Nitrogen (N) enrichment of the biosphere is an expanding problem to which arid ecosystems may be particularly sensitive. In semiarid grasslands, scarce precipitation uncouples plant and microbial activities, and creates within the soil a spatial mosaic of rhizosphere and cyanobacterial crust communities. We investigated the impact of elevated N deposition on these soil microbial communities at a grama-dominated study site located incentral New Mexico (USA). The study plots were established in 1995 and receive 10 kg ha(-1) year(-1) of supplemental N in the form of NH(4)NO(3). Soil samples were collected in July 2004, following 2 years of severe drought, and again in March 2005 following a winter of record high precipitation. Soils were [...]