Different activation and N-doping treatments were used to produce biochar-based fertilizers (BBFs) with increased N concentration and slow N release. Pristine biochars were produced by pyrolysis of olive tree pruning feedstock at low and high temperatures (400 and 800 °C). These biochars were activated either by ultrasonication, or oxidation with hydrogen peroxide (H2O2) or nitric acid (HNO3) to increase their N retention potential. Subsequently biochars were enriched with N with either urea or ammonium sulfate. The activation of low-temperature biochars with HNO3 was the most effective treatment leading to new surface carboxylic groups that facilitated the later enrichment with N. When treated with urea, BBFs reached 7.0 N%, whereas the H2O2 activation only allowed an increase up to 2.0 N%. The use of urea as the external N source was the most efficient for incorporating N. Urea treated biochars had a water-soluble fraction that represented up to 14.5 % of the total N. The hydrolyzable N fraction, composed by amides and simple N heterocycles originated by the N-doping treatments, and nitro groups generated from HNO3 activation, represented up to 60 % of the total N. This study relates the N chemical forms in the new BBFs to potential N availability in soil. The presence of water-soluble, hydrolyzable and non-hydrolyzable N implied that these BBFs may supply N that would be progressively available for plants, acting as slow-release fertilizers, We gratefully acknowledge the financial support of the projects RTI2018-099417-BI00 and PID2021-128896OB-I00 from the Spanish Ministry of Science, Innovation and Universities, co-funded with EU FEDER funds, Peer reviewed

Novel fertilization strategies, such as the use of biochar-based fertilizers (BBFs) and the co-application of biochar with mineral fertilizers, have shown promising results for mitigating nitrous oxide (N2O) emissions and reducing N losses in agroecosystems. Two greenhouse experiments were performed with radish to evaluate: (1) the mitigation of yield-scaled N2O emissions using BBFs, produced at either 400 or 800 °C and enriched with urea, compared to the co-application of raw biochars with urea; and (2) the N2O mitigation potential of low rates of raw biochars, equivalent to those used with BBFs fertilization, co-applied with low and high N rates (90 and 180 kg N ha−1). BBF produced at 800 °C reduced yield-scaled N2O emissions by 32% as compared to the urea treatment, and by 60%, as compared to the combination of raw biochar with urea. This reduction was attributed to the slow rate of N release in BBF. On the contrary, the co-application of low rates of biochar with urea increased yield-scaled N2O emissions as compared to the fertilization with urea alone. Low rates of biochar (1.4–3.1 t ha−1) reduced yield-scaled N2O emissions only with a high rate of N fertilization. High-pyrolysis-temperature biochar, co-applied with synthetic fertilizer, or used to produce BBFs, demonstrated lower yield-scaled N2O emissions than biochar produced at a lower pyrolysis temperature. This study showed that BBFs are a promising fertilization strategy as compared to the co-application of biochar with synthetic fertilizers, We gratefully acknowledge the financial support of the project PID2021-128896OB-I00 from the Spanish Ministry of Science, Innovation and Universities, co-funded with EU FEDER funds. R. Castejón-del Pino was supported by a JAE Intro fellowship by The Spanish Ministry of Science, Innovation and Universities and a FPI fellowship within the project RTI2018-099417-BI00., Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature, Peer reviewed

The agronomic use of compost and biochar as soil amendments may exhibit contrasting results in terms of soil fertility and plant nutrition. The effects of the biennial application of biochar, compost and a blend of compost:biochar (90:10; % dw:dw) on the agronomical performance of an organically managed and well established 25-year-old olive orchard was assessed 5 years after the initial application. The agronomical evaluation was based on the assessment of the soil physical, chemical, and biological characteristics, and the assessment of the soil fertility by both crop production and nutritional status of the orchard, and the bioassay with olive plantlets. Biochar mainly benefited the physical properties (bulk density, total porosity, aeration, water retention capacity) of soil, especially in the top 0-5 cm. Compost and its blend with biochar improved microbial activity, soil nutritional status (increasing the content of soluble organic C, N, and P) and favoured the formation of aggregates in soil. The bioassay conducted with young plantlets confirmed the enhanced soil fertility status in the three amended treatments, particularly in the case of biochar and its blend with compost. However, this effect was not significantly observed in the adult plants after 5 years of application, reflecting the slow response of adult olive trees to changes in fertilization. Based on these results, alongside the desirable long-residence time of biochar in soil and the ready availability of compost, the blend of biochar with compost assayed in this study is defined as a valid strategy for preparing high quality soil organic amendments, This study was financially supported by the projects TED2021-131907B-I00, PID2021-128896OB-I00 and AGROALNEXT programme from the Spanish Ministry of Science, Innovation and Universities, co-funded with EU FEDER and NextGenerationEU (PRTR-C17.I1) funds and by Fundación Séneca from Comunidad Autónoma Región de Murcia (CARM). Giovana P-F- Macan enjoyed a mobility stay funded by the Erasmus + KA1 Erasmus Mundus Joint Master Degrees Programme of the European Commission under the Plant Health Project, Peer reviewed

