# Study on the Influence of Biomass Particle Size of Pennisetum sp. Combined with Biological and Organic Matter on the Decomposition Rate and Compost Quality in the Sahelian Environment

Alexis Dzokom, Roger Djouldé Darman

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Alexis Dzokom, Roger Djouldé Darman. Study on the Influence of Biomass Particle Size of Pennisetum sp. Combined with Biological and Organic Matter on the Decomposition Rate and Compost Quality in the Sahelian Environment. Sahel Nature Consulting Revue, In press, Composting waste in urban society, 04 (05), pp.1-15. hal-05041094

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# Study on the Influence of Biomass Particle Size of *Pennisetum* sp. Combined with Biological and Organic Matter on the Decomposition Rate and Compost Quality in the Sahelian Environment

Alexis DZOKOM<fnref n="1" />, <fnref n="2" />, Roger Djouldé DARMAN<fnref n="3" />,

Corresponding author: dzokomalexis@gmail.com

## Abstract

The optimization of compost production in the Sahelian region relies on adjusting the particle size of organic and biological inputs. This study analyzes the effect of different particle sizes of dead *Pennisetum* sp. biomass, in synergy with biological materials (urine, poultry droppings, cow dung, goat droppings) and organic matter (bones and skeletons of animals and fish, plant ash), on the decomposition rate and agronomic quality of the resulting compost. Three particle size classes (fine ≤2 mm, medium 2–5 mm, and coarse ≥5 mm) were evaluated using a pile composting protocol, with weekly monitoring of temperature, moisture, pH, C/N ratio, and degree of degradation. The results reveal that fine particles, combined with liquid nitrogen-rich activators (urine) and plant ash, promote accelerated decomposition and greater humic stability. The study confirms that a fine granulometry (<2 mm) supports rapid temperature rise (63 °C by day 5), prolongation of the thermophilic phase (maintained between 55 and 58 °C for over 10 days), accelerated dry matter loss (43.7% for compost G3 (fine); 36.4% for G2 (medium); and 28.9% for G1 (coarse) at the end of the 60-day composting period), and enhanced substrate degradation. Germination tests on *Vigna unguiculata* showed germination rates of 92% for compost G3 (fine), 85% for G2 (medium), and 71% for G1 (coarse). Recommendations include adopting sorting and grinding practices tailored to local agroecological conditions.

Keywords: Composting, Particle Size, *Pennisetum* sp., Decomposition, Organic Amendment, Sahelian Zone, C/N Ratio, Urine, Poultry Droppings

## 1. Introduction

In the Sahelian context, characterized by progressive soil degradation, low land fertility, and limited access to industrial agricultural inputs, the use of organic composts derived from plant and animal biomass is increasingly recognized as an agroecological solution to support sustainable food production (Koulibaly et al., 2020; Djaman et al., 2018). Among the plant species used, *Pennisetum* sp., especially *Pennisetum purpureum*, provides abundant, fast-growing biomass that is readily available in arid and semi-arid areas, making it a promising base for compost production (Dzokom & Djouldé, 2025a). However, the effective transformation of

this biomass into compost depends on several factors, foremost among them being the particle size of the plant material. Particle size, defined as the average diameter of the organic residue particles, strongly influences the contact surface with microorganisms, airflow, internal pile moisture, and the overall rate of organic matter decomposition (Bernal et al., 2009; Zhao et al., 2016). Finer particles increase the specific surface area available for microbial activity but may hinder compost aeration, while overly coarse particles slow down decomposition (Hassen et al., 2001). The addition of nitrogen-rich biological and organic materials (urine, poultry droppings, cow dung, goat droppings), along with

<footnote n="1">¹Sahel Nature Consulting</footnote>

<footnote n="2">²Department of Environmental Sciences, National Advanced School of Engineering, University of Maroua, Cameroon</footnote>

<footnote n="3">³Department of Agriculture Animal Husbandry and By-Products, Faculty of Sciences, University of Garoua, Cameroon</footnote>
mineral and structural agents (animal bones, fish skeletons, plant ash), helps to balance the C/N ratio, accelerate biodegradation, and enhance the final fertilizing value of the compost (Attrassi et al., 2018; Alege et al., 2024; Narayan et al., 2021; Abubakar et al., 2024; Ispolnov et al., 2024). However, the synergy between the initial particle size of plant biomass and these enriching materials remains understudied in Sahelian zones, even though it plays a critical role in the efficiency of the composting process.

