Treated wastewater and mulch shape soil microbial activity in cactus-sorghum intercrops of the Brazilian semiarid

The use of wastewater for irrigation in semiarid regions represents a dual strategy for water security and nutrient cycling, yet its implications for soil microbial functioning remain poorly understood. This study evaluated the impact of treated wastewater irrigation and mulch application on soil organic carbon (SOC), microbial biomass carbon (MBC), basal respiration (CO₂-C), and metabolic and microbial quotients (q-CO₂ and q-mic) in a cactus-sorghum intercrop system in the Brazilian semiarid region. A split-plot field experiment was conducted with four irrigation depths (0%, 80%, 100%, and 120% crop evapotranspiration [ETc]) and two mulch conditions (with and without) in a randomized block design with four replicates. Wastewater irrigation influenced soil microbial dynamics and carbon-related processes, with responses varying according to soil depth, irrigation level, and crop stage. MBC was strongly affected by the interaction between irrigation, mulch, and cutting, with the highest values observed under the combined application of wastewater and mulch, particularly at 100% ETc in the final cutting. CO₂-C and q-CO₂ exhibited distinct temporal patterns, with higher emissions under both water deficit and non-mulched conditions, indicating stress-induced respiration. In contrast, mulched treatments promoted higher q-mic and lower q-CO₂, reflecting improved microbial efficiency. Additionally, microbial indicators (MBC and q-mic) became stronger predictors of sorghum productivity over time, surpassing total SOC. Overall, the integration of treated wastewater and mulch enhanced microbial functioning and strengthened the coupling between soil biological processes and crop productivity, supporting soil resilience and sustainability in semiarid forage systems.