Forest residue biochars: feedstock, pyrolysis, and properties

The European Union (EU) is fostering the bioeconomy as a strategy to achieve a climate-neutral and circular economy. In this context, valorization of forest wastes as pine sawdust into biochar is a key opportunity in Galicia (NW Spain), one of the most forested areas of the EU. Moreover, two abundant Galician shrubs, Erica cinerea L. and Ulex europaeus L. have not yet been explored as biochar feedstock, although these shrubs are associated with high biomass accumulation and an increase of forest fires in the region. Therefore, their conversion into biochar offers a sustainable waste management pathway. This study aims to characterize biochar produced from different Galician feedstocks (pine sawdust, Erica cinerea L., and Ulex europaeus L.). The biochar was produced under three pyrolysis temperatures (300, 400 and 500°C) and three pyrolysis times (30, 60 and 120 minutes). The results showed that nitrogen, carbon concentration, and C/N ratio in the biochar were greatly affected by feedstock initial composition and operating conditions during pyrolysis. Carbon concentration varied significantly among feedstocks, with all materials showing increased carbon concentration at higher pyrolysis temperatures. Sawdust consistently had the highest nitrogen concentration: up to 8% at 300 °C, after 120 min due to the preprocessing with ammonium polyphosphate (APP). The C/N ratio demonstrated distinct patterns, with sawdust maintaining a lower C/N ratio at lower temperatures, while Erica cinerea and Ulex europaeus showed a higher C/N ratio that varied with temperature and pyrolysis time. The results indicate that biochar derived from Erica cinerea L. and Ulex europaeus L. are suitable for soil carbon sequestration due to its higher and more stable C/N ratios, whereas APP processed pine sawdust biochar shows particular potential as a long-term source of nitrogen due to its consistently higher nitrogen concentration and lower C/N ratio under lower pyrolysis temperatures. However, it should be note that the effect of these biochars on soil depends on the specific soil conditions.

Biochar production is gaining interest as an important part of the European Union (EU) bioeconomy strategy, whose main environmental goal is to promote a sustainable, circular, and climate-neutral economy by reducing greenhouse gas emissions and decreasing the use of fossil resources (EU, 2025). In this context, biochar is considered an important tool because it provides a sustainable way to convert agricultural and forestry waste into a valuable resource that enhances soil health, sequesters carbon (C), and supports sustainable resource cycling. Moreover, international organizations as the FAO (2023) consider biochar a valuable amendment for sustainable soil management because it enhances nutrient release, improves crop yields, and acts as a long-term C sink that helps mitigate climate change, particularly when produced from agricultural or forestry waste. Biochar applications are particularly beneficial for dry-season farming and for restoring degraded land through synergies with biogas systems (Intergovernmental Technical Panel on Soils, 2023). Therefore, within this framework, strengthening the linkages between forest and agricultural systems has become increasingly important. This integrated approach enables more effective resource management, improves biomass supply chains, and enhances ecosystem health, as highlighted by the Agroforestry for Europe (AF4EU) project. By combining forest-derived feedstocks with farming practices, it is possible to achieve multiple benefits, including improved soil health and diversified income sources (Chamberlain and Smith-Hall, 2024).

Similarly, the European Commission (EC) is actively working to incorporate biochar into climate policies through the Carbon Removal Certification Framework. The EC is developing methodologies that recognize biochar as a permanent C dioxide removal technology when applied as a soil amendment, while excluding less sustainable uses such as steel production. This approach supports the broader objectives of the Fertilizing Products Regulation and promotes the circular bioeconomy. However, concerns about possible greenwashing highlight the need for strict certification procedures to ensure sustainability and environmental integrity (EC, 2024).

