Fostering the forest circular bioeconomy to combat climate change by producing biochar from conifer and broadleaf stands

Carbon and nitrogen play crucial roles in soil by influencing its physical, chemical, and biological properties, thereby determining soil fertility and overall health. Biochar is a carbon-rich material with additional elements that can promote plant growth by improving soil quality and nutrient availability. However, its total carbon and nitrogen concentrations vary depending on factors such as feedstock type and pyrolysis conditions. This study evaluated the effects of tree species, tree fraction, and pyrolysis time on the total carbon, total nitrogen, and carbon-to-nitrogen (C/N) ratio of biochar. Biochar was produced from three conifer species (Pinus radiata D. Don, Pinus sylvestris L., and Pinus pinaster Aiton) and three broadleaf species (Betula alba L., Quercus robur L., and Castanea sativa Mill.). In addition, three tree fractions (branches, litterfall, and green leaves) were collected from each species and subjected to pyrolysis at 300 °C for 30, 60, and 120 min. The results showed that, in general, biochar produced from conifer species had higher total carbon concentrations and C/N ratios than biochar derived from broadleaf species. In contrast, biochar from broadleaf species typically contained higher total nitrogen concentrations. Tree fraction also markedly influenced biochar composition. Branch-derived biochar exhibited the highest total carbon concentrations, while biochar produced from green leaves contained the highest total nitrogen concentrations. Moreover, regardless of tree species and tree fraction, pyrolysis time significantly influenced total carbon and nitrogen concentrations and the C/N ratio, although to a lesser extent. These findings highlight the influence of tree species, tree fraction, and pyrolysis time on biochar chemical composition, providing valuable insights into the production of biochar from forest residues with characteristics suitable for improving soil properties and supporting sustainable agricultural management.

Wood production is one of the primary sources of biomass in Europe, supported by the 159 million hectares of forests in the European Union (EC, 2020). In 2018, the EU-27 produced an estimated 489.8 million m 3 of roundwood, generating approximately 50.2 million tons of wood residues annually (Borzęcki et al., 2018). Unfortunately, only approximately 70% of these wood residues are properly managed (EC, 2020). In this context, converting forest residues into biochar offers an opportunity to improve forest waste management while promoting a circular bioeconomy. In Galicia (northwest Spain), where forests cover approximately 70% of the land area, using different tree fractions from conifer and broadleaf species, such as branches, litterfall, and green leaves, for biochar production could also strengthen the traditional link between forestry and agriculture (Waqas et al., 2018). This approach is consistent with European policies, such as the Green Deal, which pursues climate neutrality, as well as the Bioeconomy and Circular Economy strategies and the Waste Framework Directive (2008/98/EC), all of which establish pathways and a comprehensive legal framework for managing waste materials, including their recovery and disposal.

Biochar is a carbon-rich material produced through the pyrolysis of organic biomass under oxygen-limited conditions using various feedstocks, such as plant residues, animal manure, and wastewater sludge (IBI, 2015; Shoudho et al., 2024). Biochar is an extremely stable source of carbon (C) that can remain in the soil for hundreds or even thousands of years (Kapoor and Zdarta, 2024). Therefore, biochar is considered one of the most cost-effective approaches for reducing atmospheric carbon dioxide (CO 2) concentrations by enhancing C sequestration in soils. This makes biochar a promising strategy for mitigating climate change, as recognized by the Food and Agriculture Organization (FAO) (Diagne Langston et al., 2023). In addition to mitigating climate change, and depending on its composition, biochar can improve the physical, chemical, and biological properties of soil (Zhang et al., 2014; Liu et al., 2019), thereby enhancing soil fertility.

The agronomic and environmental value of biochar largely depends on its chemical composition, which is influenced by feedstock type (Anand et al., 2023) and pyrolysis conditions, including temperature and time (Balmuk et al., 2023), which can affect its potential applications. Therefore, selecting an appropriate feedstock according to the desired biochar properties is essential to achieve the specific outcomes (Wei et al., 2019; El Barkaoui et al., 2023). The suitability of biochar for climate change mitigation and/or nutrient supply depends on its C and nitrogen (N) concentrations and degree of mineralization, which is closely related to the carbon-to-nitrogen (C/N) ratio and influenced by pyrolysis conditions. Although numerous studies have investigated the effects of feedstock type or pyrolysis conditions on biochar properties, the majority of them have focused on agricultural residues or a limited number of woody feedstocks (Yize et al., 2023). Comparative studies that evaluated the combined effects of tree species, tree fraction, and pyrolysis time under the same pyrolysis conditions remain scarce, particularly for the main forest species in Galicia.

