Effects of biochar and its co-application with inorganic and organic fertilizers on soil microbial respiration and temperature sensitivity of carbon mineralization.

Biochar application to agricultural soils represents a strategy for carbon sequestration and soil quality enhancement. However, long-term field studies on the interactive effects of combined application of biochar and inorganic or organic fertilizers on temperature sensitivity of soil respiration (Q₁₀) are still scarce. To address this gap, a long-term field experiment with a randomized block design was established in a semiarid agricultural soil in Central Spain in 2012. The treatments used for this study included unamended control, amendment with biochar (B) at a rate of 20 t ha^-1 year^-1, mineral fertilizer (MF), municipal compost (MC) and sewage sludge (SS) at rates to meet N crop demands and the co-amended B+MF, B+MC and B+SS applied at the same application rates used for the individual treatments. Soil samples were taken post-harvest of 2023, eight months after the latest soil amendments. A series of soil incubations were set to a range of 5-35 °C for CO₂ measurements, which were addressed using the fully automated system Respicond respirometer X. Overall, Q₁₀ values were not affected by the co-application of biochar and fertilizers. Only soils amended with MC showed a significant increase in the Q₁₀ value in comparison to the B, MF and unamended control treatments. Furthermore, MC-amended soils showed higher respiration rates than those amended with SS. This difference could be attributed to the rapid mineralization of labile carbon fractions added with SS in preceding months or to the presence of preserved complex substrates in MC treatments that could decompose over the long-term. It could also be linked to a higher accumulation of SOC over the past 12 years in MC-treated soils compared to those treated with SS, thereby influencing the observed variations in respiration rates. On the other hand, co-application of biochar with fertilizers decreased soil CO₂ fluxes, especially for B+MC and B+SS, in comparison to the treatments applied alone. The study suggests that in semiarid soils the combined application of biochar with other fertilizers may induce beneficial synergistic effects, creating a nutrient-rich soil environment while either limiting soil CO₂ fluxes and mitigating the adverse impact on the temperature sensitivity of carbon mineralization. 

Acknowledgments: This research was supported by the Ministry of Science and Innovation (“BIOGEOCHAR” Project grant ref. TED2021-132342B-I00). M.J.C thanks to the Embassy of France in Spain for supporting her postdoctoral stay in the Biogéosciences Unit and MINECO for her “Juan de la Cierva-Formación” postdoctoral contract.