Long-term nitrogen addition enhanced soil carbon sequestration through coupled physicochemical and microbial...

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.