perform physical and chemical analyses of the soil via established standard methodologies.

Available phosphorus (P) was determined by the Melich-1 method described by [33] and analyzed according to the methodology described by [34]. Following this same methodology, exchangeable sodium (Na⁺) and potassium (K⁺) were determined via flame photometry, magnesium (Mg²⁺) via atomic absorption, and pH in water at a ratio of 1:2.5. Another component determined by the methodology mentioned above was organic matter (OM), which has a value divided by the Van Bemmelen factor (1.724) to determine the total organic carbon (TOC) content, considering that OM accounts for 58% of the total organic carbon (TOC) according to Fageria et al. [35]. Available sulfate as described by [36] and total nitrogen (N) were analyzed according to [37]. The soil density (SD), total porosity (TP), particle density (PD), and granulometric analysis results were also determined via [33], as shown in Table 1.
<table><caption>**Table 1.** Chemical and physical properties of the soil in the experimental area at 0–10 cm and 10–20 cm depths.</caption><thead><tr><th colspan="9">Chemical Features</th></tr><tr><th>Layer</th><th>P</th><th>Na</th><th>K</th><th>Mg</th><th>S</th><th>pH</th><th>N</th><th>OM</th><th>TOC</th></tr><tr><td></td><td colspan="5">mg kg<sup>-1</sup></td><td>adm.</td><td>mg kg<sup>-1</sup></td><td>%</td><td></td></tr></thead><tbody><tr><td>(0–10 cm)</td><td>14.39</td><td>46.0</td><td>43.0</td><td>25.6</td><td>35.000</td><td>6.23</td><td>480</td><td>1.28</td><td>0.74</td></tr><tr><td>(10–20 cm)</td><td>10.22</td><td>55.2</td><td>50.8</td><td>25.6</td><td>40.000</td><td>6.49</td><td>590</td><td>0.71</td><td>0.41</td></tr></tbody><thead><tr><th colspan="9">Particle size</th></tr><tr><th>Layer</th><th>SD</th><th>TP</th><th>Sand</th><th>Silt</th><th>Clay</th><th>Textural class</th><th></th><th></th></tr><tr><td></td><td>g cm<sup>-3</sup></td><td>cm<sup>3</sup> cm<sup>-3</sup></td><td>g kg<sup>-1</sup></td><td></td><td></td><td></td><td></td><td></td></tr></thead><tbody><tr><td>(0–10 cm)</td><td>1.73</td><td>0.395</td><td>924.16</td><td>35.84</td><td>40</td><td>Sand</td><td></td><td></td></tr><tr><td>(10–20 cm)</td><td>1.73</td><td>0.397</td><td>905.38</td><td>74.62</td><td>20</td><td>Sand</td><td></td><td></td></tr></tbody></table>

where P: phosphorus available; Na: exchangeable sodium; K: exchangeable potassium; Mg: interchangeable magnesium; S: sulfur available; pH: hydrogenic potential; N: total nitrogen; OM: organic matter; TOC: total organic carbon; SD: soil density; and TP: total porosity.

N was determined by sulfuric acid digestion without selenium, following the protocol of the Agronomic Institute of Campinas [37]. The method measures mainly organic and ammonium nitrogen, excluding nitrate and nitrite. Results are expressed in mg kg⁻¹.

Available sulfur (S) was determined by extraction with calcium dihydrogen phosphate [Ca(H₂PO₄)₂] solution, following the IAC method, and the results are expressed as elemental sulfur (S) in mg kg⁻¹.

## 2.2. Study Area and Environmental Conditions

The field experiment was conducted under rainfed conditions on an experimental farm in São João, Pernambuco, Brazil (Figure 1) (08°50'24" S; 36°22'49" W; 715 m altitude), during two main bean growing seasons: the rainy season (May–July) and the dry season (August–December) of 2022. The cultivars used were IAC Netuno (common bean) and BRS Tumucumaque (cowpea), following a crop rotation strategy to reflect local dryland agricultural practices.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Location of the study area in São João, Pernambuco state, Brazil, and the experimental site where field trials were conducted under rainfed conditions.</figcaption></figure>
The region's climate is classified as As' (Köppen), with well-defined dry winters and irregular rainfall averaging 782 mm annually [38,39]. In 2022, meteorological anomalies such as easterly wave disturbances (EWDs) and a favorable positioning of the Intertropical Convergence Zone (ITCZ) resulted in intense rainfall episodes over short periods [40].

Weather data (precipitation, temperature, and humidity) were monitored by an automatic meteorological station (Campbell Scientific) during the experimental period (Figure 2). The soil in the area is classified as Entisol [41], with sandy texture, low fertility, and limited water retention—common constraints in the Brazilian semiarid region.

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Meteorological conditions recorded during the experimental period in 2022, including monthly rainfall, mean air temperature, and mean relative humidity. Data were obtained from an automatic weather station (Campbell Scientific) installed at the experimental site.</figcaption></figure>

## 2.3. *Experimental Design and Treatments*

Sewage sludge (SS) was collected from a sewage treatment plant (STS) located in the study region. After air drying, the SS was subjected to slow pyrolysis for the production of SSB using a homemade vertical thermal furnace adapted from Thai farmers [42,43]. The heating source was wood combustion, and the system operated under limited oxygen conditions, without inert gas injection. The pyrolysis process reached a maximum temperature of approximately 550 °C, with a residence time of 10 h. After cooling, the resulting biochar was homogenized and sieved to a particle size smaller than 2 mm before field application. Chicken manure (CM) was obtained from poultry farms located in the same region. The chemical characteristics of the SS, SSB, and CM were determined according to the methodologies described in Section 2.1 and are presented in Table 2.

<table><caption>**Table 2.** Chemical properties of the input materials used in the treatments.</caption><thead><tr><th rowspan="2">Materials</th><th>N</th><th>OC</th><th>P</th><th>K</th><th>Ca</th><th>Mg</th><th>Na</th><th>S</th><th>EC</th><th>pH</th></tr><tr><th colspan="2">g kg<sup>-1</sup></th><th></th><th></th><th colspan="4">g kg<sup>-1</sup></th><th></th><th></th></tr></thead><tbody><tr><td>SS</td><td>10.6</td><td>124.2</td><td>11.5</td><td>3.1</td><td>20.1</td><td>2.9</td><td>1.0</td><td>20.5</td><td>3.38</td><td>3.59</td></tr><tr><td>SSB</td><td>9.7</td><td>121.1</td><td>7.2</td><td>1.2</td><td>10.2</td><td>1.6</td><td>1.8</td><td>88.6</td><td>2.20</td><td>7.92</td></tr><tr><td>CM</td><td>24.0</td><td>230.7</td><td>20.9</td><td>35.7</td><td>15.0</td><td>10.0</td><td>-</td><td>-</td><td>-</td><td>8.90</td></tr></tbody></table>

where N: nitrogen; OC: organic carbon; P: phosphorus; K: potassium; Ca: calcium; Mg: magnesium; Na: sodium; S: sulfate; EC: electrical conductivity; and pH: potential hydrogen.

