The application of biochar as an organic amendment in polluted soils can facilitate their recovery by reducing the availability of contaminants. In the present work, the effect of biochar application to acid soils contaminated by heavy metal spillage is studied to assess its effect on the quantity and composition of soil organic matter (SOM), with special attention given to soil humic acids (HAs). This effect is poorly known and of great importance, as HA is one of the most active components of SOM. The field experiment was carried out in 12 field plots of fluvisols, with moderate and high contamination by trace elements (called MAS and AS, respectively), that are located in the Guadiamar Green Corridor (SW Spain), which were amended with 8 Mg·ha⁻¹ of olive pit biochar (OB) and rice husk biochar (RB). The results indicate that 22 months after biochar application, a noticeable increase in soil water holding capacity, total organic carbon content, and soil pH were observed. The amounts of oxidisable carbon (C) and extracted HAs in the soils were not altered due to biochar addition. Thermogravimetric analyses of HAs showed an increase in the abundance of the most thermostable OM fraction of the MAS (375–650 °C), whereas the HAs of AS were enriched in the intermediate fraction (200–375 °C). Spectroscopic and chromatographic analyses indicate that the addition of biochar did not alter the composition of the organic fraction of HAs, while Cu, Fe, and as were considerably accumulated at HAs.

Today’s agriculture has the challenge of ensuring food supply for a growing population while human activity has already deteriorated about 40% of the world’s soils, reducing productive capacity and increasing reliance on mineral fertilizers. In this context, valorizing and recycling mineral and agricultural waste for use as substrates or soil supplements enhance a sustainable economy, as well as the development of activities focused on finishing the soil nutrients’ cycle. Looking for an effective solution to the massive waste generation and to enhance the agronomic qualities of soils, this study investigates the agronomic impact of contrasting inorganic and organic materials such as green compost (GC), wood biochar (WB), rice husk ash (RA), and volcanic ash (VA) as amendments to an alkaline Luvisol under controlled conditions. In this sense, barley seeds were planted and grown in a greenhouse under controlled conditions for 60 days on a soil amended with the aforementioned materials. The amendments demonstrated appropriate attributes for improving soil agronomic properties, enhancing the soil’s nutritional content with no effect on barley germination. The WB showed high aromaticity and abundance of refractory organic C. Both ash-rich amendments showed high P and K contents, which are important elements for plant development. The GC has high water retention capacity and an adequate C and N balance. Although the application of the amendments had no effect on barley yields, the plants from the ash-amended pots showed an increase of Photosystem II efficiency, indicative of a better physiological status. In terms of toxicological safety, the abundance of trace elements in soils and plants was investigated. All soils met the maximum allowable limits for these persistent pollutants. Nevertheless, longer-term tests on plants are required to determine the risk of Pb accumulation, particularly in soils amended with GC and compost-ash mixtures. The simultaneous combination of organic and inorganic amendments showed adequate agronomic attributes. WB analysis revealed its great recalcitrance and carbon sequestration potential.

The increasing demand for sustainable substrates in agriculture and urban greening calls for alternatives to peat, whose extraction poses significant environmental risks. This study assesses the potential of olive pomace biochar (OB), wood biochar (WB), and green compost (GC), alone or in combination, to partially replace peat in growing media and improve substrate properties and plant development. Ten different substrates were formulated by substituting 10–20% of a commercial peat-based substrate with these organic amendments, using the commercial substrate alone as a control. The effects of such replacements were evaluated in the following experiments: a germination test conducted in Petri dishes using four forage species (Medicago polymorpha, Lolium perenne, Festuca arundinacea, and Lolium rigidum); and two parallel pot experiments lasting 100 days each (one with M. polymorpha and L. perenne, and another with young Olea Europaea var. Arbequina saplings). This study evaluated the impact on plant development, as well as the physical properties and composition of the substrates during the incubation process. Germination and survival of forage species were comparable or improved in most treatments, except those including 20% OB, which consistently reduced germination—likely due to high electrical conductivity (>10dS/m). In the pot experiments, substrate pH and total carbon content increased significantly with biochar addition, particularly with 20% WB, which doubled total C relative to control. Both forage species (Medicago polymorpha and Lolium perenne) and the olive saplings (Olea Europaea) exhibited normal growth, with no significant differences in biomass, water content, or physiological stress indicators when compared to the control group. Nutrient uptake was found to be stable across treatments, although magnesium levels were below sufficiency thresholds without triggering visible deficiency symptoms. Overall, combining compost and biochar—particularly WB and GC—proved to be a viable strategy to reduce peat use while maintaining substrate quality and supporting robust plant growth. This approach proved effective across the different plant varieties tested, including Medicago polymorpha, Lolium perenne, and young olive plants, which together encompass a wide spectrum of agronomic and horticultural applications as well as contrasting growth and nutrient requirements. Adverse effects on early plant development can be avoided by carefully selecting and characterizing biochars, with specific attention to salinity and C/N ratio. This finding is crucial for the successful large-scale implementation of sustainable alternatives to peat.

