Enhancing soil carbon (C) storage has the potential to offset human-caused increases in atmospheric CO2. Rising CO2 has occurred concurrently with increasing supply rates of biologically limiting nutrients such as nitrogen (N) and phosphorus (P). However, it is unclear how increased supplies of N and P will alter soil C sequestration, particularly in grasslands, which make up nearly a third of non-agricultural land worldwide. Here, we leverage a globally distributed nutrient addition experiment (the Nutrient Network) to examine how a decade of N and P fertilization (alone and in combination) influenced soil C and N stocks at nine grassland sites spanning the continental United States. We measured changes in bulk soil C and N stocks and in three soil C fractions (light and heavy particulate organic matter, and mineral-associated organic matter fractions). Nutrient amendment had variable effects on soil C and N pools that ranged from strongly positive to strongly negative, while soil C and N pool sizes varied by more than an order of magnitude across sites. Piecewise SEM clarified that small increases in plant C inputs with fertilization did not translate to greater soil C storage. Nevertheless, peak season aboveground plant biomass (but not root biomass or production) was strongly positively related to soil C storage at seven of the nine sites, and across all nine sites, soil C covaried with moisture index and soil mineralogy, regardless of fertilization. Overall, we show that site factors such as moisture index, plant productivity, soil texture, and mineralogy were key predictors of cross-site soil C, while nutrient amendment had weaker and site-specific effects on C sequestration. This suggests that prioritizing the protection of highly productive temperate grasslands is critical for reducing future greenhouse gas losses arising from land use change.
Bibliographical noteFunding Information:
This work was supported by grants from the NSF Cedar Creek Long Term Ecological Research (183194) and Ecosystem Studies (1556529, 1556418, and 1556410) programs. This work was also partially supported through a National Science Foundation (NSF) award to M.A.M. at the University of Tennessee (NSF‐DEB‐1556418). Oak Ridge National Laboratory is managed by UT‐Battelle, LLC, under contract DE‐AC05‐00OR22725 with the U.S. Department of Energy. This work was generated using data from the Nutrient Network ( http://www.nutnet.org ) experiment, funded at the site scale by individual researchers. Coordination and data management have been supported by funding to E. Borer and E. Seabloom from the National Science Foundation Research Coordination Network (NSF‐DEB‐1042132) and Long‐Term Ecological Research (NSF‐DEB‐1234162 and NSF‐DEB‐1831944 to Cedar Creek LTER) programs, and the Institute on the Environment (DG‐0001‐13). We also thank the Minnesota Supercomputer Institute for hosting project data and the Institute on the Environment for hosting Network meetings. We thank Colby Carlisle, Lang DeLancey, Hanan Farah, Ingrid Holstrom, Ben Huber, Kristine Jecha, Brennan Lauer, Joe Rippke, David Sanneruud, Kaitlin Truong, Colleen Unsworth, Rylee Werden, and Esther Young for assistance in the field and laboratory. We thank three anonymous reviewers for their time and helpful comments which significantly improved the manuscript. There are no potential conflicts of interest to note.
Copyright statement: This manuscript has been authored by UT‐Battelle, LLC, under contract no. DE‐AC05‐00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid‐up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan ( http://energy.gov/downloads/doe‐publicaccess‐plan ).
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