After pyrolysis and cooling, the biochar was ground and sieved to a particle size smaller than 2 mm to ensure uniformity of application in the field. No advanced analysis of porosity or surface area was performed. However, future studies should include detailed physical characterization.
Among the materials analyzed, the SSB stood out for its high organic carbon content (121.1 g kg⁻¹), alkaline pH (7.92), and moderate electrical conductivity (2.20 dS m⁻¹), suggesting good potential for improving soil structure and nutrient retention in sandy soils. The presence of essential nutrients such as nitrogen (9.7 g kg⁻¹), phosphorus (C), and calcium (10.2 g kg⁻¹) further reinforces its agronomic value. Compared to untreated sewage sludge, the pyrolyzed product exhibited higher pH and lower electrical conductivity, indicating improved chemical stability and potential environmental safety.

The sewage sludge used in this study was collected in the final stage of treatment, where pH tends to drop due to the presence of organic acids and incomplete stabilization. Similar acidic conditions have been reported by Pereira et al. [44], who attributed low pH values in anaerobically digested sludge to operational issues during the acidogenic phase of the treatment process.

The experimental treatments included mineral fertilization (NPK), SS, CM, and four application rates of SSB. The codes and application rates used are presented in Table 3. The treatments consisted of application rates of 5, 10, 20, and 40 t ha⁻¹ (B5, B10, B20, and B40) SSB and 5 t ha⁻¹ CM and SS, in addition to a control treatment with NPK fertilizer.

<table><caption>**Table 3.** Description of treatments, materials applied, and their respective application rates.</caption><thead><tr><td>Treatment Code</td><td>Material Applied</td><td>Application Rate (t ha<sup>-1</sup>)</td></tr></thead><tbody><tr><td>NPK</td><td>Mineral fertilizer (NPK)</td><td>standard recommended dose</td></tr><tr><td>SS</td><td>Treated sewage sludge</td><td>5</td></tr><tr><td>CM</td><td>Chicken manure</td><td>5</td></tr><tr><td>B5</td><td>Sewage sludge biochar</td><td>5</td></tr><tr><td>B10</td><td>Sewage sludge biochar</td><td>10</td></tr><tr><td>B20</td><td>Sewage sludge biochar</td><td>20</td></tr><tr><td>B40</td><td>Sewage sludge biochar</td><td>40</td></tr></tbody></table>

where NPK: mineral fertilizer (standard recommended dose); SS: treated sewage sludge; CM: chicken manure; B5, B10, B20, and B40 sewage sludge biochar applied at 5, 10, 20, and 40 t ha⁻¹, respectively.

The SSB, CM, and SS treatments were manually applied to the plots and uniformly incorporated into the top 20 cm of soil using hoes, approximately 7 days before sowing. This procedure aimed to enhance the interaction of the amendments with the root zone and improve nutrient availability during crop establishment.

Basal fertilization was carried out exclusively in the control treatment, representing the conventional management adopted for common bean cultivation. The NPK fertilizer was applied at planting, at a depth of 6 cm and parallel to the planting row. The applied doses were 44 kg ha⁻¹ of N, 20 kg ha⁻¹ of phosphorus pentoxide (P₂O₅), and 20.4 kg ha⁻¹ of potassium oxide (K₂O). The sources used were urea (45% N), single superphosphate, and potassium chloride, respectively. The other treatments did not receive mineral fertilization, allowing for the evaluation of sewage sludge biochar as an alternative to conventional fertilization.

The 28 experimental plots measured 9 m² each (3 × 3 m), with 7 planting rows spaced 0.3 m between rows and 0.3 m between plants. The full experimental area covered 540 m² (18 × 30 m), including 1 m spacing between plots and between blocks to reduce edge effects. Each 9 m² plot consisted of seven planting rows spaced 0.3 m apart, with 0.3 m between plants. Three seeds were sown per hole, totaling approximately 210 seeds per plot, following local agronomic recommendations for dryland bean cultivation. SSB was applied and distributed manually in each plot. Grain yield and related variables were calculated based on the effective harvested area and then extrapolated to a per-hectare basis (kg ha⁻¹) using standard agronomic methods, as commonly adopted in field-scale experimental studies.
## 2.4. Determination of the Physical, Chemical, and Biological Properties of the Soil

After each cultivation cycle, the chemical and physical properties of the soil were analyzed again according to Section 2.1. Samples were collected at depths of 0–10 cm in all the experimental plots for analysis of microbial biomass carbon (MBC), which was determined via the irradiation process according to [45]. Quantification was performed following the methodology proposed by Vance et al. [46] and Tate et al. [47], and determination was performed via colorimetry according to [48].

The determination followed the fumigation-extraction. Fresh soil samples were divided into two subsamples: one was fumigated with ethanol-free chloroform for 24 h, and the other was kept non-fumigated as a control. Both subsamples were extracted with 0.5 mol L⁻¹ K₂SO₄ solution (soil-extractant ratio of 1:2.5). The extracted organic carbon was quantified using colorimetry after oxidation with potassium dichromate and sulfuric acid. MBC was calculated as the difference between fumigated and non-fumigated samples, applying a correction factor (KEC) of 0.38. Results were expressed in mg C kg⁻¹ soil.

Microbiological analyses were conducted exclusively on samples from the 0–10 cm soil layer, which represents the zone of greatest biological activity due to higher organic matter inputs, root exudates, and aeration. Physical and chemical analyses, including pH, exchangeable cations, and available nutrients, were performed on samples from both the 0–10 cm and 10–20 cm depths.

## 2.5. Determination of Bean Productivity and Analysis of Potentially Toxic Elements (PTE)

The grain yield of common beans, expressed in kg ha⁻¹, was determined by the relationship between the number of pods of productive plants, the number of grains per pod, and the weight of a thousand grains adjusted to 13% moisture. This adjustment follows a technical-scientific standard that reflects the ideal storage and commercialization conditions for grains produced in the tropical region of Brazil. In the second crop cycle, data were obtained from three sequential samplings carried out on the same plants, which had been previously selected at the initial phenological stage of the crop [49]. The response of common bean to SSB application rates was verified to identify the ideal dose that maximizes productivity under conditions of low water availability, such as those found in the region of the present study.

PTE levels were determined in bean grains according to [50]. Quantification was performed via atomic absorption spectrophotometry [51] following the protocols established by the Association of Official Analytical Chemists (AOAC). The metals Cd, Mn, Cr, Cu, and Ni were analyzed in the bean grains.

