Carbon and Nitrogen Decoupling Under an 11-Year Drought in the Shortgrass Steppe
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The frequency and magnitude of drought is expected to increase in the US Great Plains under future climate regimes. Although semiarid systems are considered highly resistant to water limitation, novel drought events could alter linkages among biogeochemical processes, and result in new feedbacks that influence the timescale of ecosystem recovery. We examined changes in carbon and nitrogen cycling in the last 2 years of an 11-year drought manipulation in the shortgrass steppe, and under the first 2 years of recovery from drought. We measured plant production, plant tissue chemistry, soil trace gas flux, and soil inorganic nitrogen dynamics to test the extent that this magnitude of drought altered carbon and nitrogen fluxes and how these changes affected post-drought dynamics. We found that soil inorganic nitrogen was up to five times higher under severe drought than under control conditions, but that this nitrogen may not have been accessible to plants and microbial communities during drought due to diffusion limitations. Drought plots had higher N2O flux when they received equal rainfall pulses, showing that this accumulated N may be vulnerable to loss. In addition, plants in drought plots had higher tissue nitrogen for 2 years following drought. These results show that decadal-length droughts that may occur under future precipitation regimes are likely to alter ecosystem properties through interactions among precipitation, vegetation, and N cycling. Shifts in plant N, vulnerability of nitrogen to loss, and rainfall use efficiency that we observed are likely to affect the recovery time of semiarid systems subject to droughts of this magnitude.
Keywordssemiarid grassland shortgrass steppe precipitation climate change coupled biogeochemical cycles ecosystem lags rainfall manipulations N conservation
This work was conducted at the Central Plains Experimental Range (CPER), which is administered by the USDA Agricultural Research Service (ARS) and is a Long Term Ecological Research site (SGS-LTER) funded by the National Science Foundation (NSF DEB 0823405 and NSF DEB 0217631). This work would not have been possible without the 1998–2011 SGS field crews and staff, the SGS-LTER Information Manager Nicole Kaplan, and Kenneth Murphy, who conceived and implemented the rainfall manipulation plots. We also would like to thank Dr. Joe von Fischer for assistance in trace gas analysis, Dr. Phillip Chapman for statistical advising, and two anonymous reviewers and the associate editor for their helpful comments.
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