Linking soil organic carbon characteristics, nutrient stoichiometry, and microbial community to eco-enzymatic stoichiometry within aggregates in different aged walnut plantations

Research on the variations of microbial attributes, C and nutrient properties, eco-enzymatic activities and their stoichiometry in different aged walnut orchards is essential for the sustainable development of walnut gardens. Here, four walnut orchards of various ages (0-, 7-, 14-, and 21-years) were selected in Hebei province, China, to evaluate the temporal changes in the above-mentioned indices within aggregates based on thermal gravimetric analysis, phospholipid fatty acid analysis and fluorometric assays. Results revealed that as the walnut plantation ages or aggregate sizes increased, the quantity and thermal stability of organic C exhibited increasing and decreasing trends, respectively. Long-term walnut plantation could increase C- and P- acquiring enzyme activities, and decrease N-acquiring enzyme activities in larger aggregates. Eco-enzymatic stoichiometry analyses demonstrated that the microbial C and P co-limitation increased with aggregate sizes or walnut plantation ages, although long-term walnut planting (14- and 21-years) and larger aggregates (>0.25 mm) provided more and easily available C resources for microbes. The aggravated C limitation (or P limitation) could be ascribed to the increased the ratio between microbial biomass C and organic carbon content (or the increased fungi/bacteria and soil N/P ratios) in the elder walnut plantations or larger aggregates. Overall, the study's results can provide several valuable insights (e.g., the old orchards can appropriately apply more P fertilizer) into the sustainable development of walnut gardens from the perspective of microbial nutrient demand.