## 2.6. Determination of Evapotranspiration and Water Use Efficiency

The evapotranspiration (ET) of common bean was determined via the soil water balance (SWB) method, which uses soil moisture data obtained by TDR sensors in the 0–30 cm layer. The water balance was calculated via the equation described by Silva et al. [41], which considers precipitation (P), irrigation (I), capillary rise (CR), deep drainage (D), and surface runoff (SR), all of which are expressed in millimeters (mm), representing water depth equivalents:

$$ \Delta A = P + I + CR - D + SR - ET \quad (1) $$

ET was the residual term of the equation, with P monitored by an automated rain gauge and variation in water storage (ΔA) determined by the difference between the initial and final values of the period, according to Souza et al. [52]:

$$ \Delta A = (\theta_f - \theta_i) z = (A_f - A_i) \quad (2) $$
The soil water storage (A) was obtained according to Silva et al. [41]:

$$A = \theta z \qquad (3)$$

where θ is the volumetric moisture in the soil layer in m³ m⁻³; z is the depth of the soil layer, in m.

WUE was calculated as the ratio of grain production (GY) to evapotranspiration (ET):

$$WUE = GY/ET \qquad (4)$$

where GY is the grain yield (kg ha⁻¹) and ET is the evapotranspiration (mm). Thus, WUE is expressed in kg ha⁻¹ mm⁻¹.

## 2.7. Statistical Analysis

The statistical analyses were carried out with R Studio 4.3.2 (R Foundation for Statistical Computing, Vienna, Austria). [53]. Initially, the data were tested for normality using the Shapiro-Wilk test (α = 0.05), and when necessary, appropriate transformations were applied to meet this assumption. The study followed a full factorial design and was analyzed using repeated-measures ANOVA (type III), considering two crop cycles and seven treatment levels. For each variable measured in the soil and in plants, comparisons were made using type III repeated-measures ANOVA at each soil depth.

When significant effects were observed (p < 0.05), post hoc comparisons were conducted using the Least Significant Difference (t-LSD) test with Bonferroni correction at the 5% level. The proportion of explained variance was calculated using generalized eta squared (η²g), which estimates the global effect size. According to [54], η²g values were interpreted as small (<0.01), medium (0.02–0.06), or large (>0.14).

All models were constructed using the ezANOVA function from the "ez" package. Post hoc tests and figures were produced with the help of the "agricolae", "rstatix", "tidyverse", "ggpmisc", "ggpubr", and "ggplot2" packages. Three-way repeated-measures ANOVA models were used when data included two crop cycles and two soil depths. For variables not influenced by soil depth (MBC and GY), two-way repeated-measures ANOVA was applied. Variables measured in a single cycle were analyzed separately using Tukey's HSD test with Bonferroni correction at the 5% level. Since these variables were not subjected to repeated measures or full factorial designs, the graphical representation differs from the standardized format used in other figures.

# 3. Results

## 3.1. Physical, Chemical, and Biological Properties of the Soil

The application rates of SSB (B5, B10, B20, and B40), CM, SS, and NPK are explanatory variables, and the characteristics of the soil environment represent the response variables. The analyses at different soil depths (0–10 and 10–20 cm) were performed separately; however, the bean cycles were analyzed together (Figure 3).

No statistically significant differences were detected for the variables SD and total TP (p > 0.05) in the 0–10 cm layer throughout the two crop development cycles. Furthermore, no consistent trends of increase or decrease were observed for these variables. Similar behavior was observed in the 10–20 cm layer in both cycles, without statistically significant differences or detectable trends. The ANOVA results for all the physical characteristics evaluated presented p values > 0.05, indicating the absence of relevant variations. Furthermore, the calculation of the generalized effect size (η²g) revealed low or irrelevant values (0.01 and 0.07), corroborating the absence of significant effects. These results rule out the need for additional statistical tests for the variables in question. In this study, the term
'SD' refers to bulk density (BD), a measure of the dry mass of soil per unit volume, which reflects soil compaction and porosity.

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Magnesium and pH in 0–10 and 10–20 cm soil layers under different treatments and cropping cycles with ANOVA and Bonferroni-adjusted t-tests. The asterisk (*) indicates statistically significant differences between treatments (*p* < 0.05).</figcaption></figure>

In the analyses of the soil chemical properties, the Mg²⁺ content significantly differed among the treatments in the 0–10 cm layer during cycle I (marked in the graph with *). However, the generalized effect size (η²g = 0.04) was classified as irrelevant, indicating that although a difference was detected, its practical impact was insignificant in the general context of the experiment. For the other cycles and depths, no significant differences were observed between the treatments (*p* > 0.05). pH followed the same behavior observed for magnesium, showing a statistically significant difference (*p* < 0.05), but the generalized effect size (η²g = 0.02) indicated that the effect was of little practical relevance.

