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Michelle A. Walvoord,1*Fred M. Phillips,2David A. Stonestrom,3R. Dave Evans,4Peter C. Hartsough,5,6Brent D. Newman,7Robert G. Striegl1
A large reservoir of bioavailable nitrogen (up to 104 kilogramsof nitrogen per hectare, as nitrate) has been previously overlookedin studies of global nitrogen distribution. The reservoir hasbeen accumulating in subsoil zones of arid regions throughoutthe Holocene. Consideration of the subsoil reservoir raisesestimates of vadose-zone nitrogen inventories by 14 to 71% forwarm deserts and arid shrublands worldwide and by 3 to 16% globally.Subsoil nitrate accumulation indicates long-term leaching fromdesert soils, impelling further evaluation of nutrient dynamicsin xeric ecosystems. Evidence that subsoil accumulations arereadily mobilized raises concern about groundwater contaminationafter land-use or climate change.
1 U.S. Geological Survey, Lakewood, CO 80225, USA. 2 Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA. 3 U.S. Geological Survey, Menlo Park, CA 94025, USA. 4 School of Biological Sciences, Washington State University, Pullman, WA 99164, USA. 5 Graduate Program of Hydrologic Sciences, University of Nevada, Reno, Reno, NV 89557, USA. 6 Desert Research Institute, Reno, NV 89512, USA. 7 Earth and Environmental Science Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
* To whom correspondence should be addressed. E-mail: walvoord{at}usgs.gov
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104 kilograms of nitrogen per hectare, as nitrate) has been previously overlooked in studies of global nitrogen distribution. The reservoir has been accumulating in subsoil zones of arid regions throughout the Holocene. Consideration of the subsoil reservoir raises estimates of vadose-zone nitrogen inventories by 14 to 71% for warm deserts and arid shrublands worldwide and by 3 to 16% globally. Subsoil nitrate accumulation indicates long-term leaching from desert soils, impelling further evaluation of nutrient dynamics in xeric ecosystems. Evidence that subsoil accumulations are readily mobilized raises concern about groundwater contamination after land-use or climate change. 
