Do litter decomposition and nitrogen mineralization show the same trend in the response to dry and wet years in the Patagonian steppe?

In arid environments, dry and wet years have similar number of small rainfall events but wet years result from the occurrence of a few large rainfall events. What are the implications of these different precipitation patterns on ecosystem functioning? Here, we examined the differential response of soil processes to wet and dry years. Specifically, we assessed litter decomposition and soil N mineralization responses to precipitation excess in the Patagonian steppe, and compared with responses to drought conditions previously reported for this ecosystem. Litter decomposition rates did not differ significantly between simulated wet year and control, while litter decomposition rates decreased under a drought of a similar absolute magnitude. We hypothesize that decomposition rate in this ecosystem may be driven more by number of rainfall events than by total precipitation amount. Net nitrification increased in the simulated wet year but only for a short period of time. Moreover, irrigation positive effects on nitrification were offset by negative effects on net ammonification. Consequently, net N mineralization was not affected by water availability. Our results suggest that the different microorganisms responsible of nitrification and ammonification have differential sensitivity to water availability and that the environmental controls might be overwhelmed by substrate availability in longer time scales. r 2007 Elsevier Ltd. All rights reserved.