The Na⁺, K⁺, and sulfate (SO₄²⁻) contents did not significantly differ among the treatments (*p* > 0.05), indicating the relative uniformity of these variables across the depths and cycles evaluated. However, a slight reduction in the contents of these elements was observed when the development cycles of the bean crop were compared (Table 4). This decrease can be attributed to the atypical nature of the agricultural year, especially to the recorded rainfall indices, which probably influenced the dynamics of these nutrients. Since Na⁺, K⁺, and SO₄²⁻ are mobile elements in the soil profile, excessive or poorly distributed rainfall may have promoted the transport of these ions to deeper layers, reducing their concentrations in the superficial layers evaluated.
<table><caption>**Table 4.** Mean values of exchangeable sodium (Na⁺), exchangeable potassium (K⁺), and available sulfate (SO₄²⁻) in the soil at two depths and two crop cycles under different treatments.</caption><thead><tr><th>Treatment</th><th>Cycle</th><th>Layer</th><th>Na<sup>+</sup> (cmolc kg<sup>-1</sup>)</th><th>K<sup>+</sup> (cmolc kg<sup>-1</sup>)</th><th>SO<sub>4</sub><sup>2-</sup> (hg g<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="6">NPK</td><td rowspan="6">Cycle I</td><td rowspan="6">0–10 cm</td><td>0.20 a</td><td>0.12 a</td><td>0.41 a</td></tr><tr><td>0.21 a</td><td>0.14 a</td><td>0.42 a</td></tr><tr><td>0.23 a</td><td>0.15 a</td><td>0.46 a</td></tr><tr><td>0.29 a</td><td>0.19 a</td><td>0.47 a</td></tr><tr><td>0.30 a</td><td>0.21 a</td><td>0.50 a</td></tr><tr><td>0.22 a</td><td>0.25 a</td><td>0.50 a</td></tr><tr><td rowspan="6">NPK</td><td rowspan="6">Cycle I</td><td rowspan="6">10–20 cm</td><td>0.28 a</td><td>0.14 a</td><td>0.41 a</td></tr><tr><td>0.16 a</td><td>0.13 a</td><td>0.47 a</td></tr><tr><td>0.22 a</td><td>0.15 a</td><td>0.48 a</td></tr><tr><td>0.25 a</td><td>0.17 a</td><td>0.50 a</td></tr><tr><td>0.31 a</td><td>0.20 a</td><td>0.53 a</td></tr><tr><td>0.35 a</td><td>0.22 a</td><td>0.54 a</td></tr><tr><td rowspan="6">NPK</td><td rowspan="6">Cycle II</td><td rowspan="6">0–10 cm</td><td>0.26 a</td><td>0.13 a</td><td>0.52 a</td></tr><tr><td>0.10 a</td><td>0.11 a</td><td>0.24 a</td></tr><tr><td>0.11 a</td><td>0.16 a</td><td>0.28 a</td></tr><tr><td>0.17 a</td><td>0.19 a</td><td>0.31 a</td></tr><tr><td>0.25 a</td><td>0.40 a</td><td>0.35 a</td></tr><tr><td>0.28 a</td><td>0.37 a</td><td>0.36 a</td></tr><tr><td rowspan="6">NPK</td><td rowspan="6">Cycle II</td><td rowspan="6">10–20 cm</td><td>0.16 a</td><td>0.38 a</td><td>0.37 a</td></tr><tr><td>0.22 a</td><td>0.15 a</td><td>0.24 a</td></tr><tr><td>0.09 a</td><td>0.13 a</td><td>0.31 a</td></tr><tr><td>0.12 a</td><td>0.16 a</td><td>0.32 a</td></tr><tr><td>0.18 a</td><td>0.20 a</td><td>0.34 a</td></tr><tr><td>0.25 a</td><td>0.45 a</td><td>0.45 a</td></tr><tr><td rowspan="6">NPK</td><td rowspan="6">Cycle II</td><td rowspan="6">10–20 cm</td><td>0.29 a</td><td>0.42 a</td><td>0.46 a</td></tr><tr><td>0.15 a</td><td>0.43 a</td><td>0.42 a</td></tr><tr><td>0.23 a</td><td>0.12 a</td><td>0.27 a</td></tr><tr><td>0.09 a</td><td>0.13 a</td><td>0.31 a</td></tr><tr><td>0.12 a</td><td>0.16 a</td><td>0.32 a</td></tr><tr><td>0.18 a</td><td>0.20 a</td><td>0.34 a</td></tr><tr><td rowspan="6">NPK</td><td rowspan="6">Cycle II</td><td rowspan="6">10–20 cm</td><td>0.25 a</td><td>0.45 a</td><td>0.45 a</td></tr><tr><td>0.29 a</td><td>0.42 a</td><td>0.46 a</td></tr><tr><td>0.15 a</td><td>0.43 a</td><td>0.42 a</td></tr><tr><td>0.23 a</td><td>0.12 a</td><td>0.27 a</td></tr><tr><td>0.09 a</td><td>0.13 a</td><td>0.31 a</td></tr><tr><td>0.12 a</td><td>0.16 a</td><td>0.32 a</td></tr></tbody></table>

where different letters indicate statistical differences between treatments in each column ($p < 0.05$) according to the LSD test. CM = chicken manure; SS = sewage sludge; B5, B10, B20, and B40 = sewage sludge biochar at 5, 10, 20, and 40 t ha⁻¹, respectively; NPK = mineral fertilization. Layer: Soil sampling depth. Cycle I: Common bean. Cycle II: Cowpea.

The application of SSB had a statistically significant effect on the soil P content ($p < 0.01$), highlighting the impact of the various treatments on soil fertility. In the 0–10 cm layer, the B20 treatment resulted in a significant increase in P, which was superior to the results of the treatments with lower doses (B5, B10) and NPK, with percentage increases of 56.9%, 30.1%, and 22.2%, respectively. This response reflects the efficiency of SSB in releasing P at intermediate application rates, promoting substantial gains in the short term. Furthermore, in the second evaluation cycle, the P available in B20 was comparable to that in CM, with a difference of only 1.5%. This behavior indicates that the use of SSB, even at intermediate doses, can be an efficient and viable alternative to CM in supplying P (Figure 4).

In the 10–20 cm layer, the behavior of B20 was similar to that observed in the surface layer (0–10 cm), with P contents higher than those of the other treatments (B5 and B10). However, the P concentrations in the subsurface layer were lower than those in the surface layer, confirming the more pronounced retention of nutrients in the upper layers due to the low mobility of phosphorus in the soil. Although B40 provided an additional increase in P, the marginal efficiency in relation to B20 was reduced, indicating that intermediate application rates, such as B20, can be more cost-effective for P management. These results reinforce the potential of SSB to promote increased P availability in the soil, contributing to agricultural sustainability and reducing the dependence on commercial fertilizers.
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Phosphorus in 0–10 and 10–20 cm soil layers under different treatments and cropping cycles with ANOVA and Bonferroni-adjusted *t*-tests. The asterisk (**) indicates statistically significant differences between treatments (*p* < 0.01).</figcaption></figure>

The application of SSB significantly influenced the soil nitrogen (N) content in both layers evaluated (0–10 cm and 10–20 cm, *p* < 0.05). In the surface layer (0–10 cm), treatment B20 resulted in a moderate increase in the N content, which was greater than that in the other treatments (B5 and B10), with average values of 0.05%, representing an increase of 25% compared with that in B5. Despite not reaching the values observed in B40 and CM, B20 maintained a consistent performance, indicating that intermediate application rates of SSB can provide a significant amount of N to the soil.

In the subsurface layer (10–20 cm), the B20 treatment maintained a similar pattern, with N values that were also higher than those in the NPK treatment (150% increase). These findings demonstrate that SSB at intermediate application rates can contribute to improving the N content even in deeper layers, reflecting the limited but effective mobility of nutrients in the soil profile. Although B40 and CM presented higher values in both layers, the difference from B20 was relatively small in the 0–10 cm layer (20%), suggesting that B20 may be a cost-effective option (Figure 5).

The application of SSB had a significant linear effect on the total organic carbon (TOC) content in both layers evaluated (*p* < 0.01), highlighting the potential of biochar to improve soil quality. In the surface layer (0–10 cm), during Cycle I, the B20 treatment resulted in a significant increase in TOC, which was statistically similar to that of B40, B10, and CM. Compared with NPK, B20 increased the TOC content by approximately 38.1%, demonstrating the efficiency of SSB in increasing the soil carbon content. In Cycle II, the B20 application rate continued to show consistent performance, with increases in TOC in relation to the lower rates (B5 and B10). Although B40 presented higher absolute values, the gain provided by B20 was more efficient in terms of cost and management, since it significantly increased without the need to apply maximum rates. A comparison of the performance between cycles revealed that the TOC content increased progressively, reflecting the cumulative effect of biochar in the soil.

In the 10–20 cm layer, the behavior of B20 followed the same trend, although the increases were smaller because of the lower mobility of TOC at depth. Nevertheless, the B20 treatment resulted in significant gains compared with the NPK treatment, indicating
that even intermediate application rates of SSB can positively influence the carbon content in subsurface layers (Figure 6).

