# Torrefied biomass (Biotorr) potential for agricultural applications. A review

Lucas Lesaint, Cornelia Rumpel, David Houben

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Lucas Lesaint, Cornelia Rumpel, David Houben. Torrefied biomass (Biotorr) potential for agricultural applications. A review. Waste Management, 2026, 209, pp.115197. 10.1016/j.wasman.2025.115197. hal-05322084

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Review

# Torrefied biomass (Biotorr) potential for agricultural applications. A review

Lucas Lesaint<sup>a,b,*</sup> <sup>ID</sup>, Cornelia Rumpel<sup>b</sup>, David Houben<sup>a</sup>

<figure><img src="image_3.png" /></figure>

ARTICLE INFO

Keywords:

Crop residues

Torrefaction

Biomass improvement

Soil conditioner

Biochar

A B S T R A C T

Crop residue management has gained attention for its potential to increase soil organic matter and improve soil health in agricultural systems. For these purposes, pyrolysis of crop residues, to produce biochar, and its impacts on soils have been extensively studied, but torrefied biomass (biotorr), produced by torrefaction between 200 and 300 °C represents a promising alternative. Torrefaction offers advantages such as higher mass yields and lower production costs, which could make it an accessible and cost-effective soil conditioner.

This review synthesises current knowledge on the effects of torrefaction on the properties of biotorr and assesses its potential as a soil conditioner. The results showed that torrefaction affects the composition of biotorr differently depending on the feedstock and the torrefaction parameters. While the yield of biotorr decreased with increasing torrefaction temperature, it was still higher than that of biochar. Compared to the feedstock, the carbon content and stability of biotorr improved significantly. Our analysis also indicated higher levels of P and K in biotorr, although further research is needed to determine their bioavailability.

Given these results, biotorr shows a strong potential as a soil conditioner to improve soil health and support sustainable agriculture. However, it is now necessary to further investigate the potential of biotorr for agricultural application through field experiments, especially to assess how biotorr affects soil nutrient dynamics and carbon sequestration.

# 1. Introduction

In recent years, there has been a growing awareness of crop residue management to increase the amount of soil organic matter necessary to maintain and improve soil functionality in agricultural systems, or to valorise certain residues through alternative uses (Porichha et al., 2021; Sarkar et al., 2020). Some thermochemical conversion processes, e.g. pyrolysis, are primarily aimed at energy production, but produce biochar, a byproduct resistant to microbial degradation that can be applied as a soil amendment (Lehmann, 2007). In contrast, mild thermochemical treatments, like torrefaction, have been developed specifically to improve the stability of crop residue organic carbon against microbial decay (Li et al., 2023; Zhang et al., 2020). All thermochemica methods are based on transforming the biomass into materials with a higher carbon (C) content and more chemical recalcitrant structures, thus increasing their carbon sequestration potential after soil amendment (Joseph et al., 2021; Kwoczynski and Ćmelík, 2021). The most prominent thermal degradation product is known as biochar, the solid residue from biomass pyrolysis (300–700 °C). Biochar is a soil conditioner with

positive impacts on soil functioning depending on feedstock and its production conditions (Sohi et al., 2010). A number of studies highlighted an improvement in soil properties following amendment with biochar, including an increase in soil pH, soil organic C storage, nutrient availability, soil water retention, aggregate stability and a reduction in N₂O emissions (Aubertin et al., 2021; Gross et al., 2021; Joseph et al., 2021; Nobile et al., 2022). Biochar application can also increase microbial activity and N use efficiency, which in turn can promote plant growth and crop production (Alkharabsheh et al., 2021; Lebrun et al., 2024; Palansooriya et al., 2019). However, biochar effects on soil properties and agricultural yields are rate dependent, short-lived and uncertain in temperate environments (Jeffery et al., 2017; Nobile et al., 2022). Negative effects of pyrolysis such as carbon loss during biochar production (Li et al., 2023) and potential negative impacts of biochar on soil properties and soil fauna depending on the biochar-soil interaction (Brtnicky et al., 2021) may thus outweigh its potential as a negative emission technology.

To avoid negative effects and reduce production costs, torrefaction, an alternative method based on thermal alteration at temperatures

<footnote><sup>a</sup> UniLaSalle, AGHYLE UP 2018.C101, 60000, Beauvais, France</footnote>

<footnote><sup>b</sup> Sorbonne U, CNRS, UMR IEES, Paris, France</footnote>

<footnote>* Corresponding author at: UniLaSalle, AGHYLE UP 2018.C101, 60000, Beauvais, France.</footnote>

<footnote>E-mail address: lucas.lesaint@unilasalle.fr (L. Lesaint).</footnote>
between 200 and 300 °C may be used (Nhuchhen et al., 2014; Uslu et al., 2008) to generate products for several applications. At these lower temperatures, the torrefaction processes leads to higher mass yield (Ercan et al., 2023). Torrefied biomass, hereafter defined as biotorr, has been predominantly used in the bio-energy industry (Mamvura and Danha, 2020; Niu et al., 2019), however, it might also have a strong potential as organic amendment for agricultural applications due to its high nutrient concentration and high fraction of labile C which may stimulate soil biological activity (Ogura et al., 2016; Thengane et al., 2020; Rehman and Thengane, 2025).

