Stable network complexity between bacterial community and dissolved organic matter remains under three-year consecutive nitrogen addition

Interactions between soil microorganisms and dissolved organic matter (DOM) represent critical processes in soil carbon cycle. Yet, the effects of nitrogen (N) addition on these interactions remain unclear. Over three years, we investigated bacterial communities and DOM quantity and quality at two soil depths under varying N addition rates. While N addition reduced dissolved organic carbon content and humification indices in surface soils, bacterial diversity, life-history strategies, and DOM molecular diversity remained largely unchanged. Network analysis revealed a shift from specialist to generalist taxa within bacterial–DOM associations, yet overall network complexity indicators (node number, average degree, path length) remained stable. Structural equation modeling showed that a positive covariation between bacterial diversity and DOM molecular diversity; the neutral response of both DOM molecular diversity and bacterial life-history strategies to N inputs stabilized network complexity. Our findings highlight that bacterial functions are more closely tied to DOM quality than quantity and support the “diversity begets diversity” framework. This study provides novel insights into the microbial mechanisms stabilizing soil carbon pools under N deposition.