Effects of plastic and straw mulching on soil microbial P limitations in maize fields: Dependency on soil organic carbon demonstrated by ecoenzymatic stoichiometry

Mulching rain-fed farmland ecosystems changes the soil physicochemical properties, especially soil organic carbon (SOC), but the metabolic limitations of soil microorganisms after these changes are unclear. We established a long-term experiment in 2012 with three treatments: no mulch (CK), straw mulch (SM), and plastic film mulch (FM). In 2019 the soil enzyme activities were measured in five maize growing periods: planting time, six-leaf period, silking period, milk period, and physiological maturity. Extracellular enzymatic stoichiometry models were used to examine microbial metabolic limitations. The vector length and angle were employed to determine the C and N/P limitations of soil microorganisms. Compared with CK, the average SOC and total nitrogen (TN) contents were 9.7% and 7.8% higher under SM, respectively, in each period. The SOC, TN, and total phosphorus (TP) contents were 5.6%, 4.8%, and 2.8% lower under FM, respectively. Compared with CK, the C- and N-acquiring enzyme activities were 20.5% and 5.2% lower under FM, respectively. The alkaline phosphatase enzyme activities were 2.7% and 13.5% higher under SM and FM, respectively, than CK. Soil nutrients, pH, and temperature influenced the C and P limitations of soil microorganisms. The different P limitation responses under SM and FM were mainly due to SOM. The decomposition of SOC was a key source of soil available P. The soil hydrothermal conditions under FM accelerated the decomposition of SOC in the early years, thereby increasing the P limitation. However, long-term SM increased the SOC due to the annual input of straw and its decomposition released available P to alleviate the P limitation for microorganisms. Thus, the temperature, water, pH, and SOC affect the P limitation for microbes under mulching conditions, but the SOC content of alkaline soil in arid farmland is the main factor that leads to microbial P limitation.