Effects of Contrasting Organic Amendments on Carbon Stability and Soil Carbon Dynamics in Acidic and Alkaline Soils

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.