Plant species effects and carbon and nitrogen cycling in a sagebrush-crested wheatgrass soil

Shifts in plant community structure in shrub and grass-dominated ecosystems are occurring over large land areas in the western US. It is not clear what effect this vegetative change will have on rates of carbon and nitrogen cycling, and thus longterm ecosystem productivity. To study the effect of different plant species on the decomposability of soil organic substrates and rates of C- and N-cycling, we conducted laboratory incubations of soils from a 15-yr-old experimental plot where big sagebrush (Artemisia tridentata Nutt) and crested wheatgrass (Agropyron desertorum [Fisch.] Schult.) plants had been planted in a grid pattern. Soil samples collected from beneath crested wheatgrass had significantly greater total N and NO3- concentrations and lower C-to-N ratios than samples collected beneath sagebrush plants or from interspaces. Soil carbonate concentrations beneath crested wheatgrass were intermediate between sagebrush soils and interspace soils. During the 16-week laboratory incubation, soil C mineralization rates, gross and net N mineralization rates, gross NH4+ consumption rates, net nitrification rates and soil NO3- concentrations were significantly different, with slightly higher values in the crested wheatgrass soil and lower values in the interspace soil. Gross NH4+ assimilation, gross nitrification, gross NO3-; assimilation, microbial biomass C and N and microbial growth efficiency showed no significant differences. During the 16-week incubation, microbial biomass C, microbial respiration and gross N assimilation rates declined by more than 50?-6, suggesting that the microbial biomass was C-limited. However, addition of NH4+ appeared to stimulate NH4+ assimilation. In addition, the form of N assimilated by micro-organisms shifted from predominantly NH4+ (95%) when NO3- was relatively unavailable at the beginning of the incubation, to predominantly NO3- (88-96%) as NO3- concentrations increased. Both of these latter observations suggest that microbes were NH4+-limiled. Either co-limitation by C and NH4+ or the presence of separate C-limited and NH4+-limited microsites could explain these results. Nitrifying bacteria consumed an increasing proportion of the NH4+ pool as the incubation progressed, suggesting that increased microsite structure was responsible for shifts in C and N dynamics over time. The results of this study demonstrate that different plant species can significantly influence soil C and N cycling rates; however, after 15 yr the magnitude of the: effect was still fairly small. (C) 2000 Elsevier Science Ltd. All rights reserved.