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FOREST, RANGE & WILDLAND SOILS

Mobility of Nitrogen-15-Labeled Nitrate and Sulfur-34-Labeled Sulfate during Snowmelt

^a U.S. Forest Service, PO Box 640, Durham, NH 03824
^b State Univ. of New York, College of Environmental Science and Forestry, 1 Forestry Dr. Syracuse, NY 13210
^c Univ. of Calgary, 2500 University Dr. NW, Calgary, AB, Canada T2N 1N4
^d Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545-0129

^* Corresponding author (jlcampbell{at}fs.fed.us).

The objective of this study was to investigate the winter dynamics of SO₄^2– and NO₃^– in a forested soil to better understand controls on these acidifying anions during snowmelt. In February 2004, a stable isotopic tracer solution with 93 atom% ³⁴S as H₂³⁴SO₄ and 99 atom% ¹⁵N as NH₄¹⁵NO₃ was applied to the snowpack at the Hubbard Brook Experimental Forest in New Hampshire. The chemical and isotopic compositions of throughfall, snow, snowmelt, and forest floor leachates were monitored for 10 mo following the addition of the tracers. The ³⁴SO₄^2– and ¹⁵NO₃^– tracer amounts in forest floor leachates were highest in the first fractions of meltwater and declined exponentially until returning to ambient levels in mid-May. Isotopic mass balances indicated that SO₄^2– and NO₃^– were conservative in the snowpack, with tracer recoveries near 100%. In contrast, only 54 to 62% of the ³⁴SO₄^2– and 49 to 58% of the ¹⁵NO₃^– were recovered in forest floor leachates, suggesting that much of the SO₄^2– and NO₃^– that infiltrated the forest floor during snowmelt was retained or transformed. Microbial biomass ¹⁵N values in the forest floor remained low during snowmelt and the natural abundance values of ¹⁸O–NO₃^– in forest floor leachates were indicative of an atmospheric rather than a microbial source. These results suggest that, in this study, microbial immobilization and subsequent mineralization and nitrification of snowpack NO₃^– was insignificant in the forest floor during snowmelt.

Abbreviations: DNRA, dissimilatory nitrate reduction to ammonia • HBEF, Hubbard Brook Experimental Forest • PVC, polyvinyl chloride