On the other hand, forestry activities and wood processing generate a considerable amount of waste from forest products each year. For example, in the United States alone, more than 70 million tons of solid wood waste are produced annually from manufacturing, construction, and demolition activities. A large proportion of this waste consists of recoverable materials such as sawdust, bark, and wood residues, which, despite their potential to be recycled into energy, mulch, or other products such as biochar, remain largely underutilized (Falk and McKeever, 2012). In the case of Galicia (NW Spain), the forest industry plays an essential role in the regional economy and rural development. It generates approximately €2.2 billion annually and accounts for more than 20% of industrial employment, with a strong focus on timber production and processing (Rodríguez-Vicente and Marey-Pérez, 2010). However, this high level of industry activity also leads to the generation of large amounts of waste, particularly sawdust and other wood by-products, estimated at over 1.2 million cubic meters per year from sawmills alone (Barreiro et al., 2019), creating both environmental changes and opportunities for improved resource management. In this context, Galicia can be considered a key region for circular bioeconomy initiatives because it is the main timber-producing area in Spain, contributing around 50% of national production, and therefore concentrates a large and continuous supply of forest biomass waste. In particular, in this region, the valorization of pine sawdust through biochar production represents a promising pathway to transform waste into a value-added product that improves soil quality, enhances C sequestration, and supports sustainable resource use, while also contributing to the reduction of greenhouse gas emissions and strengthening the resilience of the forestry sector (Geissdoerfer et al., 2017; Korhonen et al., 2018; Lehmann and Joseph, 2015). Therefore, one of the most significant opportunities in the Galicia region, characterized by extensive forest cover (over 70% of its land area) and predominantly acidic soils, is the sustainable production of biochar from forest residues (Cárdenas-Aguiar et al., 2024). The region’s combination of abundant forest resources and high agricultural production linked to livestock provides a valuable opportunity to link forest and agricultural systems through biomass valorization techniques (Cárdenas-Aguiar et al., 2024). These approaches not only help reduce excess biomass accumulation but also generate simultaneous benefits in terms of waste management and soil improvement (Zhang et al., 2021), whose impact depends on the type of soil.

Among the most common forest shrubs in the Galician region, Erica cinerea L. (bell heather), linked to nutrient-poor soils, and Ulex europaeus L. (gorse), sown in the past as a source of compost to mix with manure to be used as cereal fertilizer, remain largely unexplored as potential feedstocks for biochar production, despite their high abundance in Galician environments (Cárdenas-Aguiar et al., 2024). The management of these forest shrubs has been challenging due to their widespread distribution and limited economic value, and both species are commonly associated with increased fuel loads and a higher risk of recurrent wildfires (Pausas et al., 2008). In this context, biochar production from these shrubs offers a promising pathway not only for biomass valorization but also for long-term C sequestration, contributing to climate change mitigation (Woolf et al., 2010). Economic analyses suggest that biochar production from forest biomass can achieve internal rates of return between 8% to 17%, especially when low feedstock costs and carbon credit opportunities are considered (Campbell et al., 2018).

Finally, it is important to be aware that biochar is produced through a pyrolysis process, and its effect on soil properties depends on its physicochemical characteristics, which can be temporary and are influenced by both the feedstock composition and the pyrolysis conditions (Zhang et al., 2021). The C and nitrogen (N) concentrations, along with the overall chemical composition and the C/N ratio, are key parameters for evaluating biochar quality, as they strongly influence soil fertility and crop productivity (Lehmann and Joseph, 2015). In particular, the C/N ratio regulates microbial activity and nutrient cycling in soils and is considered one of the most important indicators of biochar’s agronomic quality, controlling decomposition rates and nutrient release dynamics (Ippolito et al., 2020). Since biochar can be low in N and its C concentration can vary significantly depending on production conditions, understanding the factors that influence its N and C concentration, and consequently its C/N ratio, is essential for both biochar producers and end users (Zhang et al., 2021; Wang et al., 2016).

Pyrolysis conditions, particularly temperature and pyrolysis time, further determine the physical and chemical characteristics of biochar, influencing its elemental composition, structure, and stability. Higher pyrolysis temperatures and longer pyrolysis times generally produce biochar with higher fixed C concentration, increased pH, and ash content, and more aromatic and stable C structures (Lu et al., 2021; Wijitkosum, 2023). In contrast, low-temperature pyrolysis (300–450 °C) produces biochars with higher proportions of oxygen-containing functional groups and residual nutrients (Ghorbani et al., 2022; Wijitkosum, 2023). These changes directly affect biochar performance as a soil amendment, where higher temperature biochar tends to act primarily as long-term C sinks and soil conditioners, improving soil structure and water retention due to their greater stability and porosity (Chatterjee et al., 2020; Khan et al., 2024). Overall, the interaction between pyrolysis temperature and pyrolysis time influences biochar’s C, N, C/N ratio, and porosity, thereby determining its role along a continuum from short-term soil fertility enhancement to long-term C sequestration (Lu et al., 2021; Chatterjee et al., 2020).