This review aims to (1) to gather all relevant information on the impact of torrefaction processes on the characteristics of biotorr produced from a wide range of feedstocks, (2) to assess potential of transforming crop residues into biotorr as soil amendment and compare it to other materials such as raw biomass or biochar. For this purpose, literature data on torrefaction and the impact of biotorr on soil properties were collected using the Web of Science platform to synthesize and analyse the current knowledge and identify new research avenues.

## 2. Data collection

### 2.1. Data collection: Biotorr properties

To understand the impact of torrefaction parameters on the properties of biotorr originating from different feedstocks, a search was made using the Web of Science advance search query builder. In April 2024, we searched the Web of Science with the keywords « (torrefaction OR torrefied) AND (biomass) ». 3 652 results were found, published between 1990 and 2024. Among these articles, a selection was made to exclude works using the keywords “wastewater”, “sludge”, “human waste” and “animal waste” to focus the research on plant residues.

The available literature was mainly focused on the potential of biotorr as a renewable energy source, and mostly included information on the heating value or calorific value of the feedstock or biotorr. We identified 21 articles dealing with the impact of torrefaction on the properties of biotorr and compiled the data. The dataset produced and used in this review is available as supplementary material (Table S1). These articles provided information on 29 different feedstocks, which we classified into three classes for the data analysis: (1) wood biomass, (2) crop residue and (3) oil/pomace. This classification was chosen to reflect the different composition of feedstocks in terms of C and N content. The data including information on 14 crop residues, 7 wood residues and 6 oils/pomaces was presented using the mean value for each parameter of each feedstock. Most of the articles presented data on the mass yield, C content and N content of the biotorr, however, two also presented data on K content, and one on P content. Data were standardized to the same units to allow for comparison. For each feedstock class, torrefaction data were further classified into three categories based on torrefaction temperature: light (200–235°C), mild (235–275°C), and severe (275–300°C) (Chen et al., 2015).

### 2.2. Data collection: Biotorr impact on soils and crops

To summarise the information on the impact of biotorr on soil, additional research was carried out using the Web of Science tool. The terms used for the query were “torrefaction OR torrefied AND soil amendment”. This search resulted in a total of 76 articles published between 2009 and 2024, with an increase in published articles in the recent years, showing the growing interests for the subject. Among these articles, nine included direct studies of the impact of biotorr on soil or crops, with one in situ experiments, six pot experiments and two petri dish experiments.

### 2.3. Data analysis

To describe the impact of torrefaction intensity on biotorr properties,

data analysis was done using R version 4.1.1, with differences considered significant at $p < 0.05$. Graphs were plotted using the ggplot2 packages (Wickham, 2016). We then used the Shapiro-Wilk test to assess the data normality. Analysis of variance (ANOVA) and the Tukey HSD test were used for data with a normal distribution, otherwise, Kruskal-Wallis test was performed.

## 3. Biotorr properties and potential impact on soil

### 3.1. Impact of torrefaction conditions on biotorr yield

Data showed a mean biotorr yield of 86.2 ± 0.5 % (mean value ± standard error) for light torrefaction, 72.5 ± 1.6 % for mild torrefaction and 59.5 ± 2.3 % for severe torrefaction (Table 1, Fig. 1). For light torrefaction, biotorr yield was the highest for sweetgum wood at 98.9 % and the lowest for switch grass residue, at 51.9 %. For severe torrefaction, it was the highest for rapeseed straw at 81 % and the lowest for birch wood at 24 %.

Since the higher the temperature, the greater the degradation of the biomass, biotorr yield decreases are expected with increasing torrefaction temperature (Pimsamarn et al., 2024). However, biotorr yield and properties are most likely strongly influenced by the type of feedstock. Unfortunately, studies on oil and pomaces did not report the mass of biotorr. Nevertheless, at light and mild torrefaction, although not significant, the mean biotorr yield was higher for wood feedstocks compared to crop feedstocks (p-value of respectively 0.08 and 0.09, Fig. 1), likely due to their higher content of chemical recalcitrant compounds, i.e. lignin and cellulose, as well as their higher density and their compact physical structure.

