LAUSR

Abstract Forty-four percent of the organic carbon (OC) in the world's forests is stored in soils. However, the distribution and stability of OC in forest soils along a climatic gradient remain largely unclear, hindering our understanding and the accurate prediction of biogeochemical cycles in forest ecosystems in a changing world. To address these uncertainties, we measured OC and nitrogen (N) concentrations and mineralization of OC in soils from broadleaved and coniferous forests along a wide-ranging climatic gradient in China and related these to experimental N addition and climatic conditions. An 85-day incubation was conducted under 25 °C and 60% of soil moisture at field capacity to determine the mineralization of soil OC. We hypothesized that the concentrations of OC and N would be higher but the mineralization of OC would be lower in soils from colder and drier forests and that the mineralization would be positively responsive to N addition. In support of these hypotheses, the concentrations of OC and N decreased, while the mineralization of OC measured under standard laboratory condition increased, with mean annual precipitation (MAP) and temperature (MAT). These metrics were not affected by forest type or the interaction between forest type and site. Nitrogen addition increased the cumulative mineralized OC (Cm, g kg−1) by 6–67%, and the effects varied with site and soil depth, but were similar between the broadleaved and coniferous forests. The Cm decreased with increasing soil OC concentration, C/N ratio and mineral N, while the rate constant of OC mineralization (k, day−1) showed opposite relationships with these metrics. The addition of N did not change the slopes of the relationships of Cm and k with the C/N ratio, MAP, and MAT; however, it strengthened the negative relationship of Cm with OC and mineral N concentrations. The results from this study suggested that the mineralization of OC was limited by N availability in the studied forested soils, and the response of OC mineralization to N addition was independent of climatic conditions.