Encompassing the Four Corners region of the Intermountain West, the Colorado Plateau is classified as semi-arid, with total annual precipitation ranging from 130-250mm yr-1. The Intergovernmental Panel on Climate Change report projects that temperatures at lower elevations within the Colorado Plateau will rise by 4–6C by 2100, with an overall decrease in precipitation and a shift to pulsing summer rainfall patterns. The goal of this study was to begin assessing the consequences of projected climate change for soil communities and processes on the Colorado Plateau.
Biological soil crusts play a key role in the structure and function of arid and semi-arid ecosystems. These communities, composed primarily of cyanobacteria, algae, lichens and mosses, can completely cover plant interspaces in undisturbed areas and constitute 70% or more of the living ground cover. Biological soil crusts fix atmospheric nitrogen (N) and CO2 and serve as important sources of fixed carbon (C) and N for these sparsely vegetated systems.
In a large field experiment near Moab, Utah, we set up a large field manipulation in 2005 to experimentally simulate the more extreme scenario of predicted changes in precipitation and examine the combined effects of pulsing summer precipitation and warming on soils and biological soil crusts. Five replicates of 5m2 plots were randomly assigned in the following treatments: (1) increased temperature, (2) increased temperature and summer precipitation, (3) increased summer precipitation, (4) control. Pulsing 1.5mm precipitation treatments were delivered 2-3 times per week starting in mid-June and continued until mid-September. We measured biological soil crust cover, total organic C, total dissolved N, inorganic N, and extractable soil phosphorus (P).
Results/Conclusions

We observed no changes in biological soil crust cover or soil chemistry in response to warming alone. However, we observed a dramatic reduction in moss cover in watered plots. This moss die-off was associated with reductions in soil C, N, and soil P in watered plots. Microbial biomass C and N were also lower in watered plots and inorganic N pools switched from being comprised largely of ammonium to being nitrate-dominated. Nitrification rates were also notably higher in watered plots. While the data here represent the first few years of an on-going experiment, these preliminary results suggest that changes to precipitation patterns projected for the Colorado Plateau will likely strongly affect biological soil crust composition, especially moss cover, with important consequences for soil nutrient cycling.