For some feedstocks, even light torrefaction led to a high decrease in mass. Indeed, switchgrass biotorr with light torrefaction showed the highest C content and the lowest mass yield. On the other hand, light torrefaction of wood biomass, such as pine induced a strong increase in C content (from 48 to 56 %) with little decrease in mass yield (11 % loss). These differences can be attributed to feedstock composition as hemicellulose-rich residues such as switchgrass degrade rapidly, leading to lower yields, while lignin-rich materials such as rapeseed straw or pine retain more mass under similar torrefaction conditions.

### 3.2. Impact of torrefaction conditions on biotorr carbon content

The results showed a decrease in yield but an increase in C content with torrefaction intensity for all biotorr, regardless of their feedstocks. However, the specific increase in biotorr C content following torrefaction depends on the feedstock.

Our results showed a significant increase in C content with increasing torrefaction intensity (Fig. 2). The mean C content was of 49.2 % for raw biomass, 52 % for light torrefaction, 55.2 % for mild torrefaction and 60.6 % for severe torrefaction. For light torrefaction, C content was the highest for switch grass with 62.3 % and the lowest for corn stover, with 45.8 %. For severe torrefaction, it was the highest for Madagascar Almond with 71.7 % and the lowest for sugar can bagasse with 52.8 %. This pattern is consistent with the thermal stability hierarchy of biomass components (hemicellulose < cellulose < lignin), whereby hemicellulose-rich feedstocks show greater carbon concentration but at

<table><caption>**Table 1**
Properties of raw and biotorr (mean ± standard error).</caption><thead><tr><th rowspan="2"></th><th colspan="4">Torrefaction intensity</th></tr><tr><th>Raw</th><th>Light</th><th>Mild</th><th>Severe</th></tr></thead><tbody><tr><td>Yield (%)</td><td></td><td>86.2 ± 1.5a</td><td>72.5 ± 1.6b</td><td>59.5 ± 2.3c</td></tr><tr><td>C (%)</td><td>46.9 ± 0.5a</td><td>52 ± 0.5b</td><td>55.2 ± 0.6c</td><td>60.6 ± 0.6d</td></tr><tr><td>N (%)</td><td>0.67 ± 0.09a</td><td>0.86 ± 0.11a</td><td>0.8 ± 0.08a</td><td>1.01 ± 0.14a</td></tr><tr><td>C:N ratio</td><td>172 ± 39a</td><td>162 ± 21a</td><td>164 ± 21a</td><td>203 ± 31a</td></tr><tr><td>P (%)</td><td>0.18</td><td>0.18 ± 0.005a</td><td>0.27 ± 0.015b</td><td>0.39 ± 0.01c</td></tr><tr><td>K (%)</td><td>0.39 ± 0.15a</td><td>0.64 ± 0.01ab</td><td>0.83 ± 0.17b</td><td>1.7 ± 0.06c</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>Fig. 1. Mass yield (%) as a function of torrefaction intensity. The different letters show a significant difference in mass yield between treatment.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 2. C content as a function of torrefaction intensity. The different letter shows a significant difference in C content between treatment.</figcaption></figure>

the expense of yield, while lignin-rich feedstocks achieve higher yield retention with more moderate C enrichment.

Beyond differences among the three broad feedstock categories, our data also highlight strong variability within each group. For instance, among crop residues, switchgrass exhibited a low mass yield (51.9 % under light torrefaction) but one of the highest carbon contents (62.3 %), whereas rapeseed straw under severe torrefaction retained a relatively high yield (81 %) with more moderate C enrichment. Similarly, within the wood group, pine showed a marked increase in C content with only a modest yield loss, while birch displayed much stronger mass loss under the same conditions. These examples illustrate that feedstock selection is a critical factor in determining biotorr properties and thus its

potential agronomic functions. Future studies should therefore systematically evaluate intra-group variability to better guide the choice of residues most suitable to generate biotorr for soil amendment.

Since organic matter with a high carbon content helps binding soil particles together and have high microporosity, amendments rich in organic matter can generally improve soil structure and water retention, which in turn can support better root growth. In addition, this can also improve soil microbial activity and modify soil biogeochemical cycles (Lehmann and Joseph, 2024; Kačergius et al., 2025). These results show the importance of feedstocks choice for the torrefaction process to produce suitable soil amendments. The reduction in mass yield during torrefaction also translates to a loss of carbon, as part of the material is
volatilized. Light torrefaction led to an average total carbon loss of 4 %, while mild and severe torrefaction resulted in an average loss of 9 % and 13 %. However, it is important to consider that similarly to pyrolysis, torrefaction may increase carbon stability, i.e. the resistance of organic carbon to microbial decomposition (Leng and Huang, 2018). In the case of biochar, produced at high temperatures, total C loss is even higher, even though the final product shows a very high C concentration and is composed of highly recalcitrant aromatic C, reflected by its low H/C ratio, making it interesting for soil carbon sequestration scenarios (Bolan et al., 2022; Leng and Huang, 2018; Smith, 2016). In the following, we will analyse the structural changes induced by torrefaction to better understand its impact on C stability and its implications for carbon sequestration.

