Nitrogen fixation by biological soil crusts and heterotrophic bacteria in an intact Mojave Desert ecosystem with elevated CO 2 and added soil carbon

Abstract

Fixation of N by biological soil crusts and free-living heterotrophic soil microbes provides a significant proportion of ecosystem N in arid lands. To gain a better understanding of how elevated CO 2 may affect N 2-fixation in aridland ecosystems, we measured C 2H 2 reduction as a proxy for nitrogenase activity in biological soil crusts for 2 yr, and in soils either with or without dextrose-C additions for 1 yr, in an intact Mojave Desert ecosystem exposed to elevated CO 2. We also measured crust and soil δ 15N and total N to assess changes in N sources, and δ 13C of crusts to determine a functional shift in crust species, with elevated CO 2. The mean rate of C 2H 2 reduction by biological soil crusts was 76.9±5.6 μmol C 2H 4 m −2 h −1. There was no significant CO 2 effect, but crusts from plant interspaces showed high variability in nitrogenase activity with elevated CO 2. Additions of dextrose-C had a positive effect on rates of C 2H 2 reduction in soil. There was no elevated CO 2 effect on soil nitrogenase activity. Plant cover affected soil response to C addition, with the largest response in plant interspaces. The mean rate of C 2H 2 reduction in soils either with or without C additions were 8.5±3.6 μmol C 2H 4 m −2 h −1 and 4.8±2.1 μmol m −2 h −1, respectively. Crust and soil δ 15N and δ 13C values were not affected by CO 2 treatment, but did show an effect of cover type. Crust and soil samples in plant interspaces had the lowest values for both measurements. Analysis of soil and crust [N] and δ 15N data with the Rayleigh distillation model suggests that any plant community changes with elevated CO 2 and concomitant changes in litter composition likely will overwhelm any physiological changes in N 2-fixation.