Long-term nitrogen addition enhanced soil carbon sequestration through coupled physicochemical and microbial mechanisms in a temperate forest

As critical carbon (C) sinks in terrestrial ecosystems, temperate forests exhibit complex responses to atmospheric nitrogen (N) deposition. Through a 13-year controlled field experiment, we systematically investigated the effects of three N forms (NO₃^--N, NH₄⁺-N, NH₄NO₃-N) at two deposition levels (50/150 kg N·ha^-1·yr^-1) on soil organic C (SOC) dynamics. The experimental data revealed a three-phase SOC accumulation pattern, with aggregate-associated C showing 6.23-43.54 % enhancement. Particularly, silt-clay fractions demonstrated superior C sequestration capacity (13.10 ± 0.45 g kg^-1), suggesting strengthened physical protection through aggregate stabilization. Nuclear magnetic resonance spectroscopic (NMR) analysis confirmed progressive increases in stable C pools, with aromatic and alkyl C rising by 12.44-42.94 %. Microbial necromass accumulation contributed 11.90-44.31 % to SOC stock, enhancing biochemical resistance to decomposition. Metagenomic evidence revealed N-dependent microbial community restructuring, particularly diminished abundances of ligninolytic genes, which suppressed recalcitrant SOC mineralization. These dual regulation mechanisms - physical protection via aggregate formation and biochemical stabilization through microbial metabolic constraint - synergistically increased SOC stability. Our findings elucidate the C-N coupling mechanisms in forest soils under chronic N enrichment, providing critical insights for predicting terrestrial C sink potential under global change scenarios.