There has been much recent interest in understanding biotic and abiotic controls on S and N cycling during winter. The objective of this study was to investigate winter sinks for SO42- and NO3- to better understand controls on these acidifying anions during snowmelt. In February 2004, an isotopic tracer solution was applied to the snowpack at the Hubbard Brook Experimental Forest in New Hampshire. The tracer consisted of 93% atom 34S applied as H234SO4 and 99% atom 15N applied as NH415NO3. Additionally, Br- was added (as LiBr) as a conservative tracer. The chemistry and isotopic composition of throughfall, snow, snowmelt, and forest floor leachates were monitored for one year following the addition of the chemical and isotopic tracers. The snowpack reached a maximum depth shortly after the tracer addition and melted during a three-week period ending on 22 March 2004. Nearly all (89%) of the Br- added to the snowpack surface was recovered in the snow lysimeters during spring melt. Both SO42- and NO3- behaved conservatively in the snowpack, much like Br-, with 90% of the added SO42- being recovered in the snow lysimeters and 100% the NO3-. As expected, Br- was also conservative as it leached through the forest floor. We recovered 83% of the total Br- added to the snowpack surface in forest floor lysimeters. In contrast, the forest floor was a fairly strong sink for SO42- and NO3-. Respectively, 54% and 49% of the total SO42- and NO3- added to the snowpack surface was recovered in forest floor leachates. These results indicate that almost all of the SO42- and NO3- that accumulates in the snowpack during winter infiltrates the forest floor, where a significant portion (~50%) is subsequently retained or transformed.
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