Therefore, this study aimed to evaluate the effects of tree species (Pinus radiata D. Don, Pinus sylvestris L., Pinus pinaster Aiton, Betula alba L., Quercus robur L., and Castanea sativa Mill.), tree fractions (branches, litterfall, and green leaves), and pyrolysis time (30, 60, and 120 min) at a temperature of 300 °C on the total C and N concentrations and the C/N ratio of biochar. We hypothesized that biochar produced from different tree species and tree fractions would exhibit distinct C and N concentrations and C/N ratios, and that increasing pyrolysis time would modify these properties. This information may contribute to the production of biochar with characteristics suitable for soil C sequestration and sustainable soil management.

Feedstock for biochar production was collected from representative conifer and broadleaf forest plantations located in the northwestern region of Galicia (Spain) in September 2021. This period was selected to minimize the influence of the main growing season on tree phenology and to obtain tree fractions with comparable physiological conditions. The selected tree species corresponded to the most abundant and representative forest species in Galicia and included three conifer species [ Pinus radiata D. Don (Pr), Pinus sylvestris L. (PS), and Pinus pinaster Aiton (Pp)] and three broadleaf species [ Betula alba L. (Ba), Quercus robur L. (Qr), and Castanea sativa Mill. (Cs)].

For each species, three tree fractions were collected, including branches, litterfall, and green leaves. These fractions were selected to evaluate whether different components of forest biomass exhibit similar potential for biochar production, or whether some fractions provide more suitable properties for C sequestration and soil amendment applications. After collection, the samples were manually separated in the laboratory, homogenized, and dried in an oven at 45 °C for 4 days until they reached a constant weight.

Biochar production was carried out using a muffle furnace with ceramic crucibles containing dried feedstock. A pyrolysis temperature of 300 °C was selected to evaluate whether a low-energy thermal treatment could produce biochar with suitable characteristics while reducing the energy requirements associated with higher pyrolysis temperatures. The pyrolysis process was conducted at three different residence times (30, 60, and 120 min) to determine the effect of processing duration on biochar composition and to identify whether longer pyrolysis times resulted in significant changes in biochar quality.

After pyrolysis, the resulting biochar was ground using an agate mortar and pestle, homogenized, and stored in sealed plastic bags until analysis (Gonzaga et al., 2017). The experimental design consisted of three factors: Tree species (six levels), tree fraction (three levels), and pyrolysis time (three levels). Furthermore, three independent biochar replicates were produced for each combination of species, fraction, and pyrolysis time to ensure experimental reliability and to account for variability during biochar production.

Total C and N concentrations were determined for each biochar sample using a LECO CNS-2000 elemental analyzer LECO CNS-2000: LECO Corporation, St. Joseph, MI, USA. (Kowalenko, 2001). The C/N ratio was calculated using the measured concentrations of C and N.

The total C and N concentrations and the C/N ratio were statistically analyzed using SAS software. A factorial analysis of variance (ANOVA) was conducted to evaluate the individual and interaction effects of tree species, tree fraction, and pyrolysis time on total C and N concentrations and the C/N ratio. The statistical model used was Y ijk = μ + S i + F j + T k + SF ij + ST ik + FT jk + SFT ijk + ε ijk, where Y ijk is the dependent variable, μ is the mean of the variable, S i is the tree species i, F j is the tree fraction j, T k is the pyrolysis time k, SF ij is the tree species–tree fraction interaction, ST ik is the tree species–pyrolysis time interaction, FT jk is the tree fraction–pyrolysis time interaction, SFT ijk is the tree species–tree fraction–pyrolysis time interaction, and ε ijk is the error. Before conducting the ANOVA, the data were checked for normality and homogeneity of variances to verify that the assumptions of the analysis were satisfied. When significant differences were detected, mean comparisons were performed using the least significant difference (LSD) test. A significance level of 0.05 was applied for all statistical analyses.

Tree species effect

Figure 1 shows significant differences (p < 0.05) in total biochar C and N concentrations and the C/N ratio between conifer and broadleaf species. Conifer-derived biochar had higher total C concentrations and C/N ratios than broadleaf-derived biochar. In contrast, broadleaf-derived biochar showed higher total N concentrations.