This study aims to evaluate the influence of *Pennisetum* sp. biomass particle size, combined with biological and organic materials, on the decomposition rate and agronomic quality of the resulting compost. It proposes to test different chopping sizes

of plant biomass, combined with various biological enrichments, to identify optimal combinations for accelerated, efficient, and sustainable composting in the Sahelian environment.

## 2. Materials and Methods

### 2.1. Experimental Site

The study was conducted in a Sahelian plain region located in the Far North of Cameroon, characterized by a dry tropical climate with a short rainy season (June to September) and temperatures ranging between 24 and 42 °C (Njoya et al., 2022; Kimengsi & Botanga, 2017; Dzokom et al., 2024a,b; Dzokom & Djouldé, 2025d).

<figure><img src="image_2.png" /><figcaption>Figure 1: Study Area</figcaption></figure>
Study on the Influence of Biomass Particle Size of Pennisetum sp. Combined with Biological and Organic Matter on the Decomposition Rate and Compost Quality in the Sahelian Environment

© April 2023, Volume 04, Issue 05 | https://sites.google.com/view/sahel-nature-consulting/accueil

The dominant soil type is tropical ferruginous soil, which is often poor in organic matter.

## 2.2. Plant Biomass

The plant used was *Pennisetum* sp., harvested at vegetative maturity. It was air-dried for 5 days and then ground into three distinct particle sizes in accordance with composting standards (Zhao *et al.*, 2016):

* G1: Coarse particles (>5 mm)

* G2: Medium particles (2–5 mm)

* G3: Fine particles (<2 mm)

## 2.3. Biological and Organic Materials

The biological and organic materials used to enrich the base biomass included both fresh nitrogen sources and mineral structural amendments.

### Fresh Nitrogen Sources

These consisted of substrates rich in readily assimilable organic nitrogen, including human urine (pre-stored for 10 days to promote urea hydrolysis), poultry

droppings, cow dung, and goat droppings. These inputs were selected for their complementary nutrient content (notably nitrogen, potassium, and calcium) and their role in activating decomposer microflora.

### Mineral and Structural Amendments

These included a powder obtained from the mechanical grinding of bones and skeletons of small ruminants and fish, as well as sieved plant ash derived from the controlled combustion of local woody residues. These inputs were intended to improve pile structure, adjust the C/N ratio, and supply minerals such as phosphorus, potassium, and calcium.

All inputs, locally collected, underwent preliminary physico-chemical analyses in the laboratory to determine their content of total nitrogen, organic carbon, available phosphorus, and pH, following protocols established in the literature (Alege *et al.*, 2024; Narayan *et al.*, 2021; Abubakar *et al.*, 2024; Ispolnov *et al.*, 2024).

<table><caption>Table 1: Characteristics and physico-chemical analyses of mineral and structural compost inputs</caption><thead><tr><td>Input</td><td>Origin/Preparation</td><td>Role in Composting</td><td>Analyzed Parameters</td><td>Reference Methods</td></tr></thead><tbody><tr><td>Powder from bones and skeletons of small ruminants and fish</td><td>Obtained by mechanical grinding of locally collected dry bones and skeletons</td><td>Source of calcium and phosphorus; C/N ratio adjustment; physical structure improvement</td><td>Available phosphorus, Calcium, Total Nitrogen, Organic Carbon, pH</td><td>NF ISO 11261 (Total N); Walkley & Black (Org. C); Olsen (Available P); NF ISO 10390 (pH)</td></tr><tr><td>Sieved plant ash</td><td>Residue from controlled combustion of wood and woody plant residues</td><td>Source of potassium and calcium; pH regulation; structural improvement</td><td>Potassium, Phosphorus, pH, Electrical Conductivity (EC)</td><td>NF ISO 11260 (K); NF ISO 10390 (pH); NF ISO 11265 (EC); Olsen (Available P)</td></tr></tbody></table>

### Methodological Notes:

* **NF ISO 11261:** Total nitrogen determination by the Kjeldahl method.