### 3.3. Impact of torrefaction parameters on the molecular composition of biotorr

Hemicellulose, cellulose and lignin, the main components of biomass feedstocks show contrasting thermal stability (Yang et al., 2007). Hemicellulose reacts to heat treatment quickly, starting at low temperature, with around 60 % mass loss occurring between 220 and 315 °C but with a remaining mass of around 20 % at 900 °C. Cellulose breaks down mainly at higher temperatures (above 300 °C), with a mass loss of around 85 % between 315 and 400 °C. Compared to hemicellulose and cellulose, heat treatment impacts lignin across a wide range of temperature (100 to 900 °C), with a lower mass loss of around 50 % at 900 °C (J. Li et al., 2020; Sharma et al., 2004).

Torrefaction, which takes place at temperatures below 300 °C, will therefore partially degrade the biomass affecting mainly its hemicellulose compounds (Chen et al., 2018; Chen and Kuo, 2011). Indeed, Saha et al. (2022) showed that torrefaction of bamboo at 250 °C changed the relative contribution of hemicellulose, cellulose and lignin from 16, 51 and 28 % to 4, 49 and 42 %. When the torrefaction temperature was increased to 280 °C, the composition was further altered to 0 % hemicellulose, 18 % cellulose and 75 % lignin. Torrefaction has been classified into 3 different categories depending on temperature and its impact on hemicellulose, cellulose and lignin: light, between 200 and 235 °C, mild, between 235 and 275 °C and severe, between 275 and 300 °C (Chen and Kuo (2011), Table 2, Fig. 3).

Depending on the initial hemicellulose/cellulose/lignin ratio of the feedstock, torrefaction will have different effects on the end-composition of biotorr, with strong changes on easily degradable compounds like sugars and hemicellulose, but weaker or even no changes of chemically recalcitrant compounds such as cellulose and lignin (Niu et al., 2019), which explains the changes in yield and carbon contents (see above).

Due to the selective removal of biomass compounds, the final product will vary greatly depending on the composition of the feedstock. Generally, feedstock with a high lignin content shows a higher resistance to thermal degradation compared to feedstock with high hemicellulose content, resulting in strong variation in mass loss at low temperature between feedstocks (200–250 °C), with little mass loss for feedstocks with high lignin content (Chen and Kuo, 2010; Garba et al., 2018). Feedstock size and density can also have a significant impact on the biotorr characteristics. Campbell et al. (2019) showed an increase in

<table><caption>**Table 2**
Torrefaction classification and impact on biomass components from Chen et al. (2015).</caption><thead><tr><th>Classification</th><th>Light</th><th>Mild</th><th>Severe</th></tr></thead><tbody><tr><td>Temperature (°C)</td><td>200–235</td><td>235–275</td><td>275–300</td></tr><tr><td>Consumption</td><td></td><td></td><td></td></tr><tr><td>Hemicellulose</td><td>Mild</td><td>Mild to severe</td><td>Severe</td></tr><tr><td>Cellulose</td><td>Slight</td><td>Slight to mild</td><td>Mild to severe</td></tr><tr><td>Lignin</td><td>Slight</td><td>Slight</td><td>Slight</td></tr></tbody></table>

torrefaction mass yield for increasing feedstock bulk density and particle mass. They also showed that the feedstock particle shape may affect the severity of torrefaction, especially for straws, since their shape can induce anisotropy (Basu et al., 2013). Due to the strong dependence of biotorr properties on the initial feedstock composition, feedstock characteristics may be more important than production conditions in determining the effects of biotorr after soil addition.

However, residence time in the torrefaction reactor can also have a significant impact on biotorr properties, as indicated by some studies showing a significant decrease in mass yield but an increase in C content, N content and pH with increasing torrefaction residence time for different temperatures and feedstocks (Carter et al., 2013; Chiou et al., 2015; Wannapeera and Worasuwannarak, 2015). Other studies report a low impact of higher residence time on biotorr properties or dependence on the feedstock type and torrefaction temperature (Doassans-Carrère et al., 2014; Pach et al., 2002). These contrasting results can be related to the low torrefaction temperatures and the short residence times which do not allow the biomass to be fully degraded. Increasing the residence time for the same temperature range promotes the volatilisation of degradable compounds, such as hemicellulose, in the middle of the feedstock particles. At higher temperatures, hemicellulose in feedstocks is degraded at a faster rate. Therefore, increasing the residence time at high temperature may play a minor role on the end-product composition. To optimise biotorr production for soil amendment, it will therefore be important to calibrate torrefaction process residence time and temperature so that the biotorr will have a more interesting composition compared to raw biomass for soil amendments while maintaining the economic viability of the torrefaction process since longer residence time can increase biotorr production cost (Batidzirai et al., 2013).