The effect of tree fraction on biochar composition is shown in Figure 2. Branch-derived biochar presented higher total C concentrations than litterfall- and green leaf-derived biochar in both conifer and broadleaf species (Figure 2a). In contrast, total N concentrations were higher in biochar produced from green leaves and litterfall compared to that produced from branches, particularly in broadleaf species (Figure 2b). The C/N ratio followed a similar pattern to total C concentrations, with higher values observed in branch-derived biochar compared to the other fractions (Figure 2c).

Pyrolysis time effect

Total C concentrations (Figure 3a) were significantly higher (p < 0.05) in conifer-derived biochar than in broadleaf-derived biochar at all pyrolysis times (30, 60, and 120 min) (Figure 3a). Similarly, the C/N ratio was higher in conifer-derived biochar at 30 and 60 min (Figure 3c). However, total N concentrations (Figure 3b) at 30, 60, and 120 min did not differ significantly between conifer and broadleaf species (p > 0.05). Moreover, no significant differences were observed among pyrolysis times for total C and N concentrations and the C/N ratio in both conifer and broadleaf species (p > 0.05).

Carbon: interaction between tree species and pyrolysis time

Total biochar C concentrations were significantly affected by tree species, tree fraction, and pyrolysis time (p < 0.05) (Figure 4). Total C concentrations ranged from 63.6% to 73.0% in branch-derived biochar (Figure 4a), from 59.0% to 68.8% in litterfall-derived biochar (Figure 4b), and from 61.7% to 71.2% in green leaf-derived biochar (Figure 4c).

Across all tree fractions, conifer-derived biochar generally showed significantly higher C concentrations than broadleaf-derived biochar (p < 0.05). In branches and litterfall, Pp, Ps, and Pr consistently exhibited higher C concentrations than Qr and Cs at all three pyrolysis times. In green leaves, Pp showed the highest concentrations, whereas Qr and Cs exhibited the lowest values (p < 0.05).

Pyrolysis time had a minimal effect on total C concentration, with significant differences observed only for Qr litterfall-derived biochar. In this case, the C concentration was higher after 30 min compared to 120 min (p < 0.05).

Nitrogen: interaction between tree species and pyrolysis time

Total N concentration in biochar was significantly affected by tree species in each tree fraction and by pyrolysis time (p < 0.05) (Figure 5). Total N concentrations ranged from 0.89% to 2.11% in branch-derived biochar (Figure 5a), from 1.8 to 4.3% in litterfall-derived biochar (Figure 5b), and from 2.0% to 3.6% in green leaf-derived biochar (Figure 5c).

In branch-derived biochar, after 30 and 60 min of pyrolysis, Pr showed significantly lower total N concentrations than Ps, Ba, and Qr (p < 0.05). After 120 min, Ba and Qr showed significantly higher total N concentrations than Pr and Ps (p < 0.05).

In litterfall-derived biochar, after 30 and 60 min of pyrolysis, Ps exhibited significantly higher total N concentrations than the other tree species (p < 0.05). In addition, at 30 min, Qr and Pp also showed significantly higher total N concentrations than Ba and Pr (p < 0.05). At 60 min, Pp and Qr exhibited higher total N concentrations than Cs, while Cs showed higher values than Pr and Ba (p < 0.05). After 120 min, Ps maintained significantly higher total N concentrations than Ba and Pr (p < 0.05).

In green leaf-derived biochar, Ba exhibited the highest total N concentration after 30 min of pyrolysis (p < 0.05). Moreover, at 30 and 60 min, Ps and broadleaf species showed higher total N concentrations than the conifers Pr and Pp (p < 0.05). After 120 min, Ba and Cs maintained significantly higher total N concentrations than Pp (p < 0.05).

Pyrolysis time significantly affected total N concentration only in branch-derived biochar from Pr, with lower values after 120 min than after 30 and 60 min, and in litterfall-derived biochar from Pp, with higher values after 60 min than after 30 min (p < 0.05). No significant effect of pyrolysis time was observed in green leaf-derived biochar (p > 0.05).

