Effects of experimental nitrogen deposition on soil organic carbon storage in Southern California drylands |
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| Authors: | Johann F Püspök Sharon Zhao Anthony D Calma George L Vourlitis Steven D Allison Emma L Aronson Joshua P Schimel Erin J Hanan Peter M Homyak |
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| Institution: | 1. Department of Environmental Sciences, University of California, Riverside, California, USA;2. Department of Biological Sciences, California State University, San Marcos, California, USA;3. Department of Ecology and Evolutionary Biology, University of California, Irvine, California, USA;4. Department of Microbiology and Plant Pathology, University of California, Riverside, California, USA;5. Department of Ecology, Evolution, and Marine Biology and Earth Research Institute, University of California, Santa Barbara, California, USA;6. Department of Natural Resources and Environmental Science, University of Nevada, Reno, Nevada, USA |
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| Abstract: | Atmospheric nitrogen (N) deposition is enriching soils with N across biomes. Soil N enrichment can increase plant productivity and affect microbial activity, thereby increasing soil organic carbon (SOC), but such responses vary across biomes. Drylands cover ~45% of Earth's land area and store ~33% of global SOC contained in the top 1 m of soil. Nitrogen fertilization could, therefore, disproportionately impact carbon (C) cycling, yet whether dryland SOC storage increases with N remains unclear. To understand how N enrichment may change SOC storage, we separated SOC into plant-derived, particulate organic C (POC), and largely microbially derived, mineral-associated organic C (MAOC) at four N deposition experimental sites in Southern California. Theory suggests that N enrichment increases the efficiency by which microbes build MAOC (C stabilization efficiency) if soil pH stays constant. But if soils acidify, a common response to N enrichment, then microbial biomass and enzymatic organic matter decay may decrease, increasing POC but not MAOC. We found that N enrichment had no effect on C fractions except for a decrease in MAOC at one site. Specifically, despite reported increases in plant biomass in three sites and decreases in microbial biomass and extracellular enzyme activities in two sites that acidified, POC did not increase. Furthermore, microbial C use and stabilization efficiency increased in a non-acidified site, but without increasing MAOC. Instead, MAOC decreased by 16% at one of the sites that acidified, likely because it lost 47% of the exchangeable calcium (Ca) relative to controls. Indeed, MAOC was strongly and positively affected by Ca, which directly and, through its positive effect on microbial biomass, explained 58% of variation in MAOC. Long-term effects of N fertilization on dryland SOC storage appear abiotic in nature, such that drylands where Ca-stabilization of SOC is prevalent and soils acidify, are most at risk for significant C loss. |
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| Keywords: | atmospheric nitrogen deposition carbon use efficiency extracellular enzymes fertilization mineral-associated organic matter (MAOM) particulate organic matter (POM) soil acidification soil microbes |
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