This study aims to characterize biochar produced from different Galician feedstocks (pine sawdust, Erica cinerea L., and Ulex europaeus L.) as a source of N and C for soil amendment, to improve soil fertility and long-term C sequestration. The biochar was produced under three pyrolysis temperatures (300, 400 and 500°C) and three pyrolysis times (30, 60 and 120 minutes).

In this study, in February 2022, feedstocks for biochar production, including Ulex europaeus L. and Erica cinerea L., were collected from forest areas in Galicia (NW Spain) during the species’ primary growing season. In addition, industrial pine sawdust was obtained from the Financiera Maderera S.A. group in Rábade (Lugo, Galicia) and used as a biochar feedstock. The pine sawdust was obtained from debarked pine logs that were chipped and subsequently treated with an ammonium polyphosphate (APP)-based fire-retardant formulation to produce fire-retardant wood chips. The treated material contained 4% urea, 4% phosphate, 20% gum, and 20% fire-retardant components, with the remaining fraction consisting of pine wood.

In the laboratory, the aboveground biomass of the shrub species (excluding roots) was manually separated and processed. All three feedstocks (pine sawdust (SD), Ulex europaeus L. (UE) and Erica cinerea L. (EC)) were sieved to obtain a particle size of ≤2 mm before pyrolysis. The materials were then oven-dried at 45 °C for four days.

Pyrolysis was performed in a muffle furnace (HD 430 XL PAD, HOBERSAL Furnaces and Ovens Technology, Barcelona, Spain) using covered ceramic crucibles. The furnace was not preheated; instead, each pyrolysis run started from room temperature, which was maintained at approximately 20 °C under stable laboratory conditions. The covered crucibles containing the samples were then placed in the muffle furnace, and the temperature was increased to the target value, which was reached in approximately 10 min. Once the target temperature was reached, it was maintained for the duration specified for each treatment. Each feedstock was pyrolyzed at three temperatures (300, 400, and 500 °C) and for three pyrolysis times (30, 60, and 120 min), resulting in nine temperature–time treatment combinations for each feedstock (Figure 1). In all cases, three independent replicates were performed for each feedstock × temperature × pyrolysis time combination to ensure experimental reproducibility and to account for variability among independent pyrolysis runs, thereby providing sufficient experimental replicates for the subsequent statistical analysis. No inert gas was introduced during the pyrolysis process, and the covered ceramic crucibles were used to limit direct exposure of the samples to the surrounding atmosphere. The resulting biochars were recovered after pyrolysis, ground using an agate mortar and pestle, and stored in plastic bags until further analysis (Gonzaga et al., 2017).

Carbon and N concentrations were determined for all feedstocks and biochars using a LECO CNS-2000 elemental analyzer (LECO Corporation, St. Joseph, MI, USA), following the manufacturer’s analytical procedures and the relevant ISO standard methods (Kowalenko, 2001). Quality-control standards and certified reference materials were used for C and N determinations to verify analytical accuracy and precision. The C/N ratio was subsequently calculated from the measured C and N concentrations for each treatment combination (Figure 1).

The C and N concentrations and the resulting C/N ratios were statistically analyzed using SAS software (SAS, 2001). Analysis of variance (ANOVA) was performed to assess the effects of feedstock type, pyrolysis temperature, and pyrolysis time, as well as their interactions. When significant differences were detected, a Least Significant Difference (LSD) test was used for pairwise comparisons among treatment means. Statistical significance was assessed at p < 0.05.

Carbon

The total C concentration in the biochar was significantly modified by the feedstock x pyrolysis time x pyrolysis temperature interaction (p<0.05).

At all pyrolysis temperatures and times, biochar produced from UE and EC generally showed significantly higher C concentration than biochar derived from SD.

At a pyrolysis temperature of 300° and 400°C, the C concentration in the biochar derived from EC was significantly higher at a pyrolysis time of 30 and 60 minutes compared to 120 minutes. In contrast, for UE and SD, the C concentration was significantly higher at a pyrolysis time of 60 minutes compared to 30 and 120 minutes at a pyrolysis temperature of 300 °C and compared to 120 minutes at a pyrolysis temperature of 400 °C. In the case of the pyrolysis temperature of 500 °C, the C concentration was not affected by the pyrolysis time in any of the feedstocks (Figure 2).

Nitrogen

The total N concentration in the biochar was significantly influenced by the feedstock x pyrolysis time x pyrolysis temperature interaction (p<0.001).