Meta-analyses show an overall decrease in soil N<inf>2</inf>O emissions after biochar (BC) amendment. Nonetheless, N<inf>2</inf>O mitigation with BC cannot be extrapolated to every BC-soil combination, inasmuch as an increase in soil N<inf>2</inf>O release has been occasionally reported. We hypothesized that BC characteristics are key, and performed two microcosm experiments to advance in the understanding of the properties associated. We first investigated how 22 well-characterized BCs affect N<inf>2</inf>O emissions in a calcareous soil under denitrification conditions. Whereas most BCs decreased N<inf>2</inf>O emissions, some substantially increased N<inf>2</inf>O emissions. In a second experiment, we selected and further characterized eight of the 22 previous BCs. We applied the ¹⁵N-gas-flux method to study how these BCs affect denitrification products (N<inf>2</inf>O and N<inf>2</inf>) in the same soil. Results indicate that the interaction between BC and the denitrification process depends on the temperature of pyrolysis. Whereas BCs produced at 400 °C tended to increase total denitrification (N<inf>2</inf>O+N<inf>2</inf>) by an average of 28%, BCs produced at 600 °C significantly reduced total denitrification by 53%. Nevertheless, this decline in overall denitrification did not result in a decrease of N<inf>2</inf>O emissions, as there was a strong shift in the N<inf>2</inf>O/(N<inf>2</inf>+N<inf>2</inf>O) ratio favoring N<inf>2</inf>O. A redundancy analysis revealed a direct correlation between carboxylic groups on BCs surface and N<inf>2</inf>O emissions. This research enhances our understanding of the interaction of BC with denitrification, particularly concerning the relevance of the temperature of pyrolysis, and opens up new paths for investigation, crucial for optimizing the application of BCs in different soil environments. Graphical Abstract: (Figure presented.), Public competitive calls PID2021-128896OB-I00 funded by MCIN/AEI/https://doi.org/10.13039/501100011033/ and ERDF A way of making Europe and project Nº 201840I127 from CSIC, Peer reviewed

Agronomic uses of biochar have been intensely explored in the last 15 years. Recently, a new generation of biochar-based fertilizers has been developed. Raw biochar (BCH), nitrogen-enriched biochar (N + BCH) or urea were added to a coir fiber-based substrate for tomato cultivation, to assess seed germination, growth and fruiting of two cultivars (Cuarenteno and Moneymaker), This work was supported by Grant PID2021-128896OB-I00 funded by the Spanish Ministry of Science, Innovation and Universities MICIU/AEI/10.13039/501100011033 and by ERDF/EUU funds. R. Castejón-del Pino was supported by Grant PRE2019-088308 funded by MICIU/AEI/10.13039/501100011033 and ESF Investing in your future within the project RTI2018-099417-BI00, Peer reviewed

Accurate estimation of Above-Ground Biomass Density (AGBD) is essential for assessing carbon stocks and promoting sustainable agricultural practices. This study integrates multi-source remote sensing data, including GEDI LiDAR, optical, SAR, and topographic variables, to predict AGBD in Mediterranean olive orchards using a Random Forest regression model implemented on Google Earth Engine (GEE). The proposed volumetric approach, based on GEDI L2A canopy height and dendrometric parameters, provides a scalable framework for large-scale biomass estimation using lidar technologies on satellite platforms. The model's predictive performance varied depending on data combinations, with the fully multi-source configuration achieving the most consistent results, although overall accuracy remained moderate due to sensor constraints and the inherent limitations of the proposed exploratory framework. NDBI, slope, HV polarization, and MCARI1 were identified as the most influential predictors. The spatial analysis revealed that Spain exhibited the highest total AGB stock among the studied countries, followed by Italy and Greece, reflecting their dominance in olive production. Despite its limitations in precision at fine spatial scales, this exploratory study demonstrates the potential of integrating LiDAR, optical, and SAR data to evaluate biomass distribution in low-stature vegetation. The proposed framework offers a cost-effective and scalable strategy for large-scale carbon monitoring and supports data-driven agricultural management toward more sustainable Mediterranean production systems., This work was supported by two primary sources: in part by the project PID2021-128896OB-I00 from the Spanish Ministry of Science, Innovation and Universities (co-funded by EU FEDER), and in part by the project OLIVCHAR (ref. TED2021-131907B-I00), financed by MCIN/AEI/10.13039/501100011033 and the European NextGenerationEU/PRTR. We acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI), Peer reviewed