The application of organic amendments, both traditionally utilized (e.g., compost) and more recent innovations (e.g., biochar), to degraded agricultural soils is being driven by international initiatives in the current context of global change, such as the "4 per mil initiative". The main goal is to achieve sustainable soil quality restoration, contributing to carbon (C) sequestration, while also providing a practical use and value addition to agro waste products. Despite the generally recognized benefits of such applications on soil productivity and physical properties [1,2], their effects on soil C cycling and sequestration are not as comprehensively understood. Results exhibit considerable variability depending on the type of amendment and the specific soil, emphasizing the need for a more in-depth investigation in this area [3]. Therefore, the aim of this study was to analyze the effects of contrasting organic amendments on soil carbon stability.To accomplish this, two soils commonly employed in humid grasslands of the northern region and rainfed agriculture in the southern region of the Iberian Peninsula, respectively, were amended in triplicate at 10% (w/w) with wastewater sludge biochar, olive pomace biochar , white poplar wood biochar, rice husk biochar, cow manure digestate, a mixture of cattle manure and straw digestate (CM&SD), green compost (GC), and a mix of GC and OPB. A control was also established for each type of soil. After inoculating all the vessels with 1 mL of a standard microbial solution, respiration rates (CO2 emissions) were measured every 6 h over 100 days using an automated respirometer (Nordgren Innovations, Sweden) under controlled conditions (25°C; 60% water holding capacity). The data obtained were plotted against the incubation time by an exponential curve to discern the C stability through fast and slow C pools.Our findings revealed significantly enhanced stability of recalcitrant carbon (slow C pool) in both soils treated with biochars, particularly in the case of RHB and WB. These amendments substantially extended the mean residence time of the slow C pool (MRT2) by a factor of six to nine. The overall trend observed for the studied amendments was as follows: biochar >> green compost >> digestates > native soil carbon. In contrast, the alkaline rainfed soil exhibited a faster carbon turnover rate compared to the grassland soil, resulting in a lower C MRT2.Acknowledgements: The Spanish Ministry of Science and Innovation (MCIN) and AEI are thanked for funding the project RES2SOIL (PID2021-126349OB-C22). The European Joint programme EJP SOIL from the EU Horizon 2020 R&I programme is thanked for funding the subproject EOM4SOIL (Grant agreement Nº 862695).References:[1] De la Rosa, J.M., Pérez-Dalí, S.M., Campos, P., Sánchez-Martín, Á., González-Pérez, J.A., Miller, A.Z., 2023. Agronomy, 13, 1097.[2] Doblas-Rodrigo, A., Gallejones, P, Artetxe, A., Merino, P., 2023. Sci. Total Environ., 901, 165931.[3] De la Rosa, J.M., Rosado, M., Paneque, M., Miller, A.Z., Knicker, H., 2018. Sci. Total Environ., 613-614, 969-976.