C/N ratio: interaction between tree species and pyrolysis time

The C/N ratio varied significantly among tree species within each tree fraction and pyrolysis time (p < 0.05) (Figure 6). The C/N ratio ranged from 32.47 to 68.57 in branch-derived biochar (Figure 6a), from 16.02 to 37.39 in litterfall-derived biochar (Figure 6b), and from 18.64 to 35.70 in green leaf-derived biochar (Figure 6c).

In branch-derived biochar, Pr generally exhibited the highest C/N ratio, regardless of pyrolysis time (p < 0.05). The only exception was observed after 60 min, when no significant differences were found between Pr and Pp. Moreover, after 120 min, Ps exhibited a higher C/N ratio than Ba (p < 0.05).

In litterfall-derived biochar, after 30 min of pyrolysis, Pr and Ba exhibited the highest C/N ratios, whereas Qr and Ps showed the lowest values (p < 0.05). After 60 min, Pr and Ba maintained higher C/N ratios than the other tree species. After 120 min, Pr showed higher C/N ratios than Cs, Pp, Qr, and Ps (p < 0.05), whereas Ba also exhibited higher C/N ratios than Qr and Ps.

In green leaf-derived biochar, after 30 min of pyrolysis, Pp exhibited the highest C/N ratio, followed by Pr. In contrast, Ps, Ba, Qr, and Cs showed the lowest values (p < 0.05). After 60 min, Pp maintained the highest C/N ratio (p < 0.05), while Pr also showed higher C/N ratios than Ps, Ba, Cs, and Qr. After 120 min, Pp continued to exhibit higher C/N ratios than Ba and Cs (p < 0.05).

Pyrolysis time significantly affected the C/N ratio only in branch-derived biochar. Specifically, after 120 min, the C/N ratios for Pr were higher than those after 60 min, and for Pp, the C/N ratios were higher after 30 min compared to 120 min (p < 0.05). In contrast, pyrolysis time had no significant effect on the C/N ratio in litterfall- or green leaf-derived biochar (p > 0.05).

Biochar quality, defined as its capacity to sequester C and release nutrients, was mainly determined by feedstock composition, which varied with tree species, tree fraction, and pyrolysis time (Wei et al., 2019; Kulczycki et al., 2020). Accordingly, conifer-derived biochar exhibited significantly higher C concentrations than broadleaf-derived biochar, consistent with previous studies comparing pine wood with broadleaf species such as Betula spp. and Quercus robur L. (Hagner et al., 2015; Novak et al., 2015; Jia et al., 2018; Kulczycki et al., 2020). The higher C concentrations observed in biochar derived from conifer species, particularly from branches, compared to broadleaf species and green leaves, can be attributed to their higher lignin and wax contents (Demirbas, 2004; Perakis et al., 2012). Lignin is an insoluble, complex, and highly branched phenolic polymer, with linkages to different compounds through strong carbon–carbon bonds and more labile ester linkages, forming an extensive network within the plant cell wall. These polymers are recalcitrant and reinforce the cellulose structure of plant tissues (Ortiz et al., 2009). They are also characterized by high resistance to decomposition, facilitating the development of aromatic C structures during pyrolysis, which constitute the structural framework of biochar and enhance its capacity to adsorb and retain soil organic matter (Asadi et al., 2021; Han et al., 2021). Lignin is further degraded in soil by fungi and is one of the main factors explaining the lower decomposition rates of conifers compared to broadleaves, as reported by Xiang et al. (2022) for leaf-litter decomposition in running waters. Furthermore, Li et al. (2023) also reported a positive relationship between lignin content and total C concentration in the resulting biochar. In this context, it should be noted that, in our experiment, a general trend in the C concentration of biochar derived from different tree feedstock sources was observed, regardless of tree fraction, following the order Pp > Ps > Pr > Ba > Qr > Cs. This pattern can be attributed to differences in lignin content among species. Thus, the Pinus pinaster Aiton and Pinus sylvestris L. stands in Galicia are plantations established during the 19th century, originating from Mediterranean regions of Spain. Their adaptation to recurrent wildfires is associated with thicker bark and the presence of waxes, in contrast to the northern broadleaf species. The adaptation of Pinus pinaster Aiton and Pinus sylvestris L. to wildfires could explain their higher lignin content compared to northern broadleaf species. The higher lignin and wax contents in conifer species contribute to the clear differences observed between conifer- and broadleaf-derived biochar. This distinction is particularly notable for the Pp pioneer species, but it also applies to Ps and Pr. Pp and Ps were clearly differentiated from the slow-growing species, Qr and Cs, while the pioneer species Ba occupied an intermediate position among broadleaf species, likely due to its greater adaptation to disturbances and fire. Overall, these results suggest that Pp has the highest proportion of recalcitrant C compounds (lignin and waxes), which may be associated with its Mediterranean pioneer characteristics and strong tissue protection against fire (Fernandes and Rigolot 2007; Han et al., 2021), compared with Ps, which is adapted to high-altitude Mediterranean environments, and Pr, which originates from cooler regions of Oregon, USA, where fire risk is lower. The differences in C concentration and lignin content observed among tree species and tree fractions indicate that biochar derived from different feedstocks can have distinct qualities and fulfil different objectives within the bioeconomy framework. The recalcitrant C associated with the higher lignin content of conifers confers greater potential for C sequestration and climate change mitigation. Following soil application, it also enhances soil water-holding capacity and aeration through the contribution of organic matter (mainly C), thereby improving soil physical properties (Razaq et al., 2017; Wang et al., 2025). However, the lower C content in broadleaf-derived biochar may promote faster nutrient release rates and, therefore, lead to a more direct improvement in soil chemical properties. The same principle applies to the different conifer and broadleaf fractions. Branch-derived biochar has a higher proportion of C and lignin than litterfall- and green leaf-derived biochar and may therefore contribute more to the physical improvement of soil due to the role of organic matter in enhancing soil structure (Asadi et al., 2021; Han et al., 2021). This may explain why green leaf-derived biochar is more suitable for incorporation into soil as nutrient suppliers.

