Soil Aggregate Stratification of Ureolytic Microbiota Affects Urease Activity in an Inceptisol

Ureolytic microbes play a pivotal role in the maintenance of soil fertility. Soil aggregates are supposed to provide heterogeneous habitats for microorganisms, which may result in distinct metabolic functions. However, limited information is available regarding the distribution patterns, driving factors, and activity of ureolytic microbiota at the aggregate scale. In this study, we characterized the ureolytic microbiota and urease activity of three soil aggregate fractions from an Inceptisol subjected to 5 years of different fertilization regimes. Correlations between soil chemical characteristics and ureolytic microbial communities were analyzed. The results showed that the total abundance as well as the relative abundance of copiotrophic ureolytic microbes generally increased with the increasing soil aggregate size. This trend was in line with the nutrient distribution patterns, including labile carbon, NH4+, total carbon, nitrogen, and phosphorus. Soil urease activity also increased significantly with the increasing soil aggregate size and was positively correlated with copiotrophic ureolyric microbes, such as Betaproteobacteria, Alphaproteobacteria, and Gammaproteobacteria. Thus, we speculated that larger size soil aggregates with greater availability of labile carbon support more copiotrophic ureolyric microbes with a high growth rate, leading to a high density of total ureolytic microbes and higher urease activity. Smaller aggregates with less available carbon were associated with more oligotrophs, higher abundances of total ureolytic microbes, and higher urease activity. Our results suggest that larger soil aggregates and associated ureolyric microbes play a more important role in nutrient cycling for crop growth in this Inceptisol ecosystem.