<figure><img src="image_7.png" /><figcaption>Figure 5. Total nitrogen in 0–10 and 10–20 cm soil layers under different treatments. Different letters indicate significant differences according to Tukey’s test (p < 0.05). Nitrogen values refer to total nitrogen (N-total), determined by sulfuric acid digestion, including organic and ammonium forms. Black dots indicate outliers.</figcaption></figure>

$$ \text{Anova, } F(6,18) = 17.5, p = <0.0001, \eta_g^2 = 0.52 $$

<figure><img src="image_8.png" /><figcaption>Figure 6. Total organic carbon in 0–10 and 10–20 cm soil layers under different treatments and cropping cycles with ANOVA and Bonferroni-adjusted t-tests. The asterisk (***), indicates statistically significant differences between treatments (p < 0.001).</figcaption></figure>

In the first cycle, the application of SSB did not have a statistically significant effect on MBC (p > 0.05). However, a clear trend toward increasing MBC values was observed with increasing SSB dose, especially at relatively high application rates (B20 and B40).

In the second cycle, the significant effect of SSB dose became evident (p < 0.05), with significant differences observed between treatments. B20 stood out as an efficient intermediate dose, providing significant increases in the MBC in relation to lower doses, such as B5 and B10. Although B40 presented the highest absolute values, B20 provided a consistent increase in MBC, surpassing that of SS by 113% and that of NPK by 42%.
Furthermore, MBC stocks increased overall in the second cycle, reinforcing the cumu-
lative effect of SSB application. The performance of B20 in the second cycle indicates that
intermediate application rates can be strategic for promoting microbial biomass without
the need for maximum doses, optimizing costs, and maintaining sustainability in soil
management. These results highlight that the application of SSB in the B20 treatment can be
an effective strategy to improve MBC stocks in the soil over time, contributing to biological
activity and soil quality in a sustainable manner (Figure 7).

<figure><img src="image_9.png" /><figcaption>**Figure 7.** Microbial biomass carbon (MBC) under different treatments across two cropping cycles. Asterisks indicate significant differences according to pairwise *t*-tests with Bonferroni adjustment (*p* < 0.05). The asterisk (*) indicates statistically significant differences between treatments (*p* < 0.05).</figcaption></figure>

Figure 7 shows the mean values of microbial biomass carbon (MBC) for each treatment across the two crop cycles. Although no significant treatment effect was observed in the first cycle, the second cycle revealed increased MBC values, especially in the B20 and B40 treatments, suggesting a residual and cumulative effect of sewage sludge biochar on microbial activity.

3.2. *Bean Productivity and Potentially Toxic Elements (PTE)*

B20 showed outstanding performance in terms of GY, especially in the second evalua-
tion cycle. In cycle I, although CM achieved the highest productivity (2520.7 kg ha⁻¹), B20
showed a significant increase in relation to NPK, reaching 1162.6 kg ha⁻¹, which represents
an increase of 7% compared with chemical fertilizer. Although its performance in the
first cycle was inferior to CM and B40, B20 demonstrated its ability to effectively provide
nutrients for the initial development of plants.

In Cycle II, the B20 treatment resulted in even greater results, reaching a productivity of 4002.2 kg ha⁻¹, which approached the value observed for CM (4145.8 kg ha⁻¹) and significantly surpassed those of the other treatments, with the exception of B40. Compared with NPK, B20 resulted in a 146% increase in GY, highlighting its potential as an alternative to conventional fertilizer. These results indicate that B20 is a viable and sustainable strategy to increase grain productivity, especially in the long term, providing significant increases with less dependence on commercial inputs such as NPK (Figure 8).
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Grain yield (GY) under different treatments across two cropping cycles. Asterisks indicate significant differences according to Bonferroni-adjusted *t*-tests (* *p* < 0.05, ** *p* < 0.01, *** *p* < 0.001).</figcaption></figure>

The PTE, such as Cd, Mn, Cr, Cu, and Ni, were analyzed in bean grains under specific management conditions, as presented in Table 5. The evaluation was carried out on the basis of a maximum application rate of 40 t ha⁻¹, which was chosen for its potential to maximize agronomic benefits and for offering a comprehensive view of the impact of intensive management over time. Grain samples from both B20 and B40 treatments were analyzed for PTE content. In B20, all elements were found below the maximum permissible limits for food safety (RDC N°. 722/2022), confirming the safety of this dose for human consumption.

<table><caption>**Table 5.** Concentrations of PTE in common bean grains under 20 and 40 t ha⁻¹ SSB application.</caption><thead><tr><th>Materials</th><th>Cd</th><th>Mn</th><th>Cr</th><th>Cu</th><th>Ni</th></tr><tr><td></td><td colspan="5" style="text-align: center;">mg kg<sup>-1</sup></td></tr></thead><tbody><tr><td>Common Bean Grains (40 t ha<sup>-1</sup>)</td><td>2</td><td>45</td><td>11</td><td>8</td><td>9</td></tr><tr><td>Common Bean Grains (20 t ha<sup>-1</sup>)</td><td>0.8</td><td>26</td><td>6</td><td>5</td><td>6</td></tr><tr><td>Maximum Permissible Limits (MPLs)</td><td>1</td><td>15-80</td><td>-</td><td>30</td><td>-</td></tr></tbody></table>

where Cd: cadmium; Mn: Manganese; Cr: Chromium; Cu: copper; Ni: Nickel.

The choice of an application rate of 40 t ha⁻¹ reflects a representative scenario for understanding the maximum impact of biochar on the soil-plant system under intensive management conditions. Although this dose improved soil quality and productivity, it is essential to continue monitoring the cumulative effects in the long term, especially in relation to the mobility and assimilation of PTE by grains. The continuation of this study in future cycles may deepen the understanding of the dynamics involved and consolidate the use of biochar as a viable and safe alternative in agriculture.
It is important to highlight that this study evaluated a selected set of PTE, chosen based on their mobility, agronomic relevance, and detection capacity at the time of analysis. Future studies should incorporate a broader spectrum of PTE (Pb, Zn, and As) to ensure a more comprehensive risk assessment, especially when sewage sludge-derived biochar is used in food crop systems. Zinc (Zn) was not included due to analytical limitations and prioritization of elements with greater risk potential in the context of biosolid-derived amendments.

### 3.3. Evapotranspiration, Water Use Efficiency, and Soil Water Storage

In cycle II, which was evaluated under severe drought conditions, statistically significant differences were observed for both ET and WUE, with *p* < 0.05 for both parameters. The data indicate that the treatment with 20 t ha⁻¹ of biochar (B20) performed similarly to the CM and B40 treatments in terms of WUE, reflecting the ability to optimize water use under semiarid conditions. The B20 treatment resulted in an average WUE of 11.6 mm kg ha⁻¹, representing a significant increase compared with the lower application rates of SSB (B5 and B10) and the mineral fertilizer NPK. In contrast, the highest evapotranspiration was recorded for the B5 treatment, which differed significantly from the other treatments (*p* < 0.05).