At a pyrolysis temperature of 300 °C, the biochar produced from SD showed a significantly higher N concentration than that derived from UE and EC at pyrolysis times of 30 and 120 minutes. This positive effect of SD on N concentration in the biochar was also observed at a pyrolysis temperature of 400 °C in all pyrolysis times and at 500 °C in the pyrolysis times of 30 and 60 minutes (Figure 3).

Moreover, at 300 °C, it was observed that the N concentration in the biochar from UE and EC was generally higher after 60 and 120 minutes of pyrolysis compared to 30 minutes. However, at 500 °C, the biochar derived from UE only showed significantly higher N concentration at 120 minutes compared to 30 and 60 minutes. At 400 °C, no significant differences were observed among the pyrolysis times for any of the materials used to produce biochar.

C/N ratio

The C/N ratio in the biochar was significantly influenced by the feedstock x pyrolysis time x pyrolysis temperature interaction (p<0.01).

At a pyrolysis temperature of 300 °C, the biochar produced from EC showed a significantly higher C/N ratio than from UE and SD at all pyrolysis times (Figure 4). The same result was observed at pyrolysis temperatures of 400 °C and 500°C, but only at pyrolysis times of 30 and 60 minutes.

Regarding the effect of pyrolysis time on the C/N ratio, at a pyrolysis temperature of 300 °C and for all materials used to produce biochar, the C/N ratio in the biochar was generally higher at a pyrolysis time of 60 minutes compared to the other times.

At 400 °C, for biochar from SD, pyrolysis times of 60 and 120 minutes resulted in greater increase in the C/N ratio compared to 30 minutes. However, for biochar from EC, a pyrolysis time of 30 minutes implied a higher C/N ratio than the other times. For biochar from UE, the higher C/N ratio was observed at 120 minutes compared to 30 and 60 minutes.

Finally, at 500 °C, the biochar produced from SD showed a greater increase in the C/N ratio at a pyrolysis time of 120 minutes compared to the other times. For the biochar from EC, pyrolysis times of 30 and 60 minutes resulted in higher C/N ratios than 120 minutes. In the case of UE, pyrolysis times of 60 and 120 minutes led to higher C/N ratios in the biochar compared to 30 minutes (Figure 4).

Overall, the C, N, and C/N ratio values obtained for the forest waste-derived biochars in this study highlight the importance of feedstock selection and the optimization of pyrolysis time and temperature to produce biochar for different purposes, mainly as a long-term nutrient source or as a stable C source for soil C sequestration (Hossain et al., 2020). Previous studies have shown that C is effectively retained and concentrated as temperature and pyrolysis time increase (Gul et al., 2015). In contrast, N dynamics are more complex and depend on its incorporation into large and more stable molecular structures, which appeared to be higher in UE and EC than in SD. As a result, the produced biochar may differ in their capacity to contribute to enhancing soil C retention, improving soil nutrient release as a provider of a slow-release source of N for plants and soil systems, depending on feedstock types and biochar processing temperature and time (Gul et al., 2015).

The C concentration obtained in the biochar produced in this study from EC and UE (75-80%) was within the range commonly reported for woody biomass-derived biochar (75–80%) (Jeffery et al., 2017), whereas lower values were observed for SD (65-70%) across all pyrolysis temperatures and times. In this context, it is important to note that woody biomass from EC and UE, which is usually rich in lignin, as demonstrated in other studies (Li et al., 2018), may produce biochar with high C concentration, probably due to its original lignocellulosic (lignin-rich) and recalcitrant molecular composition, and the formation of stable aromatic structures during pyrolysis (Jeffery et al., 2017). In the case of SD, this effect may have been partially reduced by the addition of chemical compounds during industrial processing to make the chipboard timber fireproof or flame-retardant. Higher pyrolysis temperatures also resulted in increased C concentration due to more aggressive carbonization reactions (Sun et al., 2014). Forest waste-derived biochar showed a higher C percentage than reported in other studies based on conventional agriculture-derived biochar feedstocks (Wang et al., 2015). For instance, corn stover biochar would contain 65-70% C at 500 °C (Wang et al., 2015), values lower than those observed for EC and UE biochars in the present study. Moreover, the high C concentrations (70-80% at 500 °C) observed in the forest waste-derived biochars suggest their potential to establish stable soil C pools, improving soil structure and water retention while indirectly supporting plant growth through enhanced soil conditions (Lehmann et al., 2015).

