Soil Preservation in Warming Climate: Organic Amendments Enhance Microbial Carbon Use Efficiency in Mediterranean Soils

Mediterranean agroecosystems are vulnerable to extreme heat‐stress, especially because of their low organic matter content. Organic amendments may enhance soil nutrient content and microbial resilience to heatwaves, whose frequency is increasing in Mediterranean regions. However, their effectiveness under these conditions is still unclear. We investigated the effect of composted organic amendments (olive mill pomace, biosolids and solid urban residue) and a mineral fertiliser (diammonium phosphate) on microbial carbon use efficiency (CUE) and soil biogeochemistry in two different soils, a calcareous Vertisol and a non‐calcareous Inceptisol, with low P availability, subjected to extreme heat‐stress. We conducted incubation experiments (20°C, 30°C, 40°C or 50°C) to monitor 14C‐glucose mineralisation and to evaluate modifications in soil biochemical properties. As a result of warming, soil microorganisms exhibited thermotolerance up to 40°C, with a critical shift in microbial respiration observed at 50°C. Consequently, microbial CUE, which was a function of the organic amendments and soil type, significantly declined from 0.47–0.65 at 20°C to 0.27–0.45 at 50°C (p < 0.05), with the unamended control decreasing by 0.010 ± 0.001°C−1 (Vertisol) and 0.007 ± 0.001°C−1 (Inceptisol). Moreover, composted olive mill pomace enhanced the resistance of soils to heat stress as they produced the highest microbial CUE at 40°C in the Inceptisol and 50°C in both soils (0.43 ± 0.02 Inceptisol vs. 0.45 ± 0.02 Vertisol). Soil biogeochemistry varied with temperature and treatment, whilst available P in soils treated with diammonium phosphate was reduced with temperature in both soils, but it was increased with biosolids for all temperatures in the Inceptisol. In conclusion, organic matter‐rich organic amendments (composted olive mill pomace) may enhance the resistance of Mediterranean agricultural soils subjected to extreme heat‐stress events (50°C).