Long-term N addition accelerated organic carbon mineralization in aggregates by shifting microbial community composition

Nitrogen application in agroecosystem maintains and improves soil fertility while affecting organic carbon mineralization (Cmin). Soil aggregates play a crucial role in soil organic carbon (SOC) turnover. However, the cumulative Cmin and underlying microbial mechanisms responses to long-term N addition at aggregate scales remain unclear. Hence, a 16-year field N addition field experiment was conducted on winter wheat (Triticum aestivum L.) at three N levels: 0 (N0), 180 (N180), and 360 (N360) N ha-1. Overall, long-term N addition significantly altered the Cmin, bacterial and fungal community composition and bacterial and fungal cooccurrence patterns in four aggregate fractions (large macroaggregates, small macroaggregates, microaggregates, and silt-clay fractions). Specifically, N addition facilitated a more copiotrophic microbial community, with a significant increase in the relative abundances of Proteobacteria, Gemmatimonadota, and Ascomycota, and a significant decrease in the relative abundances of Actinomycetales, Acidimicrobiales, and Basidiomycota. The addition of N led to more complex and tight microbial networks with more nodes, higher average degrees, shorter average path lengths, and greater connectivity. These microbial changes accelerated Cmin in aggregates; however, the main microbial mechanisms varied with aggregate size and Cmin in silt-clay fractions was mainly influenced by microbial community composition. These results indicate that the spatial heterogeneity of resources available in different aggregate sizes is strongly selected for microbial life strategies and influences the distribution of microbial communities, thereby affecting the Cmin processes. Overall, our study provides a fundamental understanding of the microbial regulation of SOC turnover at an aggregate scale and highlights the importance of network topological patterns.