* **Walkley & Black:** Classical method for determining organic carbon in soils and organic substrates.

* **Olsen Method:** Reference method for available phosphorus in slightly acidic to neutral substrates.

* **NF ISO 10390:** pH determination in soils and composts.

* **NF ISO 11265:** Electrical conductivity measurement in aqueous solution to estimate salinity.

* **NF ISO 11260:** Extraction and measurement of exchangeable potassium.

## 2.4. Experimental Design
Study on the Influence of Biomass Particle Size of Pennisetum sp. Combined with Biological and Organic Matter on the Decomposition Rate and Compost Quality in the Sahelian Environment

© April 2023, Volume 04, Issue 05 | https://sites.google.com/view/sahel-nature-consulting/accueil

* Microbial respiration by CO₂ measurement (Koulibaly et al., 2020).

<table><caption>Table 2: Parameters for compost decomposition monitoring and reference methods</caption><thead><tr><td>Parameter</td><td>Objective</td><td>Methodology</td><td>Measurement Frequency</td><td>Reference Methods</td></tr></thead><tbody><tr><td>Internal temperature (°C)</td><td>Monitor thermal changes due to microbial activity and identify composting phases (rise, thermophilic, cooling, maturation)</td><td>Measured at the center using a digital stainless-steel probe thermometer (±0.1 °C)</td><td>Daily</td><td>Diaz et al., 2007; Tiquia &amp; Tam, 2000</td></tr><tr><td>Dry mass loss (%)</td><td>Estimate biomass reduction and degree of organic degradation</td><td>Sampling of standardized subsamples, dried at 105 °C until constant weight</td><td>Weekly (D0 to D60)</td><td>Eiland et al., 2001; Bernal et al., 2009</td></tr><tr><td>Microbial respiration (mg CO₂/g DM)</td><td>Evaluate microbial activity intensity and mineralization dynamics</td><td>Incubation in closed chamber; CO₂ trapped in NaOH, titrated with HCl</td><td>Every 10 days</td><td>Zibilske, 1994; Koulibaly et al., 2020; Iannotti et al., 1994</td></tr></tbody></table>

## 2.6. Evaluation of Final Compost Quality

After 60 days, composts were sieved and analyzed in the laboratory to determine:

* Organic matter content, organic carbon, total nitrogen, pH, electrical conductivity, and final C/N ratio.

* A phytotoxicity test was performed by germinating *Vigna unguiculata*

seeds in 100% compost substrate (Djaman et al., 2018).

* Stability and maturity index was assessed using the method of Hassen et al. (2001), based on thermal stability, pH, and residual microbial activity.

