The priming effect of soluble carbon inputs in organic and mineral soils from a temperate forest

The priming effect (PE) is one of the most important interactions between C input and output in soils. Here we aim to quantify patterns of PE in response to six addition rates of (13)C-labeled water-soluble C (WSC) and determine if these patterns are different between soil organic and mineral layers in a temperate forest. Isotope mass balance was used to distinguish WSC derived from SOC-derived CO2 respiration. The relative PE was 1.1-3.3 times stronger in the mineral layer than in the organic layer, indicating higher sensitivity of the mineral layer to WSC addition. However, the magnitude of cumulative PE was significantly higher in the organic layer than in the mineral layer due to higher SOC in the organic layer. With an increasing WSC addition rate, cumulative PE increased for both layers, but tended to level off when the addition rate was higher than 400 mg C kg(-1) soil. This saturation effect indicates that stimulation of soil C loss by exogenous substrate would not be as drastic as the increase of C input. In fact, we found that the mineral layer with an WSC addition rate of 160-800 mg C kg(-1) soil had net C storage although positive PE was observed. The addition of WSC basically caused net C loss in the organic layer due to the high magnitude of PE, pointing to the importance of the organic layer in C cycling of forest ecosystems. Our findings provide a fundamental understanding of PE on SOC mineralization of forest soils and warrant further in situ studies of PE in order to better understand C cycling under global climate change.