Broadleaf-derived biochar generally showed significantly higher N concentrations than conifer-derived biochar. The N concentration observed in this study (0.89%–4.4%) was within the range (0.2%–5%) reported by Novak et al. (2015), Jia et al. (2018), and Kulczycki et al. (2020) for biochar derived from different feedstocks, including Pinus sylvestris L., Pinus taeda L., cotton straw, and rice husk. The higher N concentration observed in broadleaf-derived biochar can be attributed to the higher N content in the broadleaf feedstock compared to conifers (Leng et al., 2020). In addition, a consistent pattern in biochar N concentration was observed, regardless of tree fraction, following the order Ps > Qr and Ba > Cs > Pp > Pr. This pattern can be attributed to the presence of specific amino acids, such as arginine, which contain amide groups that can readily be converted into gaseous products, such as NH 3, during pyrolysis. As N-containing compounds accumulate, they contribute to the formation of additional N-functional groups, ultimately leading to higher N concentrations in the resulting biochar (Leng et al., 2020). Nitrogen plays a vital role in plant growth (Ye et al., 2022), and a higher N concentration in biochar may reduce the need for synthetic fertilizers (Solaiman et al., 2020). Moreover, species-specific growth strategies may also explain the differences in N concentrations in biochar. Thus, Ba is a pioneer and fast-growing broadleaf species compared to all conifer species. This may explain the higher N concentration in Ba branch-derived biochar than in Ps and Pr, and the sampling was carried out in early autumn when N translocation had already started. This interpretation is supported by the lower N concentration observed in Ba litterfall and the higher N concentration in green leaves, which act as a source of N for translocation to the branches. On the other hand, Pinus sylvestris L., the slowest-growing conifer species, showed higher N concentrations in its green leaves than Pp and Pr. This may explain the higher total N concentration in branch-derived biochar and the higher N concentration in litterfall-derived biochar. This could be attributed to the higher initial N content in the leaves before translocation and their lower capacity to translocate all N to the branches. Consequently, Ps litterfall-derived biochar showed higher N concentrations than those of the other conifer and broadleaf species regardless of pyrolysis time. This pattern can be associated with the fact that Ps is an evergreen and slow-growing species. In contrast, Pinus pinaster Aiton, the species with the highest C concentration and lignin content as associated with adaptation to wildfires, consistently showed the lowest N concentration in green leaves, reflecting a greater investment in structural compounds than in growth under Mediterranean conditions.