The 20 t ha⁻¹ rate is particularly advantageous in semiarid environments, as it balances the need for efficient water use with the provision of favorable conditions for plant growth. The proximity of the B20 results to those of B40 and CM reinforces the effectiveness of this application rate as a sustainable and economically viable alternative, avoiding the excessive use of inputs and maintaining high levels of productivity under water stress conditions (Figure 9).

<figure><img src="image_11.png" /><figcaption>**Figure 9.** Evapotranspiration (ET) and water use efficiency (WUE) in response to different treatments. Different letters indicate significant differences according to Tukey's test (*p* < 0.05). Black dots indicate outliers.</figcaption></figure>

As shown in Figure 10, the data from cycle II indicate that the higher application rates of SSB (B10, B20, and B40) promoted greater soil water storage than did the NPK and B5 treatments. This trend reflects the ability of biochar to improve soil physical-hydric properties, increasing water retention and availability for plants, especially in environments with low rainfall.
<figure><img src="image_12.png" /><figcaption>**Figure 10.** Soil water storage (SWS) and rainfall during the crop cycle under different treatments. Lines represent SWS dynamics; bars represent daily rainfall.</figcaption></figure>

# 4. Discussion

## 4.1. *Physical, Chemical, and Biological Properties of the Soil*

Although the benefits associated with the application of biochar to Regosols are widely recognized, especially in studies such as [43], significant gaps remain with respect to investigations under field conditions. Experimental results have demonstrated wide variability, particularly in relation to changes in the physical properties of the soil subjected to the incorporation of biochar, due to the complex interactions between the characteristics of the applied material and the intrinsic attributes of the soil. In this study, the applied biochar had a particle size of less than 2 mm. This factor may play a relevant role in the dynamics of soil pore spaces, since relatively small biochar particles (<2 mm) can accumulate in soil micropores, resulting in potential adverse impacts on the soil bulk density (SD) and total porosity (TP) or even neutral effects [55].

Importantly, although sandy soils generally respond less significantly to biochar application than clay soils do, the impacts of biochar evolve over time. The aging of biochar in soil, particularly in sandy environments, can generate positive changes in soil physical properties, such as changes in soil composition and structure, due to chemical and physical evolution of the material. Wang et al. [56] highlighted that, over time, biochar can develop a greater specific surface area and reactivity, expanding its ability to interact with soil aggregates, influence water retention, and improve structural stability.

Mg²⁺ levels progressively increased throughout the crop cycles, with more noticeable increases in the surface layer (0–10 cm). This behavior can be attributed to the gradual release of alkaline cations present in the applied material, such as Mg²⁺, which becomes available because of the dissociation of oxides present in the biochar ash. This dynamic is in line with what is expected for ash-rich materials, which contribute to the supply of exchangeable bases to the soil, improving its fertility. Previous studies, such as that by Woicichowski et al. [57], indicate that biochar can meet nutritional needs in agricultural crops, especially in the early stages of development, in addition to promoting soil moisture retention. However, in the present study, the statistical effect size for magnesium was classified as irrelevant (η²g = 0.04) in specific analyses, suggesting that, despite the observed increase, the practical impact in the short term is limited.

Soil pH behavior reflects the alkaline properties of biochar and its ability to alter soil chemical conditions. Biochar is recognized for its alkaline character, which is due mainly to
the presence of ash rich in oxides formed during the pyrolysis process [43]. These oxides, when dissociated in the soil, release ions that neutralize acidity, resulting in increases in pH. Although the influence of pH observed in the present study was significant, the effect size ($\eta^2$g = 0.02) indicates a limited practical impact.

Soil pH is a key variable that directly influences fertility, regulating nutrient availability, microbial activity, and chemical reactions in the rhizosphere [58]. Increasing pH can improve the soil environment by reducing the toxicity of ions such as Al³⁺, increasing nutrient uptake by plants, and promoting interactions with beneficial microorganisms. Studies have reported significant increases in soil pH after biochar application, increasing from 4.93 to 6.04 at relatively high doses [25]. The results of the present study, however, do not fully corroborate previous observations, showing discrete increases in soil pH and limited practical effects. This can be explained by differences in edaphoclimatic conditions, soil characteristics, and evaluation times. Furthermore, the increase in pH tends to be more evident in soils with high initial acidity, which was not a characteristic of the Entisol studied [59].

The Na⁺, K⁺, and SO₄²⁻ contents did not significantly differ among the treatments, indicating uniformity in the conditions evaluated. However, the reduction observed in the levels of these elements throughout the crop development cycles may be related to the high rainfall rates recorded during the experimental period, especially in sandy textured soils, which are characterized by high susceptibility to leaching. The decrease in Na⁺ content in the soil is particularly beneficial, considering the negative impacts that high concentrations of this cation can have on the soil structure, such as increased SD and reduced physical and microbiological quality of the soil environment [60,61]. The natural removal of sodium through leaching, favored by high rainfall volumes, contributes to the maintenance of soil structure, avoiding problems such as compaction and salinization, which are often harmful to plant growth.

The behavior observed for K⁺ and SO₄²⁻ followed the same pattern, with reduced levels in the surface layer of the soil (0–10 cm) and a relative increase in the deepest layer (10–20 cm). This distribution reflects the mobility of these ions in the soil profile, especially under conditions of high rainfall, which intensifies leaching. The natural mobility of sulfur, which is predominantly present in the soil in the form of SO₄²⁻, makes it particularly susceptible to losses in environments with excessive drainage. Since up to 90% of the sulfur in the soil is associated with organic matter, its reduction in the surface layer may be related to the lower retention capacity in the sandy soil evaluated. Previous studies corroborate the results of this work. Lima et al. [62] did not observe significant changes in soil K⁺ levels after biochar application. Divergent results were found by Woicichowski et al. [57], who observed significant increases in soil K⁺ levels following biochar application.

The increase in soil P levels can be attributed to the high concentration of this element in SSB and its ability to adsorb and release P gradually. During the pyrolysis process, the P present in the biomass becomes stable, even at high temperatures, while more volatile elements, such as carbon, hydrogen, and oxygen, are lost [57,60]. The stability of P in biochar allows it to act as a continuous and sustainable source of phosphorus for agricultural crops, replacing soluble mineral sources and minimizing losses due to erosion and leaching. In sandy soils, the addition of biochar has shown promising results in increasing soil P levels, as also observed by Lima et al. [62]. Furthermore, biochar can function as a tool to mitigate climate instability in semiarid regions, providing a stable source of P and increasing the efficiency of nutrient use by plants. Thus, the contribution of high doses of sewage sludge biochar (SSB), rich in phosphorus, was already expected in sandy soils with low fertility, promoting an increase in available phosphorus levels in the soil, as observed by Fristák et al. [63]. These authors highlight the effectiveness of SSB in the gradual
release of phosphorus in environments with low retention capacity. Moreover, compared to other organic sources, SSB stands out not only for supplying high concentrations of phosphorus but also for providing it in chemical forms with varying degrees of stability, which reduces immediate solubilization and prolongs nutrient availability in the soil [17,26]. These mechanisms help explain and support the results obtained in the present study.

