Carbon Flow from Plant Detritus and Soil Organic Matter to Microbes-Linking Carbon and Nitrogen Cycling in Semiarid Soils

We used in situ C-13-labeling of an annual grass to examine the decomposition rate and fate of detrital C over 17 mo in a sagebrush ecosystem. We coupled these measurements with N-15 pool dilution and tracer measurements to examine seasonal changes in gross N cycling rates in bulk soil and organic matter density fractions. Annual grass fine roots decomposed rapidly (0.23 mo(-1)) over the 17 mo, substantially faster than rates typically reported for other semiarid ecosystems using litterbags. Despite a long summer dry period, recent detrital C was rapidly respired and incorporated into microbial biomass and nonmicrobial fractions between summer and autumn. Decomposition of recent detritus accounted for 13% of mineralized C and 9% of microbial C produced during the summer following plant labeling. These proportions declined by 50% in autumn, indicating that microbes relied more heavily on recent detritus for C and energy during the dry summer than during the following moist autumn months. In spite of this, decomposition rates of recent plant detritus were faster during autumn than summer due to higher rates of microbial activity. Nitrogen immobilization into soil organic matter (SOM) fractions increased from summer to autumn and was fastest in spring. Rates of N immobilization into the supposedly recalcitrant heavy-fraction (HF) SOM were faster than into the light fraction; however, rates in light and heavy fractions were similar when expressed per unit organic C. Rates of N immobilization were positively correlated with the C-13 content of density fractions, suggesting that inputs of new plant C were an important driver of N immobilization. While addition of NH4+ stimulated nitrification rates in both summer and autumn, NH4+ addition only stimulated rates of N immobilization during spring, indicating that heterotrophic microbial growth was N limited during spring but C limited during summer and autumn when plants were not actively growing. Our results indicate that root decomposition in semiarid soils is more rapid than previously thought, and is not likely to be N limited. Moreover, HF SOM is not as recalcitrant as is frequently assumed, but instead, it is a strong sink for recent plant C inputs and a driver of N immobilization in soil.