Cover cropping and chemical fertilizer seasonally mediate microbial carbon and phosphorus metabolisms in an apple orchard: Evidence from the enzymatic stoichiometry method

The extracellular enzymatic stoichiometry in soil in terms of the ratios of soil enzyme activities related to the acquisition of nutrients such as carbon (C), nitrogen (N), and phosphorus (P) can reflect the demand for resources by microorganisms. However, it is not clear how cover cropping and chemical fertilization might shift nutrient limitations for microbes in terms of the soil enzymatic stoichiometry in an apple orchard. Thus, we determined the metabolic limitations for microbes in soil based on enzymatic stoichiometry and clarified their effects on the carbon use efficiency (CUE) for soil microbes. The experiment was conducted in two main orchard plots under cover cropping and no cover cropping, where two subplots received no fertilizer or N, P, and K fertilizer application. The activities of C, N, and P acquiring enzymes (β-1,4-glucosidase, leucine aminopeptidase, β-1,4-N-acetylglucosaminidase, and acid or alkaline phosphatase), soil nutrient contents, plant nutrient contents, and microbial biomass were determined. We found that cover crop and fertilization significantly influenced the soil properties, microbial biomass, and soil enzyme activities and stoichiometry in the apple orchard, and the effects varied at different developmental stages (bud breaking, fruit swelling, and fruit maturity stages) and distances (soil samples were collected at 30, 60, 90, 120, and 150 cm from the trunks of apple trees). Soil extracellular enzymatic stoichiometry analysis showed that microbial metabolism was affected by relative P limitation in the soil throughout the growth of apple trees, but it varied among different treatments, stages, and sampling distances. Microbial carbon limitation was affected by the cover crop and fertilization at different stages and distances. Random forest analysis suggested that the soil available P and available N contents were the two most important factors that affected microbial C metabolism in the germination, fruit swelling, and mature stages. Microbial CUE was also significantly correlated with microbial P and C limitation. However, microbial C metabolism was affected by the soil available P as well as the soil C. An increase in the soil available P could have made more P available for microbial energy production and increased microbial C metabolism. Overall, our results indicate that the soil available P and available N regulated microbial C and P metabolism at different tree development stages, thereby providing insights into how microbial C and P metabolism and microbial C turnover are affected by the soil enzymatic stoichiometry in an apple orchard under cover crop and fertilization.