Pulse additions of soil carbon and nitrogen affect soil nitrogen dynamics in an arid Colorado Plateau shrubland

Biogeochemical cycles in arid and semi-arid ecosystems depend upon the ability of soil microbes to use pulses of resources. Brief periods of high activity generally occur after precipitation events that provide access to energy and nutrients ( carbon and nitrogen) for soil organisms. To better understand pulse-driven dynamics of microbial soil nitrogen (N) cycling in an arid Colorado Plateau ecosystem, we simulated a pulsed addition of labile carbon ( C) and N in the field under the canopies of the major plant species in plant interspaces. Soil microbial activity and N cycling responded positively to added C while NH4+ - N additions resulted in an accumulation of soil NO3-. Increases in microbial activity were reflected in higher rates of respiration and N immobilization with C addition. When both C and N were added to soils, N losses via NH3 volatilization decreased. There was no effect of soil C or N availability on microbial biomass N suggesting that the level of microbial activity ( respiration) may be more important than population size ( biomass) in controlling short-term dynamics of inorganic and labile organic N. The effects of C and N pulses on soil microbial function and pools of NH4+ - N and labile organic N were observed to last only for the duration of the moisture pulse created by treatment addition, while the effect on the NO3-- N pool persisted after soils dried to pre-pulse moisture levels. We observed that increases in available C lead to greater ecosystem immobilization and retention of N in soil microbial biomass and also lowered rates of gaseous N loss. With the exception of trace gas N losses, the lack of interaction between available C and N on controlling N dynamics, and the subsequent reduction in plant available N with C addition has implications for the competitive relationships between plants species, plants and microbes, or both.