Ecoenzymatic stoichiometry reflects the regulation of microbial carbon and nitrogen limitation on soil nitrogen cycling potential in arid agriculture ecosystems

Purpose Water and nutrient availability are critical factors that affect soil biological processes in agroecological systems, which are regulated by extracellular ecoenzymatic activity; however, the response of these enzymes to water and nutrient levels is poorly understood. Methods A 4-year field experiment with three levels of irrigation (high, 400 mm; medium, 300 mm; and low, 200 mm) and two levels of fertilization (high, 600 kg/ha P2O5 + 300 kg/ha urea; and low, 300 kg/ha P2O5 + 150 kg/ha urea) was conducted to investigate the microbial metabolic status in an arid agroecological system in China using the model of extracellular enzymatic stoichiometry. Results Higher C-acquisition enzyme activity and lower N-acquisition enzyme activity were observed during the crop growth stage, suggesting promoted C limitation and alleviated N limitation for microbes by the combination of medium irrigation and low fertilization. Increased microbial C limitation surged the abundance of amoA-AOA and amoA-AOB genes involved in nitrification and strengthened this process. Decreased microbial N limitation hindered the denitrification potential by reducing the abundance of the involved nirK, nirS, nosZ, and narG genes. Increased microbial C limitation was due to the elevated soil water content, which further promoted the activity of C-acquiring enzymes and facilitated microbial decomposition of organic matter. The decreased microbial N limitation was largely related to the increased soil N availability. Conclusions These findings suggest that a combination of medium irrigation and low fertilization is effective for organic matter decomposition, by promoting microbial C metabolism and reducing the risk of N loss via alleviation of microbial N limitation. Our results emphasize the roles of stoichiometry-regulated microbial metabolism in soil nutrient transformation and have implications for agricultural practices in arid fertigation agroecosystems.