Determining the effects of global change on soil nitrogen cycling in arid ecosystems

The responses of soil nitrogen (N) and carbon (C) dynamics to global change drivers such as elevated atmospheric CO2 and exotic species invasion threaten the stability of arid ecosystems worldwide by altering resource availability, trophic structure, and biodiversity. Responses of arid ecosystem nutrient dynamics to global change are not known, largely because we have little understanding of the belowground C and N dynamics. To more fully understand soil C and N dynamics, there is a need to relate the sources of C utilized by heterotrophic microorganisms to decomposer community composition and soil N cycling. The effects of global change drivers on the mechanistic controls of C and N availability on ecosystem N dynamics in two and ecosystems were investigated using a series of field and laboratory experiments. Effects of elevated CO2 were studied in the Mojave Desert, and exotic species invasion was studied in the Colorado Plateau. Available soil C was the primary limiting factor to ecosystem N cycling with N sometimes a secondary factor. Elevated atmospheric CO2 showed the potential to increase the proportion of microbial to extractable organic N, and to increase nitrous oxide N fluxes under the canopies of deciduous shrubs. Exotic species invasion resulted in a net increase in inorganic and labile organic N. Both elevated CO2 and exotic species invasion led to changes in microbial community composition, and increases in microbial activity and gross rates of soil N cycling. Changes in microbial community structure were linked to changes in C substrate utilization that occurred in response to recent C inputs in the case of elevated CO 2, and to increased organic matter inputs from exotic invaders. Under elevated atmospheric CO2, soil fungi utilized recent C inputs and the fungal: bacterial ratio increased on a seasonal basis. In contrast, soil bacteria preferentially utilized C inputs from an exotic invasive species and the fungal:bacterial ratio decreased with invasion in a C 4 grassland. These shifts in microbial community dynamics appear to lead to shifts in ecosystem dynamics, such as altering the amounts of plant available N and rates of organic N deposition, in these and ecosystems.