Due to the additions of ammonium phosphate (APP) during chipboard processing, N concentration was generally two or three times higher in SD-derived biochar than in UE- and EC-derived biochar across all pyrolysis conditions, but, especially at pyrolysis temperatures of 300 °C and 400 °C and at shorter pyrolysis duration. This result can be explained by the fact that lower temperatures and shorter pyrolysis duration preserve the original feedstock composition, particularly with regard to N, which has a high volatilization rate at high temperatures than other elements (Sun et al., 2014). Sawdust biochar produced at 300 °C retained up to 8% N, which is substantially higher than the 0.5-2% typical reported for wood-based biochars (Jassal et al., 2015), whereas the EC- and UE-derived biochars contained around 0.5-3% N. The higher N concentration of the studied feedstocks may be due to the addition of APP in the case of SD, but also to the presence of leaves as part of the shrubs, which were included due to the lack of economic feasibility for feedstock commercial transformers to separate the leaves from the branches. Therefore, these differences compared with exclusive woody perennial materials can be explained by the specific feedstock composition and thermal behavior of the feedstocks (Bruun et al., 2012; Li et al., 2023). Shrub biomass, usually rich in lignin (Li et al., 2023), probably undergoes worst under thermal decomposition pathways during pyrolysis compared with SD, which had previously been treated with APP. APP addition to stabilize SD makes the SD- derived biochar richer in N and therefore with a higher potential to release this nutrient when the biochar is used with crops (Castejón del Pino et al., 2023). Longer pyrolysis durations may reduce N concentration due to volatilization of nitrogenous compounds during anoxic thermal treatment to produce biochar (Gaskin et al., 2008; He et al., 2023), as observed in SD biochar. For example, at 500 °C, SD biochar showed higher N concentration at shorter pyrolysis times (30 and 60 minutes) than at 120 minutes, indicating that excessive pyrolysis time may cause N losses. In contrast, at lower temperatures (300 °C), N concentration in UE- and EC-derived biochars generally increased with longer pyrolysis times, possibly due to the volatilization at higher temperatures associated with more complete thermal decomposition of proteinaceous compounds present in woody perennial tissues as biochar processing conditions change (Bruun et al., 2012). The N and C retention patterns observed in UE and EC biochars differed markedly from those of conventional woody feedstocks. While pine wood biochar typically shows progressive N loss and increased C densification during pyrolysis (Kloss et al., 2012), UE and EC displayed more complex N retention patterns, particularly at intermediate temperatures. This is especially relevant when considering that UE is a legume. This suggests that the chemical composition and cellular structure of shrub material involve specific thermal decomposition pathways that influence both C preservation and N stabilization (Bruun et al., 2012). Nitrogen concentration in all studied biochars also appears to depend significantly on pyrolysis temperature. At 300 °C, approximately 60-70% of the N in SD biochar may remain in more available forms, such as amino acids and proteins, whereas at 500 °C, this percentage decreases to 30-40%, with the remaining N probably being incorporated into more recalcitrant aromatic structures (Li et al., 2018). These temperature-dependent differences in N concentration suggest that biochar production conditions can be optimized according to specific agronomic and nutritional requirements, or as a more C mitigation option, as highlighted by the meta-analysis of Li et al. (2023).

Regarding the C/N ratio, significant differences were observed depending on the interaction among feedstocks, pyrolysis temperature, and pyrolysis time. Biochar produced from EC showed the highest C/N ratio at 300 °C across all pyrolysis times, while different patterns were observed at higher temperatures. Overall, the C/N ratios obtained in this study (20-80) were similar to those reported by Bruun et al. (2012) for biochars derived from different feedstocks under varying pyrolysis conditions. At 400 and 500 °C, higher C/N ratios were generally observed at shorter pyrolysis times (30 and 60 minutes), reflecting the complex dynamics of C and N transformations during pyrolysis. The increase in the C/N ratio observed in EC-, UE- and SD-derived biochars with increasing temperature was mainly associated with the preferential loss of N relative to C, as previously reported by Li et al. (2018) for lignocellulosic biomass. While traditional agricultural residues biochars produced at 500 °C commonly exhibited C/N ratios between 20-40 (Hossain et al., 2020), the UE- and EC-derived biochars in this study reached values between 45-60, indicating greater C stability and lower N release rates. This result suggests that forest residue-derived biochars may be more suitable for long-term C sequestration than traditional agricultural feedstocks. The optimal C/N ratios for specific applications depend on the intended use: low C/N ratios (< 20) are most appropriate for faster nutrient release to plants, while high C/N ratios (> 30) are most suitable for long-term C sequestration applications. The C/N ratio and its relationship with nutrient release in our biochars are of the utmost importance for their practical application. Biochars with C/N < 20 (e.g., SD at 300 °C) enable quick N mineralization, making N available to plants in 2–4 weeks after soil incorporation (Gul et al., 2015). On the other hand, biochars with C/N ratios of over 30 (such as EC and UE at higher temperatures) initially immobilize N in the soil but provide extended C benefits and long-term nutrient release over 2–3 years (Hossain et al., 2020). Carbon stability in biochars with high C/N ratios contributes to soil organic C pools with residence times ranging from 100 to 1,000 years, compared with 1–10 years for low-C/N biochars (Wang et al., 2016).

