Concentration and mineralization of organic carbon in forest soils along a climatic gradient

Forty-four percent of the organic carbon (OC) in the world's forests is stored in soils. However, the distribution and stability of CC 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 CC and nitrogen (N) concentrations and mineralization of CC in soils from broad-leaved 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 degrees C and 60% of soil moisture at field capacity to determine the mineralization of soil CC. We hypothesized that the concentrations of CC and N would be higher but the mineralization of CC 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 CC and N decreased, while the mineralization of CC 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 CC (C-m, g kg(-1)) by 6-67%, and the effects varied with site and soil depth, but were similar between the broad-leaved and coniferous forests. The C-m decreased with increasing soil CC concentration, C/N ratio and mineral N, while the rate constant of CC mineralization (k, day(-1))showed opposite relationships with these metrics. The addition of N did not change the slopes of the relationships of C-m and k with the C/N ratio, MAP, and MAT; however, it strengthened the negative relationship of C-m with CC and mineral N concentrations. The results from this study suggested that the mineralization of CC was limited by N availability in the studied forested soils, and the response of CC mineralization to N addition was independent of climatic conditions.