The microbial-driven C dynamics within soil aggregates in walnut orchards of different ages based on microbial biomarkers analysis

Soil aggregates and microbial attributes are of great importance to clarify soil C dynamics in agroecosystems. However, whether and how walnut plantation with different ages affects microbial-driven soil C dynamics, especially at the aggregate scales, remains unclear. In this study, the microbial community (as measured by phospholipid fatty acids analysis), exo-enzyme activities (micro-plate enzyme method), and microbial residues (amino sugar analysis) were determined in soil aggregates, which were collected from surface soils in walnut plantations of various ages (0-, 7-, 14-, and 21-year) in the areas of Lincheng county, China. According to the optimal-moisture sieving method, soil aggregates were divided into three parts, including > 2 mm, 2–0.25 mm, and < 0.25 mm aggregates. Results showed that long-term walnut cultivation (14- and 21-year) improved soil aggregate stability as well as its associated nutrient conditions, and these changes were mainly reflected in the larger (>2 mm and 2–0.25 mm) aggregates. Besides, long-term walnut cultivation was favorable to promoting microbial growth (as indicated by total PLFAs), functions (hydrolase activities), and metabolism (microbial residues C, MRC) within aggregates. Irrespective of the walnut plantation ages, the above-mentioned microbial attributes increased with increasing soil aggregate sizes, whereas oxidase activities decreased. We also found that the increased plantation ages and decreased aggregate sizes could enhance the microbial contributions to soil organic C (SOC) accumulation (as indicated by the MRC/SOC ratios), although higher SOC contents have been found in the larger aggregates. Notably, the partial least squares path model further emphasized that walnut plantation ages and aggregate sizes had strong effects on microbial attributes (e.g., microbial communities, exo-enzyme activities, and microbial residues), as well as their importance for soil C dynamics. Consequently, the mature walnut plantations were beneficial for improving soil structure, microbial attributes, and SOC sequestration in the East Taihang Mountains in Hebei, China.