Long-term Nutrient Fertilization Increased Soil Carbon Storage in California Grasslands
Elevated nutrient deposition often increases primary productivity in terrestrial ecosystems and thus has the potential to increase the flux of carbon (C) into soils. An important step toward greater understanding of nutrient effects on C storage involves assessing effects on different fractions of the soil C pool across a range of soil types. We quantified the combined effects of 8 years of nitrogen (N), phosphorus (P), potassium (K), and micronutrient fertilization on the C storage in bulk soil and in density fractions at four grassland sites in California. When averaged across sites, fertilization increased soil light fraction C by 64% relative to the control in the 0–10 cm depth. The increase in light fraction C likely resulted from the fertilization-induced increase in plant C input to soil, as aboveground net primary productivity also consistently increased with fertilization across sites. Effects of fertilization on heavy fraction C were highly site specific, having positive, negative, or no effect at individual sites. The response of heavy fraction C to fertilization appeared to be related to mean annual precipitation and soil bulk density. Overall, bulk soil C concentration showed a marginally significant increase of 6% with fertilization when averaged across sites (P = 0.07). Our results indicate that biomass production and soil light fraction are generally sensitive to fertilization across grasslands in California, likely contributing to increases in soil C storage. Responses of heavy fraction C, on the other hand, vary greatly among sites and may depend on climate and soil characteristics.
KeywordsNutrient Network (NutNet) particulate organic matter (POM) mineral-associated organic matter (MAOM) priming C/N ratio Mediterranean climate
We thank Summer Ahmed, Heather Dang, Beatrix Jimenez, Keri Opalk, Andrew Saunders, Whendee Silver, Frank Wang, and Kana Yamamoto for their support in field and laboratory work. We thank Elizabeth Borer, Stan Harpole, Anita Krause, Eric Lind, and Eric Seabloom from the NutNet headquarters for facilitating this study. We also appreciate the help from Hannah Bird, Anthony Jackson, Catherine Koehler, and Kate McCurdy for providing access to the sites and logistical support. This study benefited from discussion with Joseph Blankinship, Sarah Hobbie, Peter Homyak, and Charlotte Riggs. We thank the subject-matter editor and two anonymous reviewers for their comments that improved this manuscript. This project was supported by a Faculty Research Grant to JYK and YL from the Academic Senate, UCSB, and by funding to C. D’Antonio from UCSB and the Schuyler Endowment.
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