Forty-year-old orchards promote carbon storage by changing aggregate-associated enzyme activities and microbial communities

Soil organic carbon (SOC) is closely related to ecosystem functions and services, such as biodiversity conservation and the mitigation of climate change. Soil aggregates are among of the most important microbial hotspots and are an important material for C cycling. However, the effects of microbial properties such as enzyme activities and microbial communities on SOC across different soil aggregates remains unclear. Here, we evaluated the SOC, microbial biomass carbon (MBC) content, hydrolase enzyme activities (i.e., 13-glucosidase, alpha-glucosidase, and sucrase), and microbial communities in different soil aggregates in 7-, 20- and 40-years orchards and newly established orchards in southern China to determine the relationships between the SOC and microbial properties. The aggregate-associated SOC, MBC contents and three enzyme activities were unevenly distributed. The average SOC and MBC contents in > 0.25 mm aggregates increased by 40% and 26%, respectively, compared with those in the other aggregates (< 0.25 mm). The 13-glucosidase and alpha-glucosidase activities in the 0.053-0.25 mm aggregates were the lowest among different aggregates, but the sucrase activities did not differ significantly across soil aggregates of different sizes. The average SOC and MBC contents, and the three enzyme activities in the > 0.25 mm aggregates in the forty-year orchard increased by 2.53-, 1.41- and 2.09-fold, respectively, compared to those in the newly established orchard. Positive correlations with the soil aggregate size were observed for Chloroflexi, and negative correlations were found for Actinobacteriota, Proteobacteria and Acidobacteria. The relative abundance of Ascomycota increased, and the relative abundance of Basidiomycota decreased with increasing orchard age. Our study demonstrated that the increased orchard age (40 years old) accelerated C cycling in soil aggregates of different sizes by increasing aggregate-associated enzyme activities and changing the microbial community composition, and greater SOC accumulation occurred in macroaggregates (> 0.25 mm) than in microaggregates (< 0.25 mm).