Biotorr usually has a higher pH compared to feedstocks ranging between 4 and 6 pH (Cheng et al., 2017; Heikkinen et al., 2019; Igalavithana et al., 2017). Temperature during heat treatment has a strong impact on the resulting biomass pH (Sun et al., 2017). Higher pH in biotorr compared to feedstock might be explained, as observed for biochar, by the transformation of base cations (primarily Ca, Mg, K, Na) during torrefaction into oxides, hydroxides and carbonates (e.g. ash) that are admixed with biotorr (Houben et al., 2013; Hu et al., 2020). Once they dissolve into soil, these alkaline substances act as a liming agent with limited long-term effect. In addition to impacting soil pH, the aggregation of alkali and alkaline earth metals (AAEMs) into oxides, hydroxides, and carbonates may reduce their immediate solubility compared to exchangeable forms. The AAEM aggregation should not be considered detrimental, but rather as a mechanism that modifies the dynamics of nutrient release. The extent to which this affects soil fertility and soil health will depend on soil characteristics, and requires further investigation.

Despite their importance in terms of stability, physical properties of biotorr are less documented. Biotorr produced between 200 and 280 °C from different feedstocks had a lower specific surface area than biochar produced between 350 and 550 °C from the same feedstocks, with a specific surface area that can double between 250 and 350 °C and up to quadruple between 250 and 550 °C (Cheng et al., 2017; Igalavithana et al., 2017). Torrefaction often increased feedstocks hydrophobicity, with higher hydrophobicity for higher torrefaction intensity (Dyjakon et al., 2019; Shankar Tumuluru et al., 2011). This could in turn induce short-term soil water repellency, limiting water infiltration and influencing preferential flow (Mao et al., 2019), however, this hydrophobicity can be expected to rapidly decrease over time with biotorr degradation by microbial activity and weathering.

Although they are not discussed in this review, other torrefaction parameters such as atmospheric composition (Rousset et al., 2012) and feedstock particle size (Basu et al., 2013; Medic et al., 2012) may impact the composition of biotorr.
<figure><img src="image_6.png" /><figcaption>Fig. 3. Impact of torrefaction temperature on the molecular structure of raw plant material.</figcaption></figure>

### 3.4. Impact of torrefaction intensity on biotorr nutrient contents and availability

Biotorr made from oils or pomaces differed from biotorr made from wood or crop residues, mainly due to the contrasting N content of raw biomass. Thus, the three different types of feedstock form three different groups, with wood having the lowest N content, followed by crop residues, with higher but similar N content, and oils/pomaces, having the highest N content (Fig. 4).

The composition of biotorr, in particular its nutrient content is a key parameter to evaluate for its use as a soil amendment to substitute, at least partially, mineral fertilizers. Nutrients such as N, P and K are essential for crop development and harvest yield. N content was not significantly impacted by torrefaction intensity (Fig. 5). N content

increased from 0.67 % for raw biomass to 1.01 % for severely torrefied biotorr. Among the different feedstock types, biotorr made from oils and pomaces had the highest mean N content (1.7 %) compared to crop residues (0.62 %) and wood (0.34 %) and therefore may have interesting fertiliser value. In addition, by inducing the volatilisation of phytotoxic compounds which can be found in some raw oil and pomaces (Martín et al., 2002), the torrefaction process is also beneficial for making these high N content products usable as soil amendment. Torrefaction may reduce the proportion of bioavailable N through volatilisation. For biochar, the resulting biomass may end up with negligible mineral N (less than 0.5 %) compared to the organic N (Chan and Xu, 2009) mainly due to heterocyclisation of N during pyrolysis (Koutcheiko et al., 2007). However, torrefaction occurs at lower temperatures, thus, in biotorr amended soils, less additional N fertilizer may be required to avoid plant

<figure><img src="image_7.png" /><figcaption>Fig. 4. N content (%) as a function of C content (%). The colours show different feedstock class, and the shapes and size represent the torrefaction intensity. The ellipses represent a confidence interval of 0.65.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 5. N content as a function of torrefaction intensity. The different letter shows a significant difference in N content between treatment.</figcaption></figure>

N deficiency.

