Nitrogen addition effects on soil mineral-associated carbon differ between the valley and slope in a subtropical karst forest.

• N addition increased POC via accelerating aboveground litter input in the valley. • N addition increased MAOC via increasing CUE and necromass formation in the valley. • N addition enhanced POC by increasing fine root biomass on the slope. • N addition inhibited MAOC by declining microbial necromass on the slope. While the responses of soil organic carbon (SOC) dynamics to atmospheric nitrogen (N) deposition have been widely investigated, little is known in terms of if, and how the responses are modulated by topography. Here, a N addition experiment with three N addition levels, i.e., 0, 50 and 100 kg N ha−1 yr−1, was carried out in the valley and on the slope of a subtropical karst forest. Soil type was Cambisols on the slope but Luvisols in the valley developed on a mixture of limestone and dolomite. Bulk SOC content in the valley was marginally (p < 0.1) enhanced attributed to the stimulation of both particulate organic C (POC) and mineral-associated organic C (MAOC), but that on the slope was not significantly altered owing to enhanced POC but suppressed MAOC under N addition. High N addition (100 kg N ha−1 yr−1) significantly increased POC through accelerating aboveground litter input, and increased MAOC via increasing carbon use efficiency (CUE) and necromass formation in the valley. On the slope, high N addition significantly stimulated POC owing to increase of fine root biomass, but inhibited MAOC due to decrease in microbial necromass accumulation. Our results suggest that the impacts of N addition on SOC formation and hence persistence likely modulated by topography, so that provide a novel view in underlying the differential responses of SOC pool to N addition. [ABSTRACT FROM AUTHOR]