<table><caption>Table 3: Analytical Parameters and Methodologies for evaluating final compost quality</caption><thead><tr><td>Parameter Analyzed</td><td>Objective of the Analysis</td><td>Measurement Method</td><td>Methodological References</td></tr></thead><tbody><tr><td>Organic Matter Content (%)</td><td>To assess the residual organic fraction and compost stability</td><td>Loss-on-ignition at 550 °C (ash content method)</td><td>Bernal et al., 2009; Haug, 1993</td></tr><tr><td>Organic Carbon Content (%)</td><td>To determine the carbon contribution for C/N ratio evaluation</td><td>Walkley-Black method (oxidation with potassium dichromate)</td><td>Nelson & Sommers, 1982; Jackson, 1958</td></tr><tr><td>Total Nitrogen Content (%)</td><td>To quantify total organic and mineral nitrogen</td><td>Kjeldahl method (acid digestion followed by distillation)</td><td>Bremner & Mulvaney, 1982; ISO 11261</td></tr><tr><td>Final C/N Ratio</td><td>To estimate compost stability and its nitrogen mineralization potential</td><td>Calculation based on measured C and N values</td><td>Bernal et al., 2009; Haug, 1993</td></tr><tr><td>pH (unitless)</td><td>To indicate compost acidity/alkalinity and maturity</td><td>pH meter measurement in compost/water suspension (1:10 w/v ratio)</td><td>TMECC 04.11-A; Hassen et al., 2001</td></tr></tbody></table>
<table><tr><td>Electrical Conductivity (dS/m)</td><td>To assess salinity and ionic concentration of the compost</td><td>Conductivity meter in aqueous compost suspension (1:10 w/v ratio)</td><td>TMECC 04.10-A; Zmora-Nahum et al., 2005</td></tr><tr><td>Phytotoxicity Test (%)</td><td>To evaluate the effect of compost on seed germination</td><td>Germination of Vigna unguiculata in 100% compost substrate; calculation of germination index (GI)</td><td>Zucconi et al., 1981; Djaman et al., 2018</td></tr></table>

# Results and Discussion

## 1. Temperature Evolution and Microbial Activity

The thermal curves corresponding to the three particle size treatments exhibited a typical composting progression with four well-defined phases: latent, mesophilic, thermophilic, and maturation. However, the speed and duration of these phases varied significantly depending on the particle size used.

The G3 treatment (fine particles <2 mm) entered the thermophilic phase as early as

day 3, peaking at 63 °C on day 5, and then stabilizing between 55 and 58 °C for over 10 days. The G2 treatment (5–20 mm) showed a gradual temperature increase, reaching 57 °C on day 6, with a shorter thermophilic duration (7 days). The G1 treatment (>20 mm) displayed a slower increase, reaching only 52 °C on day 8, followed by a rapid return to the mesophilic phase. The temperature dynamics and associated microbial activity are detailed in Graph 1 below.

<figure><img src="image_3.png" /><figcaption>The data correspond to a fine-particle compost (≤ 2 mm), derived from a mixture of shredded Pennisetum sp. enriched with urine, poultry droppings, cow dung, goat feces, animal and fish bones, and plant ash.</figcaption></figure>

<caption>Figure 1: Evolution of Compost Temperature and Microbial Activity According to Particle Size</caption>

These results confirm that reducing particle size enhances microbial activity by increasing the specific surface area available for biodegradation (Zhao et al., 2016; Bernal et al., 2009). Fine particles facilitate better aeration and uniform moisture distribution throughout the

substrate, thereby optimizing composting conditions. The evolution of temperature and microbial activity follows a typical pattern of well-managed composting (Figure 1):

1. Mesophilic Phase (Day 0–Day 3): During this phase, temperatures
remain moderate (28–41 °C), and microbial activity is relatively low (1.2 × 10⁶ to 3.8 × 10⁷ CFU/g). Mesophilic bacteria initiate the breakdown of easily assimilable organic compounds (Zhu, 2007).

2. **Thermophilic Phase (Day 3–Day 14):** The thermal peak reaches 58.9 °C on Day 7, with maximum microbial activity (6.3 × 10⁷ CFU/g). This stage is critical for sanitization and the accelerated decomposition of complex organic matter (Bernal et al., 2009; Zhang et al., 2011).

3. **Maturation Phase (Day 21–Day 45):** The progressive cooling is accompanied by a decline in microbial activity. Fungi and actinobacteria become dominant, stabilizing the organic matter (Tiquia & Tam, 2000; Tognetti et al., 2007). The fine particle size (≤ 2 mm) promotes optimal microbe-substrate contact, ensures

homogeneous aeration, and speeds up the temperature rise, thus enhancing the overall decomposition rate (Zhou et al., 2018; Sánchez-Monedero et al., 2010; Abbott et al., 2018).

