Fertilization and cultivation management alleviate microbial nitrogen limitation in purple soil sloping farmland: Evidence from ecoenzymatic stoichiometry

Fertilization and cultivation management strongly affect crop productivity, alter soil nutrient availability, and influence microbial communities, leading to substantial stoichiometric imbalances. However, how these practices reflect the potential nutrient limitation of soil microbes in agricultural ecosystems remains unclear. Herein, soil samples (0–10 and 10–20 cm) from a maize crop subjected to a 15-year long-term field experiment considering five different treatments (no fertilizer + downslope cultivation, combined manure and mineral fertilizers + downslope cultivation, mineral fertilizer alone + downslope cultivation, 1.5-fold mineral fertilizer + downslope cultivation and mineral fertilizer + contour cultivation representing CK, T1, T2, T3 and T4, respectively) were deployed on a 15° purple soil sloping farmland to explore the potential microbial resource limitation using various extracellular enzyme stoichiometry (EES) approaches. Our results revealed that fertilization practices (i.e., T1, T2, T3, and T4) significantly influenced extracellular enzyme activity (EEA), particularly in T1 and T3 at the 0–10 and 10–20 cm soil depths. The mean natural logarithms of the EES ratio across the treatments were 1.23:1.34:1.00 at 0–10 cm and 1.23:1.32:1.00 at 10–20 cm depths, deviating from the overall global mean of 1:1:1, suggesting an imbalance in microbial resources. Based on the calculations of threshold elemental ratio (TER) and available resource ratios (R_C:N – TER_C:N > 0), scatter plots of EES (below the 1:1 line) and vector angle (<45°) revealed that fertilization and cultivation management alleviated microbial N limitation. Furthermore, a strong homeostasis analysis of N:P and a significant increase in the N:P stoichiometry imbalance also synthetically supported N limitation from soil microbes. Heatmap correlation and random forest analysis showed that C:N, EES_C:N and N:P stoichiometry imbalances were the main factors influencing microbial N limitation. Based on partial least squares path modeling (PLS-PM), soil EEA was the driving factor that induced microbial N limitation. These findings enable greater comprehension of the status of microbial resource limitation by considering the EEA stoichiometry approach under fertilization and cultivation management and provide insight into regulating soil nutrient cycling (i.e., N cycle) mediated by soil ecological processes and adjusting their management in similar intense agroecosystems worldwide.