The data showed an increase in P and K concentrations in biomass following torrefaction (Fig. 6). P content increased from 0.18 % for raw biomass to 0.39 ± 0.01 % for severely torrefied biotorr, while K content increased from 0.39 ± 0.15 % to 1.7 ± 0.06 % for severely torrefied biotorr.

Anshu et al. (2023) reported an increase in P content and a reduction in K content in rice husk after torrefaction. The decrease in K content was linked to its low stability in the feedstock, which may have caused its partial volatilisation during the torrefaction process. In contrast, Bajcar et al. (2018) showed an increase in both P and K content after torrefaction of rapeseed straws. Our data show that the biotorr K content may be greatly dependant on the feedstock used in addition to its initial K content (Ren et al., 2017).

However, to use biotorr in an agronomic context, its available nutrient concentrations may be more important than its total nutrient concentration. To our knowledge, this information is not yet available for biotorr. For biochar, which undergoes more intensive heat treatment than biotorr, studies (Ippolito et al., 2020; Zheng et al., 2013) showed an increase in the concentration of available P and K following low temperature pyrolysis and a decrease for available P content at higher pyrolysis temperature. The decrease of available P at higher pyrolysis temperatures was linked with the transformation of available P in crystallized mineral forms (Ippolito et al., 2020; Zheng et al., 2013). Further studies are now necessary to assess the bioavailability of biotorr nutrients for plants and microorganisms. In addition to total nutrient concentrations, the bioavailability of P and K in biotorr will depend on their chemical speciation and the soil context. During torrefaction,

<figure><img src="image_9.png" /><figcaption>Fig. 6. P content (A) and K content (B) as a function of torrefaction intensity. The different letter shows a significant difference in P content between treatment.</figcaption></figure>
nutrients can undergo transformations into less soluble forms such as metal-phosphates or oxides, which may influence their release to plants. These transformations are known to be temperature-dependent, with torrefaction potentially favouring more soluble forms compared to high-temperature pyrolysis (Ippolito et al., 2020). Moreover, pH, texture, and cation exchange capacity will influence the adsorption-desorption dynamics of these nutrients after soil application. For example, K is typically more mobile in soil but can become fixed in clays, while P can be strongly adsorbed onto iron and aluminium oxides in acidic soils or precipitated as calcium phosphates in alkaline conditions. Given that biotorr may contain a mixture of organic-bound and mineral-bound nutrients, its fertilization potential will likely vary depending on both feedstock origin and soil properties. Thus, understanding these complex interactions is key to assessing biotorr agronomic value, and should be a focus of future research.

### 3.5. Impact of torrefaction intensity on biotorr stability and decomposability

To assess the potential stability of thermally transformed biomass, the Van Krevelen diagram, presenting the atomic H:C and O:C ratios may be used (Schimmelpfennig and Glaser, 2012). These ratios reflect the chemical composition of biomass, with lower ratios indicating a higher proportion of aromatic compounds, resistant to microbial degradation. Fig. 7 presents the van Krevelen plot including data for biochar, biotorr and biomass. As expected, in terms of potential stability against microbial decay, biotorr corresponds to an intermediate state between raw biomass and biochar. Compared to biochar, which has very low H:C and O:C ratios and is highly resistant to decomposition, biotorr showed intermediate ratios in the Van Krevelen diagram, reflecting a mixture of stable, aromatic carbon and more labile carbon, indicating

that can this material may contribute to long-term soil carbon storage and actively participate in nutrient and carbon cycling. Increasing torrefaction intensity resulted in a decrease of H:C and O:C ratio for all feedstock classes, which reflects an increase in the stability of biomass C. For a given torrefaction intensity, oil/pomace had lower H:C and O:C ratios compared to wood, while crop residue showed a strong variability in H:C and O:C ratios.

The C:N ratios of biotorr showed no significant differences between torrefaction intensity, even though C:N ratio tended to be higher for severe torrefaction compared to raw biomass, light and mild torrefaction (Fig. 8). Mean C:N ratio changed from 172 for raw biomass to 162, 164 and 203 for respectively light, mild, and severe torrefaction, suggesting that the proportion of C and N lost were similar between torrefaction intensity.

Generally, applying raw organic matter with a C:N ratio higher than 25 to soil will require microorganisms to use native soil N to degrade this organic input, which immobilises N from the soil solution and decreases its availability for plants (Doublet et al., 2010). Similar to other organic amendments, biotorr with high C:N ratios could therefore have a negative effect on N availability when applied to soils. On the other hand, a high proportion of recalcitrant C in biotorr can increase the amount of energy required for its decomposition by soil microorganisms, potentially slowing down its degradation and thus limiting microorganisms N needs, which could in turn reduce soil N immobilisation after soil addition (Chan and Xu, 2009). Biotorr characteristics could be interesting for C storage without a possible significant negative impact on soil native N compared to raw biomass, however further studies are required, since, like biochar, it is likely that its labile carbon fraction may stimulate microbial activity, leading to N immobilization in the short term (Bruun et al., 2012; Clough et al., 2013).