In terms of the C/N ratio, conifer-derived biochar exhibited a significantly wider range of C/N values (16.1%–68.6%) than broadleaf-derived biochar (18.1%–40.8%), reflecting the generally higher C concentrations and lower N concentrations in conifer species. The different types of linkages and incorporation of C and N into organic compounds, such as carbohydrates and proteins, subsequently influence the C/N ratio. Reported biochar C/N ratios vary widely, ranging from approximately 10 to 250 (Phillips et al., 2022). However, when biochar is used as a soil amendment, it is recommended that the C/N ratio of the applied material falls within the range of 20–30, although a value close to 10 is considered optimal for adequate mineralization rates (USDA Natural Resources Conservation Service, 2020). Therefore, the C/N ratios obtained in this study indicate that biochar has high potential for long-term soil C sequestration and soil structural improvement. Its application together with N-rich fertilizers may be advisable to support plant nutrient requirements. Furthermore, in this study, a consistent pattern in the biochar C/N ratio was observed, regardless of tree fraction, following the order Pr > Pp > Ps > Ba > Cs > Qr. This pattern reflects the differences in C and N concentrations among tree species. High C/N ratios promote N immobilization and may explain the limited understory growth observed when full canopy cover and leaf fall are produced in Pinus radiata stands (Mosquera-Losada et al., 2009). They also favor longer soil C residence times, thereby reducing N leaching and volatilization (Prasad and Shih, 2016). A similar pattern was observed among tree fractions. Regardless of tree species and pyrolysis time, branch-derived biochar exhibited the highest C concentration (68.6%) and C/N ratio (45.0), followed by litterfall-derived biochar (65.5% C and a C/N ratio of 27.4) and green leaf-derived biochar (66.4% C and a C/N ratio of 23.5). In contrast, N concentration increased from branches (1.64%) to litterfall (2.58%) and green leaves (2.97%). These results are consistent with the limited available literature reporting biochar properties according to tree fraction. Konaka et al. (2021) and Asadi et al. (2021) also found that branches-derived biochar generally exhibits higher C concentrations and C/N ratios than leaf-derived biochar. This difference can be attributed to the higher lignin content and greater degree of carbonization of woody tissues compared to litterfall and green leaves (Keiluweit et al., 2010; Al-Wabel et al., 2013; Wei et al., 2017). However, the higher N concentration in green leaf-derived biochar is likely related to the greater abundance of arginine and other N-containing compounds in leaves (Yang et al., 2020).

Finally, pyrolysis time significantly affected C concentration, N concentration, and the C/N ratio only in a few feedstocks, namely the C concentration of Qr litterfall-derived biochar, the N concentration of Pr branch- and Pp litterfall-derived biochar, and the C/N ratio of Pr and Pp branch-derived biochar. The C concentration of Qr litterfall-derived biochar significantly decreased after 120 min of pyrolysis compared to that obtained after 30 min. This may indicate that most of the organic matter was carbonized within the first 30 min at 300 °C, so extending the pyrolysis time promoted additional C losses (Wang et al., 2019). In contrast, the N concentration in Pr branch-derived biochar decreased after 120 min of pyrolysis, whereas the N concentration in Pp litterfall-derived biochar increased from 30 to 60 min. These results could be attributed to the differences in the degree of feedstock decomposition and the concentration effect during pyrolysis (Gaskin et al., 2008). Authors such as Leng et al. (2020) and He et al. (2023) also reported that excessive pyrolysis time could lead to a decrease in N content in biochar due to N volatilization through the release of gaseous compounds such as NH 3. This highlights the importance of selecting an appropriate pyrolysis time to optimize biochar quality while minimizing N losses. The observed changes in N concentration with increasing pyrolysis time were also reflected in the C/N ratio, which was significantly affected only in branch-derived biochar from Pr and Pp. Overall, these results reflect the importance of both pyrolysis time and feedstock type in determining biochar quality and its potential applications.

The C and N levels and the C/N ratio of biochar were influenced by several factors, such as tree species and tree fractions, while pyrolysis time had a lower effect. Conifer-derived biochar showed higher C and lower N concentrations than broadleaf-derived biochar, probably due to the higher lignin content in conifer species. These differences highlight the importance of feedstock selection to obtain biochar with targeted properties.

Optimizing feedstock selection is crucial for producing biochar suitable either as a soil amendment or for C sequestration to mitigate climate change. Appropriate feedstock mixtures should therefore be considered to balance the C/N ratio according to the requirements of each application, whether for soil improvement or C sequestration. Further studies should evaluate the long-term effects of biochar derived from different species, tree fractions, and pyrolysis times on soil characteristics under field conditions from both productive and soil C sequestration perspectives.