In the case of N, the positive results can be explained by the retention of N in the soil induced by biochar. This retention occurs due to the increase in the cation exchange capacity (CEC) of the soil and the increase in organic matter levels promoted by biochar [55]. Owing to the negative charges inserted during the pyrolysis process, biochar contributes to the adsorption of ammonium ions (NH₄⁺) on its surface, reducing leaching losses and increasing the availability of N for plants. This mechanism provides economic benefits by reducing the need for nitrogen fertilization and environmental benefits by reducing the risk of nitrate contamination in groundwater. The residual effect of biochar on N is also relevant since it contributes to the increase in this element over time, as demonstrated by Guimarães et al. [64]. The ability of biochar to minimize inorganic N losses through leaching reinforces its role as a sustainable solution in agroecosystems, especially in sandy-textured and low-fertility soils.

The rapid increase in soil TOC levels observed in this research can be attributed to the presence of labile forms of carbon in the biochar after pyrolysis. Although biochar is widely recognized for its recalcitrance, approximately 20% of the carbon contained in it is easily oxidized and made available in the soil [65]. This labile fraction is readily accessible to soil microorganisms, favoring the formation and increase of MBC, as evidenced in this study.

Elevated soil TOC levels have direct implications for improving soil fertility and quality. Soils with higher TOC levels have better water and nutrient retention capacity, promoting a more suitable environment for root growth and microbial activity. In sandy soils, which are typical of semiarid regions, the application of biochar is especially beneficial since these soils have low carbon and nutrient retention capacities. Thus, biochar not only contributes to the immediate increase in TOC but also plays a crucial role in building sustainable agricultural systems through long-term carbon sequestration.

Despite the positive results observed in this research, other studies have presented divergent results. For example, Lima et al. [62], when evaluating the effect of biochar on a Red Argisol in a dry tropical region of Brazil, did not find significant effects on TOC levels. The results obtained in this study are consistent with those of El-Naggar et al. [66], who investigated the potential of biochar in carbon mineralization and the rehabilitation of infertile soils. These authors reported a 72% increase in carbon stock in a sandy soil after the application of 30 t ha⁻¹ of biochar, highlighting its effectiveness in improving soil quality and enhancing organic carbon accumulation in environments with naturally low fertility.

The increase in MBC stocks observed in this study reflects the crucial role of biochar as a soil conditioner, especially in environments with low natural fertility, such as sandy soils. This increase can be attributed to the contribution of biochar to providing labile forms of carbon and essential nutrients, which serve as energy sources for soil microorganisms [67]. During the pyrolysis process, biochar not only retains a significant recalcitrant fraction of carbon but also forms labile compounds, which are rapidly mineralized and absorbed by the microbiota.

The labile fraction of carbon present in biochar has an immediate function, stimulating microbial activity in the short term. This rapid availability of carbon favors the multiplication of microorganisms and the intensification of metabolic activities in the rhizosphere. In parallel, the recalcitrant fraction, with its stable aromatic structure and rich functional groups, plays a long-term role, providing structural support and protection
against degradation and oxidation processes [65]. Another relevant aspect is the porous structure of biochar, which increases soil aggregation and creates microhabitats that shelter microorganisms. These pores offer protection against predators and adverse environmental conditions, in addition to improving water and nutrient retention in the soil, which are fundamental conditions for the sustainability of microbial activity in sandy soils.

Although biochar clearly benefits microbial biomass, the literature still lacks an in-
depth understanding of the mechanisms by which biochar interacts with the different
functional groups of microorganisms in the soil. Studies such as that by Medeiros et al. [68]
highlight the recent attention given to the influence of biochar on soil microbiological
structure, reinforcing the need for further research to unravel the complexity of these
interactions. In addition to recalcitrant carbon, pyrolysis also produces more labile carbon
fractions that can be readily assimilated by soil microorganisms. Thus, biochar contains a
small labile carbon fraction, which is rapidly mineralized, and a predominant stable carbon
fraction characterized by an aromatic structure resistant to oxidation and enriched with
functional groups [65]. Moreover, the highly porous structure of biochar enhances soil
aggregation and provides microhabitats that shelter microorganisms. Within these pores,
essential biological processes occur, including oxygen and moisture retention, protection
from predators, and nutrient supply [66].

4.2. *Bean Productivity and PTE*

The increase in GY observed in this study highlights the potential of biochar as a
promising technology for improving agricultural productivity, especially in regions with
low natural fertility. This increase can be explained by the cumulative improvement in soil
physical, chemical, and biological properties promoted by biochar application. By favoring
soil water storage (SWS) and reducing ET, biochar optimizes plant WUE. This interaction
directly contributes to a more stable environment in the root zone, promoting healthy crop
development. In agricultural systems characterized by water restrictions, such as those in
semiarid regions, the ability of biochar to retain water and nutrients is especially valuable,
reducing the impact of adverse weather conditions on crop yield.

Furthermore, biochar acts as an additional source of nutrients and as a catalyst for the retention and availability of essential elements, such as phosphorus and nitrogen, whose positive influence on agricultural productivity is well documented. The improvement in soil fertility, associated with the greater cation exchange capacity and stimulation of microbiota, creates a favorable environment for crop development, positively impacting GY. Previous studies corroborate the findings of this study. Research such as that by Lima et al. [43] and Figueiredo et al. [17] demonstrated that the use of biochar, even at moderate doses, significantly increases agricultural productivity and is recommended as an alternative or complement to mineral fertilizers. Lima et al. [62], for example, reported increases of up to 150% in the GY of beans in soils treated with biochar, reinforcing its viability as a sustainable strategy for the management of sandy soils. Figueiredo et al. [17], investigating the influence of sewage sludge biochar (SSB) on maize cultivation, reported that biochars produced at 300 °C and 500 °C significantly increased grain yield. The authors emphasized the potential of SSB as a substitute for mineral fertilizers in agricultural systems. In a subsequent study, Figueiredo et al. [26] indicated that biosolid biochar produced at 300 °C led to higher maize productivity over four consecutive growing seasons, highlighting its positive residual effects.

Although the results of this research demonstrate the benefits of SSB in improving the
soil environment and increasing GY, it is essential to address the potential risks associated
with PTE. Among the elements evaluated, cadmium (Cd) presented concentrations that
exceeded the maximum permissible limit (MPL) at the highest application rate (B40),
whereas nickel (Ni) and chromium (Cr) also presented high levels, warning of possible impacts on food and environmental security.

