Introduction

Global climate change in the Mojave Desert will likely result in a greater intensity of summer (monsoon) rain events and greater N deposition. The nitrogen cycle has already been significantly altered by human activities to the extent that anthropogenically released N now equals natural terrestrial biological fixation (Vitousek et al. 1997; Galloway 1998). Because most bryophytes receive the bulk of their nutrients from direct atmospheric deposition (Bates 2000), this influx of N can affect the productivity of individual species and thus may alter bryophyte community structure and function. In addition to N deposition, global change models for the southwestern USA predict significant increases in summer precipitation in the northern Mojave Desert (Taylor & Penner 1994; Higgins & Shi 2001). The interaction between increased N deposition and an increased monsoon effect on bryophytes in the arid southwestern USA is largely unknown. Although growth rates of desert bryophytes are relatively low compared with bryophytes in more mesic ecosystems, the contribution of biological soil crusts (a community of cyanobacteria, mosses, lichens, algae, and fungi) to the global cycling of trace gases can be significant in regard to global budgets (Zaady et al. 2000).

Most field studies have found a rapid negative effect of N fertilization on the growth and productivity of mosses, with nutrient uptake a function of desiccation regime, temperature, and light. For several bryophyte species, high experimental N deposition rates decreased biomass production except in a widely tolerant species of Sphagnum (Jauhiainen et al. 1998).