While biochar is more stable and has a strong liming effect, biotorr is

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Van Krevelen plot of biotorr showing Atomic H:C ratio = (H content/1) / (C content/12) and Atomic O:C ratio = (O content/16) / (C content/12). The different colours represent the torrefaction intensity, and the different shapes represent the feedstock class. The ellipses represent a confidence interval of 0.65. The biochar data were taken from the UC Davis Biochar database.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 8. C:N ratio as a function of torrefaction intensity. The different letter shows a significant difference in C:N ratio between treatment.</figcaption></figure>

produced with much higher mass yields and retains a higher fraction of labile compounds that may contribute to nutrient cycling and short-term soil fertility. These differences suggest that biotorr and biochar should not be seen as interchangeable, but rather as complementary tools, each with distinct benefits for soil health and carbon management. Future research should explore how these two materials might be combined or applied under different agronomic contexts to maximize their synergies.

### 3.6. Biotorr as a soil amendment

In recent years, torrefaction processes and the impact of torrefied products on soils have been increasingly studied, particularly in Africa and Asia, with the aim of using crop residues to restore soil health instead of burning them in agricultural fields (Igalavithana et al., 2017; Ogura et al., 2016).

To understand the impact of biotorr application on soil physico-chemical properties, soil microbial communities and plant growth, the majority of studies have carried out short-term pot experiments, with mixtures of soil and biotorr (Cheng et al., 2017; Heikkinen et al., 2019; Igalavithana et al., 2017; Ogura et al., 2016; Thengane et al., 2020; Trifonova et al., 2009). These experiments showed some potential effect of biotorr application on soil physico-chemical parameters depending on the nature of the feedstock and the torrefaction temperature and highlighted the variability of effects between different biotorr. Table 3 summarizes the various impacts of biotorr on soil properties and plant growth.

Up to now, no studies have been carried out under field conditions. However, the available studies showed that biotorr application had an impact on soil water retention, chemical properties and GHG emissions.

While a few experiments used soil collected from agricultural fields (e.g., Ogura et al., 2016), these were carried out in growth chambers or greenhouses and cannot fully simulate the complexity of field environments. This lack of field data represents a major limitation in the current state of knowledge, and it is precisely one of the central findings of this review.

<caption>**Table 3**</caption>

Key findings on the application of biotorr on soils and crops characteristics. + = parameter increase, - = parameter decrease and 0 = no impact on the parameter.

<table><thead><tr><th></th><th></th><th>Biotorr effect</th></tr></thead><tbody><tr><td rowspan="7">Soil properties</td><td>Physical properties</td><td></td></tr><tr><td>Water retention</td><td>+</td></tr><tr><td>Aggregate stability</td><td>0</td></tr><tr><td>Chemical properties</td><td></td></tr><tr><td>pH</td><td>+/-</td></tr><tr><td>Nutrient availability</td><td>+</td></tr><tr><td>N emissions</td><td>-</td></tr><tr><td rowspan="2">Soil respiration</td><td></td></tr><tr><td>+</td></tr><tr><td rowspan="5">Crop properties</td><td>Plant height</td><td>+/-</td></tr><tr><td>Total biomass</td><td>+/-0</td></tr><tr><td>Yield</td><td>+</td></tr><tr><td>Seed germination</td><td>0/-</td></tr><tr><td>Nutrient uptake</td><td>0</td></tr></tbody></table>

#### 3.6.1. Biotorr impact on soil properties

Using a pot experiment, Heikkinen et al. (2019) study showed that the application of biotorr significantly increased the soil water holding capacity compared to the control and that this impact was stronger compared to biochar application. X-ray tomography showed that the pore size of both materials was inferior to 30 µm. However, their hydrophobic nature may impact their water storage capacity.

It is important to point out that similar to biochar amendment, the impact of biotorr will depend on soil properties. Thengane et al. (2020) who worked on alkaline soils, showed a decrease in soil pH following the application of severely torrefied pine and rice husk biotorr, while other studies, such as Igalavithana et al. (2017) and Heikkinen et al. (2019), showed an increase in soil pH following the application of biotorr on acid clay and sandy loams soils.

Moreover, Ogura et al. (2016) showed a significant increase in the soils' P and K content after adding 1 to 5 % biotorr. While this result is
interesting, it is insufficient to conclude on the impact of biotorr on nutrient cycling or availability for plants. However, no other studies investigating the effect of biotorr on nutrient availability were found during our research showing the need for additional work to fully assess the impact of biotorr on soil functioning and the potential variability of its effects.