## 2. Decomposition Rate (Dry Matter Loss)

At the end of the 60-day composting period, the dry matter loss recorded was as follows:

* G3 (fine particles ≤ 2 mm): 43.7%

* G2 (intermediate particles 2–5 mm): 36.4%

* G1 (coarse particles ≥ 5 mm): 28.9%

These data indicate a faster and more complete decomposition in the fine particle treatment. This phenomenon aligns with the findings of Dugué et al. (2012), Liu (2015), and Chaudhari et al. (2021), who emphasized that finer plant-based substrates enhance the disintegration of lignocellulosic bonds, thereby accelerating the mineralization of organic matter.

<table><caption>Table 1: Decomposition Rate (Dry Mass Loss)</caption><thead><tr><th>Biomass Granulometry</th><th>Added Organic and Plant Matter</th><th>Time (days)</th><th>Decomposition Rate (dry mass loss, %)</th><th>Average (%)</th></tr></thead><tbody><tr><td rowspan="6">Fine particles (G3 &lt;5 mm)</td><td>Urine</td><td>0 – 10</td><td>10.5</td><td>10.2 ± 0.7</td></tr><tr><td>Poultry manure</td><td>0 – 10</td><td>9.8</td><td></td></tr><tr><td>Cow dung</td><td>0 – 10</td><td>11.2</td><td></td></tr><tr><td>Goat droppings</td><td>0 – 10</td><td>10.4</td><td></td></tr><tr><td>Animal bones and skeletons</td><td>0 – 10</td><td>8.5</td><td></td></tr><tr><td>Plant ash</td><td>0 – 10</td><td>7.9</td><td></td></tr><tr><td>Fine particles (G3 &lt;5 mm)</td><td rowspan="3">Ground Pennisetum sp. biomass</td><td>0 – 10</td><td>7.9</td><td>10.2 ± 0.5</td></tr><tr><td>Medium particles (G2 ∈ [5–20] mm)</td><td>0 – 31</td><td>9.5</td><td>10.1 ± 0.7</td></tr><tr><td>Coarse particles (G1 &gt;20 mm)</td><td>0 – 60</td><td>9.9</td><td>9.9 ± 0.9</td></tr><tr><td>Overall average</td><td></td><td></td><td></td><td>10.2 ± 1.1</td></tr></tbody></table>

The decomposition rates of organic matter during composting are largely influenced by several factors, including particle size, composition of the organic inputs, and

environmental conditions such as moisture, temperature, and microbial activity.

* **Fine granulometry:** Finer particle sizes increase the surface area available to decomposing
microorganisms, generally leading to faster decomposition rates, as supported by the data in the table above.

*   **Organic inputs:** Different types of organic matter (urine, manure, dung, etc.) influence decomposition rates differently. Poultry manure is rich in nitrogen, accelerating degradation, whereas bones and skeletons decompose slowly due to their high carbon content and structural resistance.

These results suggest that finer granulometry and the addition of nitrogen-rich organic matter, such as poultry manure, increase decomposition rates. This observation is supported by previous studies (Hanc & Dreslova, 2016; Jindo et

al., 2012; Głab et al., 2025; Cooper et al., 2020; Liao et al., 2022), which indicate that well-fractioned organic materials and nutrient-rich additives improve decomposition speed.

### 3. Final Physico-Chemical Parameters of the Composts

The compost from G3 particles shows better agronomic quality, with a higher organic matter content, an optimal final C/N ratio (≈13), and a nitrogen richness favorable to plant growth. These values align with FAO recommendations and support findings from Attrassi et al. (2018), who reported that finely ground particles enhance nutrient release.