In contrast, application rates below 20 t ha⁻¹ demonstrated that Cd levels were maintained at safe levels, which reinforces the importance of adequate management strategies to minimize associated risks. Biochar at adequate application rates has the capacity to immobilize PTE with negative surface charges, reducing their bioavailability, which is affected mainly by the high aromaticity and specific surface area of the biochar, as well as its ability to increase the soil pH [69]. Considering the PTE levels in grains remained within safe limits for B20, this intermediate dose of sewage sludge biochar can be considered agronomically and toxicologically safe, without risk of contamination for human consumption.

### 4.3. Evapotranspiration, Water Use Efficiency, and Soil Water Storage

The application of biochar as a soil conditioner has shown promising results in increasing WUE and reducing ET, especially compared with conventional methods such as NPK and the use of SS. Biochar, especially when applied at intermediate doses (B20), has shown significant potential for optimizing water resource management in agricultural environments, particularly in semiarid regions where water scarcity is critical. The 148% increase in WUE with the use of B20 compared to NPK fertilization, and the 92.8% increase compared to SS, highlights the efficiency of the technology in improving agricultural productivity without compromising water resources. The difference between B20 and CM, which was only 2%, suggests that biochar can be a sustainable and efficient alternative for soil water management, contributing to environmental sustainability and reducing operational costs. Furthermore, the results obtained are reinforced by previous research, such as the study by Lima et al. [25], which demonstrated substantial increases in the WUE and productivity of bean crops when biochar was used. This effect is attributed to the ability of biochar to stimulate stomatal closure and reduce the transpiration rate, improving plant resistance to water stress. These findings are consistent with those reported by Zhang et al. [70], who studied the effects of wheat straw biochar on soybean and found that it significantly increased water use efficiency (WUE) by up to 27.5%. The authors recommend the application of biochar as a promising strategy for achieving future sustainable agriculture. Similarly, Liu et al. [71], working with wheat straw biochar in tobacco cultivation, concluded that biochar addition led to higher WUE compared to the control treatment.

The positive effect of adding biochar on WUE occurs by stimulating the closure of stomatal pores and reducing the transpiration rate, even under possible external stress. In this context, WUE indicates a plant's resistance to water stress. Water stress alters the physiological parameters of plant leaves, such as decreased photosynthetic processes, transpiration, and stomatal conductance, thereby reducing crop productivity [72]. Although the relationship between particle size and the amount of biochar applied for soil evaporation is not yet fully understood, studies such as that by Wang et al. [73] highlight the positive impact of biochar on altering soil physical characteristics, such as porosity and pore distribution. These characteristics are crucial for water retention, especially in sandy soils where the storage capacity is limited.

B20 SSB treatment positively affected SWS. Sandy soil has a low water storage capacity throughout the soil profile. Water stress severely affects soil attributes, interfering with the decomposition of organic matter, as well as nutrient mineralization. This factor occurs mainly in semiarid regions, where ET is greater than precipitation [20]. The increase in the organic fraction of the soil increases water storage, providing a solution for water retention in sandy soils. Biochar mimics the action of organic matter because its specific surface area is larger than that of sand, consequently increasing the specific surface area of the soil [73].
**5. Conclusions**

This study confirmed the potential of SSB as a sustainable practice for improving agricultural systems under semiarid conditions. Compared with conventional NPK fertilization, the application of SSB resulted in significant increases in water use efficiency (WUE), reaching a 148% increase. In addition, the treatment with 20 t ha⁻¹ SSB (B20) stood out as a sustainable and efficient alternative, promoting bean grain productivity, which was 146% greater than that of NPK in the second cycle, while maintaining low levels of evapotranspiration and increasing soil water retention. The 75% increase in TOC content and the improvement in soil fertility also highlighted the role of biochar in rainfed agricultural systems. These results reinforce that SSB can be a viable substitute for traditional inputs, with reduced environmental impact and greater sustainability.

Although higher rates of SSB (such as 40 t ha⁻¹) showed additional agronomic benefits, elevated levels of PTE, such as cadmium, were observed in bean grains, exceeding the MPLs. On the other hand, the application of 20 t ha⁻¹ was shown to be effective in maximizing agronomic benefits, such as grain yield and improving soil chemical and physical-hydraulic properties, without exceeding the PTE limits. These findings emphasize the importance of adequate management strategies to balance agronomic benefits with environmental risks, consolidating the safe and efficient use of SSB as a tool to address the challenges of agriculture in semiarid regions.

The application of 20 t ha⁻¹ of sewage sludge biochar is recommended as the optimal rate for bean cultivation in dryland agricultural systems. This dosage demonstrated a positive balance between improved productivity, water use efficiency, and environmental safety preservation, making it a viable and sustainable strategy for farmers in semiarid regions.

**Author Contributions:** Conceptualization, J.R.d.S.L., É.V.d.M., G.P.D. and R.E.d.M.; methodology, R.E.d.M., M.F.d.A.T.A. and V.P.d.S.; software, D.P.d.C. and R.E.d.M.; validation, E.D.B., J.H.d.S.J. and A.P.M.F.; formal analysis, R.E.d.M., M.F.d.A.T.A., V.P.d.S. and M.H.L.L.; investigation, R.E.d.M., E.D.B. and J.H.d.S.J.; resources, A.C.D.A., J.R.d.S.L., G.P.D. and É.V.d.M.; data curation, R.E.d.M., A.P.M.F. and M.C.d.B.S.; writing—original draft preparation, R.E.d.M., M.F.d.A.T.A., V.P.d.S., D.P.d.C., E.D.B., J.H.d.S.J., A.P.M.F., M.H.L.L., G.P.D., É.V.d.M. and M.C.d.B.S.; writing—review and editing, R.E.d.M., M.F.d.A.T.A., V.P.d.S., D.P.d.C., E.D.B., J.H.d.S.J., A.P.M.F., M.H.L.L., G.P.D., É.V.d.M. and M.C.d.B.S.; visualization, A.C.D.A., J.R.d.S.L. and C.H.; supervision, J.R.d.S.L. and C.H.; project administration, J.R.d.S.L.; funding acquisition, A.C.D.A. and J.R.d.S.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** The data presented in this study are available upon request from the corresponding author.

**Acknowledgments:** The authors would like to thank the Pernambuco Science and Technology Support Foundation (FACEPE) for providing a scholarship to the first author (IBPG-1549-5.00/21). The authors are grateful for the support of the BINSAH Project (Biochar and the Food and Water Security Nexus) and the National Institute of Science and Technology—National Observatory of Water and Carbon Dynamics in the Caatinga Biome (INCT/ONDACBC). The authors also thank COMPESA (Companhia Pernambucana de Saneamento) for providing the sewage sludge.

**Conflicts of Interest:** The authors declare no conflicts of interest.

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