Contrasting Bacterial Biogeography and Life-History Strategies in Paddy and Upland Mollisols of Northeast China

Distinct hydrological management under rice paddy field (RS) and corn upland field (CS) cultivation reshapes soil bacterial communities in agricultural ecosystems. Yet large-scale paired comparisons disentangling how these two land-use types jointly modulate bacterial biogeographic patterns, life-history strategies, and co-occurrence associations remain poorly documented for the typical Mollisol region of Northeast China. Using 30 pairs of geographically adjacent RS and CS soils, this study integrated high-throughput 16S rRNA gene sequencing, qPCR quantification, and rrn copy number derived trait proxies alongside co-occurrence network analysis to compare bacterial community characteristics and nutrient-cycling-related signatures between the two habitats. RSs harbored higher bacterial α-diversity, total bacterial abundance, and niche width relative to CSs; while both land-use types exhibited significant distance–decay relationships, environmental filtering exerted stronger control over community turnover than geographic distance in both systems. RSs had significantly higher NH4+-N content, whereas CSs exhibited higher NO3−-N, total phosphorus (TP), total potassium (TK), and available potassium (AK) contents, together with a higher multi-nutrient-cycling index (MNC). In contrast to the r-strategist-biased trait signals observed in RSs, CSs was dominated by putative K-strategist oligotrophic taxa, and clear latitudinal gradients in community life-history traits were detectable in CSs yet largely obscured in RSs by redox fluctuations. Conversely, CSs exhibited more complex and cohesive bacterial co-occurrence networks with higher simulated robustness derived primarily from oligotrophic subgroups, and the sets of keystone taxa and their functional linkage to multi-nutrient cycling diverged sharply between RSs and CSs. These contrasting community, trait, and network signatures indicate that hydrological differences between paired paddy and upland soils drive divergent bacterial contributions to nutrient cycling, offering microbial-based references for land-use targeted soil management in Mollisol agroecosystems.