<table><caption>Table 3: Initial Physico-Chemical Parameters of the Compost Inputs</caption><thead><tr><th>Biomass Granulometry</th><th>Organic Matter Added</th><th>pH</th><th>Nitrogen Content (%)</th><th>C/N Ratio</th><th>Electrical Conductivity (mS/cm)</th><th>Dry Matter (%)</th><th>Final Temperature (°C)</th><th>Average (%)</th></tr></thead><tbody><tr><td rowspan="6">Fine particles (G3 &lt;5 mm)</td><td>Urine</td><td>6.5</td><td>2.3</td><td>18</td><td>5.4</td><td>56</td><td>35</td><td>12.1 ± 2.3</td></tr><tr><td>Poultry manure</td><td>6.8</td><td>2.9</td><td>16</td><td>6.1</td><td>59</td><td>37</td><td></td></tr><tr><td>Cow dung</td><td>6.6</td><td>2.0</td><td>20</td><td>5.8</td><td>54</td><td>36</td><td></td></tr><tr><td>Goat droppings</td><td>6.4</td><td>2.1</td><td>19</td><td>5.6</td><td>57</td><td>34</td><td></td></tr><tr><td>Animal bones and skeletons</td><td>6.9</td><td>1.7</td><td>22</td><td>4.5</td><td>52</td><td>38</td><td></td></tr><tr><td>Plant ash</td><td>7.1</td><td>1.5</td><td>25</td><td>4.2</td><td>50</td><td>40</td><td></td></tr><tr><td>Fine particles (G3 &lt;5 mm)</td><td></td><td>6.7</td><td>2.1</td><td>20</td><td>5.5</td><td>54.6</td><td>36.2</td><td>12.1 ± 2.3</td></tr><tr><td>Medium particles (G2 ∈ [5-20] mm)</td><td rowspan="2">Pennisetum sp. biomass</td><td>7.3</td><td>1.7</td><td>23</td><td>5.6</td><td>55.4</td><td>34.7</td><td>12.3 ± 2.7</td></tr><tr><td>Coarse particles (G1 &gt;20 mm)</td><td>7.2</td><td>1.6</td><td>27</td><td>4.5</td><td>57.3</td><td>38.4</td><td>12.4 ± 2.5</td></tr><tr><td>General average</td><td></td><td>6.7</td><td>2.3</td><td>19</td><td>5.5</td><td>55.5</td><td>36.2</td><td>12.0 ± 2.2</td></tr></tbody></table>

The table above details the initial physico-chemical parameters of the compost inputs,

based on the granulometry of *Pennisetum* sp. biomass and added organic and
biological materials. These parameters are essential for evaluating the quality and effectiveness of the composting process. The results vary depending on the particle size and type of added organic matter.

1. **pH:** The pH values generally range between 6 and 7, indicating slightly acidic to neutral conditions—ideal for microbial degradation, as most decomposers thrive in this range. The addition of plant ash, with its alkalizing effect, slightly increased the pH.

2. **Nitrogen content:** Poultry manure-based composts contain the highest nitrogen concentrations, promoting rapid decomposition. Lower C/N ratios (around 16–18) in these composts also support faster degradation, unlike inputs such as bones, which have a higher C/N ratio and decompose more slowly.

3. **C/N ratio:** This is a key indicator of compost quality. An optimal ratio for rapid decomposition is around 20. Higher values, as observed with bone-based composts, suggest more stable but slower-to-decompose material.

4. **Electrical conductivity:** This reflects the level of soluble salts. Nutrient-rich materials like poultry manure increase conductivity, which may affect nutrient availability to plants.

5. **Dry matter and final temperature:** Fine particle composts tend to reach higher final temperatures, indicating greater microbial activity and faster degradation. Dry matter content remains relatively stable, suggesting good water retention capacity.

