Do composted bioamendments enhance the resistance of Mediterranean agricultural soils and their microbial carbon use efficiency to extreme heat-stress events?

Abstract. Mediterranean agroecosystems are vulnerable to extreme heat-stress, especially because of their low organic matter content. Bioamendments may enhance soil nutrient content and microbial resilience to heatwaves. However, their effectiveness under these conditions is still unclear. We investigated the effect of bioamendments (composted olive mill pomace, biosolids and solid urban residue) and a conventional 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 warming experiments (20, 30, 40, or 50 °C), to monitor ¹⁴C-glucose mineralization and to evaluate modifications in soil biochemical properties. As result of warming, both soils microorganisms exhibited thermotolerance until 40 °C, with a critical shift in microbial respiration observed at 50 °C. Consequently, microbial CUE, which was a function of the bioamendments and soil, significantly declined from 0.47–0.65 at 20 °C to 0.27–0.45 at 50 °C (p < 0.05), with the control decreasing by 0.010 ± 0.001 °C^-1 (Vertisol) and 0.007 ± 0.001 °C^-1 (Inceptisol). Moreover, composted olive mill pomace-treated soils 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, while available P in soils treated with diammonium phosphate was reduced with temperature in both soils, and available P added with bioamendments was not affected by temperature but was increased with biosolids for all temperatures in the Inceptisol. In conclusion, organic matter rich bioamendments (composted olive mill pomace) may enhance the resistance of Mediterranean agricultural soils subjected to extreme heat-stress events (50 °C).