Land use shapes the microbial community structure by altering soil aggregates and dissolved organic matter components

The agro-pastoral ecotone epitomizes the ecologically fragile semi-arid zone, where the soil microbiomes play a pivotal role in regulating its multifunctionality. However, whether and how changes in soil structure and organic matter composition under different land uses affect microbial community structure remain unclear. Here, land-use types in the agro-pastoral ecotone, including shrubland（BF）, artificial grassland（ArG）, abandoned grassland（AbG）,and maize farmland（MA）, were chosen to explore the response relationships between soil microbial communities and the aggregates and dissolved organic matter（DOM） composition. The results showed that compared to MA, the macroaggregates in BF, AbG, and ArG were increased by 123.0, 92.79, and 63.71%, respectively, while MA soil had the greatest abundance of <100 μm particles. The higher aromatic carbon with high aromaticity and molecular weight in BF soil DOM contributed to its highest mineral-associated organic carbon level(12.61 g kg–1), while MA soil organic carbon had highly efficient decomposition due to its high content of aliphatic and carboxy carbon, so it is prone to loss from the active carbon pools. The transition in land use from shrubland to grassland and farmland has facilitated the conversion of stable aromatic carbon to unstable carboxy carbon. The taxonomic analysis revealed that soil bacterial and fungal communities in the four land uses were dominated by Proteobacteria, Actinobacteriota, Chloroflexi, and Ascomycota. More taxonomic groups from phylum to family were enriched in BF soil. The DOM components and organic carbon are crucial variables shaping the composition of soil bacterial communities, jointly explaining 61.66% of the variance, while aggregates are important variables driving the composition of fungal communities, with an explanation rate of 20.49%. Our results suggest that DOM components and aggregates impact the soil microbial structure; and the transition in land use from agricultural land to grassland and shrubland in the agro-pastoral ecotone enhances aggregate stability, carbon sequestration potential, and microbial diversity.