Mobility of nitrogen-15-labeled nitrate and sulfur-34-labeled sulfate during snowmelt

The objective of this study was to investigate the winter dynamics of SO42- 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% S-34 as (H2SO4)-S-34 and 99 atom% N-15 as (NH4NO3)-N-15 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 (SO42-)-S-34 and (NO3-)-N-15 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 SO42- and NO3- were conservative in the snowpack, with tracer recoveries near 100%. In contrast, only 54 to 62% of the (SO42-)-S-34 and 49 to 58% of the (NO3-)-N-15 were recovered in forest floor leachates, suggesting that much of the SO42- and NO3- that infiltrated the forest floor during snowmelt was retained or transformed. Microbial biomass delta N-15 values in the forest floor remained low during snowmelt and the natural abundance values of delta O-18-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.