Agricultural soils in the Mediterranean basin face a critical state of degradation, characterized by the depletion of organic matter and a high dependence on external inputs. Faced with this scenario, and the urgent need for practices to guarantee economic and environmental viability, the RESIOLIVA Operational Group and the RES2SOIL project propose an agroecological and circular management. The main objective is to safeguard the sustainability of certified olive groves and increase their adaptive capacity to climate stress.A field trial was established in La Campana (Seville, Spain) to monitor 3,169 certified olive trees (cv. Sikitita-2 and Arbosana). A factorial experimental design was used to evaluate six treatment combinations across replicated plots, testing (i) the application of green compost + biochar produced from olive pomace and (ii) inoculation with beneficial Trichoderma spp. with plant growth–promoting activity, both individually and in combination. In addition, the influence of prior mycorrhization of the planting material was assessed.The monitoring strategy combined soil physicochemical properties, elemental composition, water retention capacity, thermogravimetric analysis, and infrared spectroscopy with measurements of biological activity, as well as plant physiological assessments based on photosynthetic yield and chlorophyll content. Preliminary results showed that the organic-amended plots had statistically significant increases in soil organic carbon, mainly stable fraction, supporting its role as a carbon sink, while most of the physicochemical parameters remained stable. Besides, amended plots consistently maintained higher soil moisture levels at 0-20 cm depths. However, the trial was severely disrupted by extreme floodingin February 2025, accumulated rainfall exceeded 600 mm, causing prolonged waterlogging and anaerobic conditions and leading to higher-than-expected plant mortality, which reduced the statistical robustness of the first vegetative cycle, resulting in lower survival rates compared to controls. Notably, the rainfed-bread Sikitita proved highly susceptible to root asphyxia, whereas the irrigation-adapted Arbosana showed superior tolerance to high precipitation rates. Monitoring continues to assess amendment stability, C dynamics, and soil structural interactions, providing key information for designing rainfed scenarios.ACKNOWLEDGEMENTSThe Resioliva Operational Group (GOPO-CO-23-0004) gratefully acknowledges funding from the European Agricultural Fund for Rural Development (FEADER) and the Regional Government of Andalusia through the Ministry of Agriculture, Fisheries, Water and Rural Development, in accordance with Article 18.4 of Law 38/2003 of November 17 (https://agriculture.ec.europa.eu/index_es). This work was funded by the RES2SOIL project (PID2021-126349OB-C22) of the MCIN/AEI/10.13039/501100011033.

The management of dredged riverine sediments represents a significant environmental challenge due to the massive volumes generated annually [1]. Reusing these sediments as agricultural amendments offers a promising opportunity to improve crop yields and plant quality [2]. In the Guadalquivir marshlands (Seville, Spain), soils also face severe physical constraints due to their clay-rich texture and low soil organic carbon content. In this context, targeted organic amendments, such as plant-based composts, are proposed to enhance nutrient availability and overall functionality in formulated Technosols.Within this context, a six-month field trial was established in a fine-textured soil near the Guadalquivir River, characterized by compaction and poor drainage, under the framework of the SARECO Project, which formulated Technosols through the co-application of two riverine dredged sediments from the Guadalquivir River (supplied by the Port of Seville) and green compost, mixed with the native soil, as a strategy to restore soil health. Winter wheat was cultivated, and over the six-month period (until harvest) we monitored soil physical properties, organic matter composition and respiration rates, carbon dynamics, and crop productivity.The results showed that the Technosols significantly improved soil physical properties, with lower soil bulk density and penetration resistance. These improvements were accompanied by sustained organic carbon enrichment throughout the trial. Soil respiration rates increased initially in the Technosols but declined over time. The combined application of sediments and compost produced a synergistic effect on yield, exceeding the control in grain weight and grains per head, while total biomass and protein content remained comparable to the control soil.Overall, these findings confirm that combining riverine dredged sediments with organic amendments in fine-textured, carbon-poor soils is a sustainable strategy that increases soil organic carbon stocks while maintaining competitive crop yields. This approach provides a viable pathway for dredged-sediment valorization and the restoration of degraded agricultural soils.REFERENCES [1] Renella, G. Sustainability (2021), 13(4),1648.[2] De la Rosa, J. M., Pérez-Dalí, S. M., Campos, P., Sánchez-Martín, Á., González-Pérez, J. A., & Miller, A. Z. Agronomy (2023), 13(4), 1097.ACKNOWLEDGEMENTS: Authors gratefully acknowledge CSIC for funding the SARECO project (COCRE24015; CSIC COCREA 2024 – circular economy call). RES2SOIL project (PID2021-126349OB-C22) and P. Campos (PTA2023-023661-I) thank funding from MICIU/AEI (10.13039/501100011033) and the EU (FEDER, NextGenerationEU/PRTR) We thank Verónica Asensio and Rubén Leboreiro for their support, and Antonio Bejarano Moreno (Autoridad Portuaria de Sevilla) for providing access to the field plot area.