[EN] The agronomic use of compost and biochar as soil amendments may exhibit contrasting results in terms of soil fertility and plant nutrition. The effects of the biennial application of biochar, compost and a blend of compost:biochar (90:10; % dw:dw) on the agronomical performance of an organically managed and well established 25-year-old olive orchard was assessed 5 years after the initial application. The agronomical evaluation was based on the assessment of the soil physical, chemical, and biological characteristics, and the assessment of the soil fertility by both crop production and nutritional status of the orchard, and the bioassay with olive plantlets. Biochar mainly benefited the physical properties (bulk density, total porosity, aeration, water retention capacity) of soil, especially in the top 0-5 cm. Compost and its blend with biochar improved microbial activity, soil nutritional status (increasing the content of soluble organic C, N, and P) and favoured the formation of aggregates in soil. The bioassay conducted with young plantlets confirmed the enhanced soil fertility status in the three amended treatments, particularly in the case of biochar and its blend with compost. However, this effect was not significantly observed in the adult plants after 5 years of application, reflecting the slow response of adult olive trees to changes in fertilization. Based on these results, alongside the desirable long-residence time of biochar in soil and the ready availability of compost, the blend of biochar with compost assayed in this study is defined as a valid strategy for preparing high quality soil organic amendments., This study was financially supported by the projects TED2021-131907B-I00, PID2021-128896OB-I00 and AGROALNEXT programme from the Spanish Ministry of Science, Innovation and Universities, co-funded with EU FEDER and NextGenerationEU (PRTR-C17.I1) funds and by Fundacion Seneca from Comunidad Autonoma Region de Murcia (CARM). Giovana P-F-Macan enjoyed a mobility stay funded by the Erasmus + KA1 Erasmus Mundus Joint Master Degrees Programme of the European Commission under the Plant Health Project.

[EN] BACKGROUND: Agronomic uses of biochar have been intensely explored in the last 15 years. Recently, a new generation of
biochar-based fertilizers has been developed. Raw biochar (BCH), nitrogen-enriched biochar (N + BCH) or urea were added to
a coir fiber-based substrate for tomato cultivation, to assess seed germination, growth and fruiting of two cultivars
(Cuarenteno and Moneymaker).
RESULTS: BCH stimulated seed germination and early radicle growth, possibly because of the presence of karrikins detected in
both BCH and N-BCH (0.039 and 0.044 mg kg¿1
, respectively). However, BCH reduced growth in adult plants in both cultivars,
probably because of ammonium retention, causing low-N-stress-related symptoms such as accumulation of flavonoids in the
leaf. Urea was toxic for seed germination because of the fast release of ammonium, but caused a positive effect on adult plant
growth and yield, increasing chlorophyll in both cultivars, quantum yield and ascorbic acid in cv. Cuarenteno, and decreasing
flavonoids and peroxide in leaves of both cultivars. Unlike urea, N + BCH showed a positive impact on plant growth and yield,
but without releasing high amounts of ammonium or negatively affecting seed germination. Nitrogen-rich amendments
reduced phosphorus and increased iron leaf content in both cultivars.
CONCLUSION: BCH can be effectively used as a growth medium constituent in nurseries for seedling production, whereas N
+ BCH offers a promising alternative to urea or other nitrogen mineral fertilizers for crop cultivation., This work was supported by Grant PID2021-128896OB-I00 funded by the Spanish Ministry of Science, Innovation and Universities MICIU/AEI/10.13039/501100011033 and by ERDF/EUU funds. R. Castejón-del Pino was supported by Grant PRE2019-088308 funded by MICIU/AEI/10.13039/501100011033 and ESF Investing in your future within the project RTI2018-099417-BI00. We acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).