Differential drivers in organic and mineral layers: unraveling the mechanisms of soil organic carbon stabilization under long-term nitrogen addition in a temperate forest

The soil carbon (C) and nitrogen (N) cycles in forest ecosystems are closely coupled, and N inputs, such as atmospheric deposition, significantly influence soil organic C (SOC) stability. However, the extent and underlying mechanisms of this influence remain uncertain.

The organic layer and mineral topsoil (0–5 cm) were sampled from a temperate forest in Northeast China subjected to nine consecutive years of NH₄NO₃ addition at rates of 0, 20, 40, and 80 kg N ha⁻¹ yr⁻¹. The effects of long-term N addition on SOC stability and the key drivers were investigated. SOC fractions based on density and particle size were examined, SOC chemical structures were characterized by ¹³C solid-state NMR, and microbial C source utilization was assessed using Biolog EcoPlates.

High N treatment (80 kg N ha⁻¹ yr⁻¹) increased SOC stability in the organic layer and topsoil. Specifically, it significantly increased the proportion of heavy fraction SOC, the ratios of alkyl C/O-alkyl C and hydrophobic C/hydrophilic C, aromaticity, and the C quality index. The drivers of SOC stability in the organic layer included pH, soil cellulase, total N, and polysaccharides, whereas total C and hydroxy acids were key factors in the mineral topsoil.

Long-term high N input enhanced SOC stability in both the organic horizon and the topsoil of the temperate forest. However, the key drivers of this stabilization differed between the two layers. These findings provide theoretical insights into the response mechanisms of SOC pools in temperate forests to atmospheric N deposition.