Earthworms Reduce Biotic 15-Nitrogen Retention in Northern Hardwood Forests
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Invasive exotic earthworms are significantly influencing understory community composition, soil, and ecosystem processes in northern hardwood forests in North America, but their effect on the retention of nitrogen (N) has been inconclusive. We examined this in two northern hardwood forest sites in New York state, USA through a tracer study. In both spring and fall, we added tracer amounts of 15N as nitrate—to simulate atmospheric deposition—with the biologically less active tracer bromide (Br−) to areas both with and without large populations of invasive earthworms. Total recovery of 15N was lower in earthworm-invaded plots, largely due to less retention in litter and upper soil horizons. Although the strong relationship between retention in the upper soil horizons and total 15N recovery suggests that earthworm destruction of the forest floor may be one mechanism reducing the capacity for N retention, in some cases the mineral soil in earthworm-invaded plots retained substantial N. Biotic pools, particularly litter and microbial biomass, retained significantly less 15N in earthworm-invaded plots than in their uninvaded counterparts. In plots invaded by earthworms, negative effects of earthworms on trees were revealed through root-uptake assays suggesting somewhat greater plant demand for ammonium in the spring and in lower 15N recovery in maple seedlings the year following tracer addition. Although similar patterns of Br− movement across treatments suggested that earthworms had smaller effects on hydrologic tracer movement than expected, they appear to have significant effects on the biological processes that underlie N retention.
Keywordsammonium earthworms hardwood forest hydrology invasive species nitrate nitrogen retention nutrient cycling microbes soils
We thank David Lewis, Jessica Kriebel, Amanda Lindsey, Julie Hart, Will Kessler, Tali Lee, Rebecca Huskins, Art Gold, and Catherine Shields for help with fieldwork and laboratory and data analyses. Soil extracts and lysimeter collections were analyzed in the Cary Institute Analytical Lab by Denise Schmidt and analytical lab staff. David Harris at the UC Davis Stable Isotope Lab and the Oregon State University Stable Isotope Research Unit in the Department of Crop and Soil Science were responsible for 15N analyses. Pamela Templer provided important perspective on 15N recovery. This research was supported by Grants from the National Science Foundation (DEB-9726869 and DEB-0542065) and the Andrew W. Mellon Foundation.
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