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One of the more uncertain aspects of nutrient cycling in desert ecosystems is the extent to which decomposition is controlled by water availability. Some of this uncertainty may be partly related to the duration of the studies and/or differences in the chemical composition of the decaying litter. We conducted a 3-year field study of rainfall to determine the impact of summer (June September) drought on decomposition of leaf and root litter of two shrub species (Larrea tridentata and Prosopis glandulosa) in the northern Chihuahuan Desert (New Mexico, U.S.A.). In both species, leaf litter decayed at a faster rate and was less effected by drought than root litter. Drought had no influence on the rates of decomposition of leaves of either species during the first 18 months, but caused decay rates to decline by about 25% during the latter half of the study. Drought decreased decay of root litter in both species by about 25% throughout the 3 years. There was a general increase in percent nitrogen (%N) of decomposing leaf litter in both species. Root litter %N declined slightly in Larrea but remained relatively constant in Prosopis. Using a modified version of the CENTURY model, we were able to predict most aspects of observed leaf and root litter mass loss and nitrogen dynamics. Overall, the results of this study suggest that relatively large changes in precipitation produce comparatively small changes in rates of decay of both leaf and root litter. (C) 2002 Elsevier Science Ltd.