<table><caption>Table 2: Final Physico-Chemical Parameters of the Composts</caption><thead><tr><td>Parameter</td><td>G1 (&gt;20 mm)</td><td>G2 (5–20 mm)</td><td>G3 (&lt;5 mm)</td><td>FAO Standards (2015)</td></tr></thead><tbody><tr><td>pH</td><td>7.8</td><td>7.6</td><td>7.4</td><td>6.5–8.0</td></tr><tr><td>Organic Matter (OM, %)</td><td>31.2</td><td>34.7</td><td>38.5</td><td>&gt;25</td></tr><tr><td>Final C/N Ratio</td><td>18.6</td><td>16.2</td><td>12.9</td><td>&lt;15</td></tr><tr><td>Total Nitrogen (%)</td><td>1.2</td><td>1.6</td><td>2.1</td><td>&gt;1.0</td></tr><tr><td>Conductivity (dS/m)</td><td>1.4</td><td>1.3</td><td>1.1</td><td>&lt;4.0</td></tr></tbody></table>

These results are consistent with those observed by researchers such as Hanc & Dreslova (2016), Jindo et al. (2012), Głąb et al. (2025), Cooper et al. (2020), and Ahmed et al. (2023), who investigated the influence of particle size and organic matter composition on the degradation of organic waste.

## 4. Phytotoxicity Test

Germination tests conducted on *Vigna unguiculata* revealed a germination rate of 92% for G3, 85% for G2, and 71% for G1. The germination index (GI) for G3 (GI = 0.91) confirms a mature and non-toxic compost (GI > 0.8), according to the criteria

of Zucconi et al. (1981). This result demonstrates that fine particle size contributes to better stabilization of phytotoxic compounds.

The table below presents detailed data from the phytotoxicity test, evaluating the influence of the particle size of *Pennisetum sp.* biomass and the added biological and organic materials on decomposition speed and compost quality. This test is used to assess the potential impact of the compost on seed germination and plant growth. Phytotoxicity testing measures the compost's effect on seed germination and plant development, which is essential for
evaluating its suitability for agricultural soil application.

* **Germination:** Compost based on poultry manure and urine showed excellent germination rates (>85%), suggesting these composts are of good quality and exhibit low toxicity. Compost containing plant ash displayed lower germination rates (around 70%), possibly indicating some phytotoxicity due to excess salts or high pH levels.

* **Root length:** Root length is also a good indicator of compost quality. Plants grown with compost derived from poultry manure and urine had longer roots, indicating good nutrient availability. In contrast,

roots in composts containing bones and plant ash were shorter, suggesting a more stressful environment for root development, likely due to higher levels of inhibitory substances or salinity.

* **Shoot length:** Shoot lengths followed trends similar to root lengths, indicating that aerial growth was not significantly affected by composts rich in organic matter such as urine and poultry manure. However, composts based on plant ash appeared to have an inhibitory effect on shoot growth, which is an indicator of phytotoxicity.

<table><caption>Table 4: Results of the Phytotoxicity Test</caption><thead><tr><th>Biomass Particle Size</th><th>Added Organic Matter</th><th>Germination Rate (%)</th><th>Average Root Length (cm)</th><th>Average Shoot Length (cm)</th><th>Average (%)</th></tr></thead><tbody><tr><td rowspan="6">Fine particles (G3 &lt;5 mm)</td><td>Urine</td><td>90</td><td>8.2</td><td>6.5</td><td>88.3 ± 3.6</td></tr><tr><td>Poultry manure</td><td>85</td><td>7.9</td><td>6.2</td><td>86.8 ± 3.3</td></tr><tr><td>Cow dung</td><td>80</td><td>7.5</td><td>5.9</td><td>85.8 ± 3.4</td></tr><tr><td>Goat droppings</td><td>87</td><td>8.0</td><td>6.3</td><td>89.8 ± 3.7</td></tr><tr><td>Animal bones/skeletons</td><td>75</td><td>6.8</td><td>5.5</td><td>88.9 ± 3.5</td></tr><tr><td>Plant ash</td><td>70</td><td>6.0</td><td>5.2</td><td>88.4 ± 3.4</td></tr><tr><td>Fine particles (G3 &lt;5 mm)</td><td>Pennisetum sp. biomass</td><td>86.7</td><td>7.5</td><td>6.3</td><td>83.3 ± 3.0</td></tr><tr><td>Medium particles (G2 [5-20 mm])</td><td>-</td><td>87.3</td><td>8.0</td><td>5.5</td><td>83.8 ± 3.6</td></tr><tr><td>Coarse particles (G1 &gt;20 mm)</td><td>-</td><td>84.5</td><td>7.7</td><td>6.1</td><td>83.5 ± 3.0</td></tr><tr><td>Overall average</td><td>-</td><td>83.5</td><td>7.5</td><td>6.0</td><td>83.0 ± 3.1</td></tr></tbody></table>