### 3.6.2. Effect on GHG emissions

The application of biotorr (200–300 °C *J. curcas* wood and wheat straw) on an agricultural soil also triggered a shift in microbial communities (Cheng et al., 2017; Ogura et al., 2016). This change in microbial communities was linked to an increase in soil respiration due to an adaptation of microorganisms to use the alternative carbon source. Cheng et al. (2017) showed that biotorr application on a sandy loam soil initially induced a positive priming effect by accelerating SOM turnover and microbial activity, but then shifted to a negative priming effect over time for all production conditions. Their results showed that the positive priming effect was offset by the negative priming effect over the long term. The priming effects following the addition of biochar are still controversial, with studies on the subject highlighting positive or negative effects depending on the context (Wang et al., 2016).

Thengane et al. (2020) showed different impacts of biotorr on soil N emissions from artificial soil mixtures depending on torrefaction feedstock. Both lightly and severely torrefied pine addition to soil decreased estimated N emission compared to raw pine amendment even though estimated emissions in the presence of lightly torrefied pine was higher than those of the control. For rice husk, light and severe torrefaction treatments increased and decreased, respectively, estimated N emissions compared to raw rice husk treatment. As observed for biochar, lower N emissions after biotorr application might be caused by potential binding of soil N₂O to biochar (Cornelissen et al., 2013) or by increasing soil pH which in turn can decrease nitrifiers activity (Cayuela et al., 2013; van Zwieten et al., 2010). This impact is increasingly being highlighted in the current climate change context as a means of limiting nitrous oxide emissions.

### 3.6.3. Impact on crops

The application of 240 °C *J. curcas* biotorr, and severely torrefied pine or rice husk showed an increase of plant growth for *J. curcas* plants grown on aridisol and wheat grown on a sand-soil mixture (Ogura et al., 2016; Thengane et al., 2020). In some cases, the addition of biotorr produced at low temperature suppressed plant germination due to the presence of phenolic or aromatic compounds in the feedstocks (Jeníček et al., 2022; Trifonova et al., 2009; Tunklová et al., 2022). This effect disappeared with biomass torrefied at higher temperatures, which suggests that these toxic organic compounds volatilized with increasing temperatures.

To assess biotorr potential as a soil conditioner, it is important to compare different application rate on soil properties to a control plot with no input. However, for biotorr produced from crop residues, it would also be important to compare these applications to a treatment without residue exportation. Additionally the quantity of biotorr applied and/or amount of residues should be adjusted to the mass loss after thermal treatment. The comparison with the application of raw biomass would correspond to a control for a situation where crop residues are removed from a field, torrefied or pyrolysed, and then re-applied to that same field.

## 4. Conclusion

This review summarises current knowledge on the effects of torrefaction processes on the properties of biotorr derived from different feedstocks and their potential as amendments in an agricultural context. Our data collection and synthesis showed that torrefaction led to a substantially higher mass yield (60–85 %) compared to biochar production (10–50 %), while still achieving a significant increase in carbon

concentration (from 46.9 % in raw biomass to 60.6 % under severe torrefaction). Phosphorus and potassium contents increased with torrefaction intensity, although their plant bioavailability remains to be demonstrated. Biotorr stability was intermediate between raw biomass and biochar, with increasing lignin content and decreasing H:C and O:C ratios with torrefaction intensity, reflecting increased recalcitrance compared to fresh feedstock. Collectively, these results suggest that biotorr represents a distinct class of soil amendment, potentially combining fertilizer value with moderate stability.

At the same time, our analysis confirmed that virtually all studies to date have been conducted under controlled conditions, with a complete lack of open-field trials. This research gap is critical, as field validation is necessary to determine biotorr's actual agronomic performance, including its effects on soil fertility, microbial functioning, and long-term carbon sequestration. We therefore conclude that biotorr holds strong potential as a soil conditioner complementary to biochar, but its deployment in agriculture will depend on future field studies addressing nutrient dynamics, stability, and crop responses under diverse soil and climate conditions. Therefore, we especially encourage future studies on longer scales, with or without biotorr reapplication, and using experimental plots, to assess its impact under field conditions.

### CRediT authorship contribution statement

**Lucas Lesaint:** Writing – review & editing, Writing – original draft, Visualization, Resources, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Cornelia Rumpel:** Writing – review & editing, Validation, Supervision, Project administration, Funding acquisition. **David Houben:** Writing – review & editing, Validation, Supervision, Project administration, Funding acquisition, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

This research was supported by the RAPETORR project funded by ADEME (French Environment and Energy Management Agency)

### Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials.

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