Therefore, considering the results of this study from an agronomic perspective, the C concentration indicated that all feedstocks evaluated in this study may enhance C sequestration under high-temperature pyrolysis conditions when applied to soil and may further increase crop production (Han et al., 2023). The degree of impact of biochar addition also depends on soil type, with effects usually being higher in sandy soils (Utomo et al., 2018). A meta-analysis carried out by Han et al. (2023) showed that key biochar factors for improving soil are associated with the soil organic C, total N, and the biochar C/N ratio, as described in our study. The EC- and UE-derived biochars exhibited very good potential for long-term C storage. SD biochar contained 65-70% C at 500 °C, while EC- and UE-derived biochars contained 75-80% C under high-temperature conditions (500°C). Based on their composition, EC and UE biochars suggest a greater potential for C retention and mitigation than SD biochar in the present experiment, as well as compared with other woody biomass feedstocks (Jeffery et al., 2017). Additionally, the inputs of C with the different types of biochar into the soil provide clear long-term C sequestration potential due to their high C/N ratios (Zhang et al., 2021). Moreover, the different biochar characteristics and pyrolysis conditions tested in this study may also influence soil-plant interactions through several mechanisms. On the one hand, the addition of biochar to soil introduces N into soil pools that can be utilized by soil microorganisms, which use nutrients for such biological processes such as C sequestration linked to nutrient cycling. In this regard, SD showed particular potential as a N source when produced at low pyrolysis temperatures due to its pre-processing with APP, making it more suitable as a N source for some types of soils. On the other hand, the biochar matrix can immobilize C and N in various chemical forms in the soil, creating a continuous, slow nutrient-release system that supplies nutrients to plant and soil biota over long periods of time. The effect depends on soil type. In this context, the C/N ratio plays a key role in balancing short-term plant demand with long-term C stabilization in soil (Zhang et al., 2021). Nitrogen and C concentrations were highest when SD biochar was produced at 300 °C and 500 °C, respectively. Therefore the pyrolysis temperature used for SD biochar may be selected according to different soil management objectives, depending on whether N addition to improve crop production or C sequestration to enhance soil health and mitigate climate change is sought (Wang et al., 2015). The low C/N ratios observed in SD biochar indicate more rapid nutrient mineralization and release (Mukherjee and Zimmerman, 2013), while the high C/N ratios in EC- and UE-derived biochars suggest long-term C sequestration with slow nutrient release (Alburquerque et al., 2013). Overall, biochars produced from EC, and UE presumably with high lignin contents, appear to promote the formation of stable C forms associated with long-term soil C storage, although with lower short-term nutrient release compared to many agriculturally derived biochars (Jeffery et al., 2017).

Feedstock origin and preprocessing, pyrolysis temperature, and time influence N and C concentrations in biochar, as well as its C/N ratio. Biochar derived from sawdust showed the highest N concentration due to the addition of ammonium polyphosphate (APP) during chipboard processing, particularly when pyrolysis was carried out at lower temperatures, making it more suitable as a nitrogen source for soils. All feedstock-derived biochar showed significant increases in C concentration at higher temperatures, especially those derived biochars from Erica cinerea L. and Ulex europaeus L., making them suitable for C sequestration when applied to soils. Therefore, variations in feedstock type and pyrolysis conditions make it possible to produce biochar for different specific soils uses, such as soil C sequestration, nutrient release and consequently soil health improvement through organic amendments, depending of soil type. However, future research should focus on field trials to evaluate the long-term stability of these biochars in different soil types, as well as their agronomic performance under different crops and soil conditions.