Dual Isotopes Tracing Carbon and Nitrogen Dynamics during Leguminous and Non-Leguminous Litter Decomposition under Controlled Precipitation

Plant litter decomposition figures importantly in the cycling of C and N pools in terrestrial ecosystems. We investigated how C and N fluxes changed during the decomposition of leguminous and non-leguminous leaf litters, and how these processes responded to different precipitation regimes. We used the dual-isotope tracing method to investigate differences in leaf and soil C and N, along with d(13)C and d(15)N, in the soil of the Loess Plateau in China. The d(15)N and d(13C) values were 3604%o and 56%o for Robinia pseudoacacia (Leguminosae) and 8115%o and 452%o for Populus tomentosa (Salicaceae) leaf litters. Through decomposition, d(13)C decreased in all litters, and d(15)N in the leguminous litter increased while it decreased in the non-leguminous litter. In the surface soil, the fraction of litter derived N (14%) from the leguminous litter was significantly higher than that of the non-leguminous litter after 16 months. The C and N concentrations of both litters and soil always had a positive correlation during decomposition, and the responses of N to C changes in soil were reduced by the litter cover. Increased precipitation enhanced the litters' C and N correlation. The 600 mm precipitation treatment most benefited litter C's transformation to SOC, and drought conditions promoted the transformation of legume litter N to soil TN, but inhibited non-legume litter N. In the soil and both litters, C and N changes always had a positive correlation. After 16 months, the proportion of soil N from legumes was higher than that from non-legumes. Reduced precipitation could promote leguminous N in soil. Our results provide a scientific basis for accurately predicting the C and N cycles in terrestrial ecosystems.