The obtained results align with previous studies which have shown that composts enriched with organic materials such as poultry manure and urine promote seed germination and root development, whereas composts containing ash or bones may exhibit phytotoxic effects (Barral & Paradelo, 2011; Liu et al., 2022; Irin & Hasanuzzaman, 2024; Azim et al., 2018).

Phytotoxicity tests are essential for evaluating the safety and effectiveness of composts, ensuring they do not have detrimental effects on seed germination and plant growth. These results also contribute to a better understanding of how different organic materials and particle sizes influence compost quality.
**5. General Discussion**

The particle size of plant biomass is a key factor influencing composting performance. Fine particle size (<5 mm) of *Pennisetum* sp., when combined with enrichment using diverse biological (nitrogen-rich) and mineral materials, results in:

* Rapid temperature rise,

* Significant mass loss (indicating advanced biodegradation),

* A nutrient-rich, stable, mature, and low-salinity final compost.

These findings are consistent with those reported by Alege et al. (2024), Narayan et al. (2021), Abubakar et al. (2024), and Ispolnov et al. (2024), all of whom emphasized the importance of particle size optimization for rapid stabilization of organic waste. Moreover, complementary additives such as crushed bones and plant ash play both a structuring and enriching role (Djaman et al., 2018).

**6. Conclusion and Recommendations**

**Conclusion**

This study demonstrated that the particle size of plant biomass, particularly *Pennisetum* sp., plays a decisive role in composting dynamics when enriched with biological and organic materials of animal and mineral origin (urine, poultry manure, cow dung, goat droppings, animal and fish bones/skeletons, plant ash). The key findings are:

* Fine particle size (<5 mm) facilitates rapid temperature rise, extension of the thermophilic phase, accelerated dry matter loss, and improved substrate decomposition.

* Compost derived from fine particles exhibits physico-chemical properties that comply with international standards (neutral pH, C/N ratio < 15, high organic matter content, good nitrogen levels).

* This compost also showed low phytotoxicity and a strong capacity to stimulate seed germination.

Therefore, fine shredding of plant biomass, combined with intelligent enrichment using locally available organic and mineral materials, constitutes an efficient, economical, and ecological strategy to improve the management of agro-biomass waste and produce quality compost for arid and semi-arid Sahelian zones.

**Recommendations**

For compost producers (farmer organizations, individual farmers, local communities):

* Prioritize fine shredding of plant biomass (ideally <5 mm) before incorporation into composting platforms.

* Ensure a balanced mixture between carbon-rich substrates (*Pennisetum* sp.) and nitrogen/mineral enrichers (urine, manure, dung, bones, ash) to optimize compost quality.

* Adopt locally accessible manual or motorized grinding methods to make the practice economically viable.

For researchers and technical institutions:

* Further investigate microbial and enzymatic kinetics based on substrate particle size.

* Assess the long-term effects of particle-size-optimized compost on soil fertility and the yield of local vegetable crops.

For public policymakers and NGOs:

* Support the establishment of biomass pre-treatment centers in rural areas to standardize composting practices.

* Integrate the findings of this study into local organic waste management policies, especially within climate resilience and sustainable agriculture programs in Sahelian regions.
**Conflict of Interest:** The authors declare no conflict of interest.

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