Natural abundance of ¹⁵N as a tool for assessing patterns of nitrogen loss from forest ecosystems

Abstract

Stable isotopes provide an integrated measure of the nitrogen cycling history of a site. Among ecosystems with contrasting nitrate loss patterns, the [delta]15 N of soil and plant material should be higher at sites with higher nitrate losses. An underlying assumption in natural abundance isotope studies is that soil [delta]15 N is at steady-state over time. I found that [delta]15 N was not at steady state in either the Oie or Oa horizon for the period 1969 t[delta]1992 for the reference watershed (W6) at the Hubbard Brook Experimental Forest (HBEF); when nitrate losses were high, [delta]15 N increased. I measured the [delta]15N of soils from 28 soil pits at Watershed 5 at the HBEF before and after clear-cutting in order t[delta]test the hypothesis that elevated nitrification and nitrate loss induced by clear-cutting would be associated with a concurrent increase in soil [delta]15 N. A mass-balance model confirmed that increases in nitrification and nitrate loss after clear-cutting could explain the increase in soil [delta]15N (l .6%0 in the Oie horizon and 1. 1 o/o[delta]in the Oa horizon) in the organic horizons after 3 years. I tested the hypotheses: (1) that foliar [delta]15 N will be higher in a clear-cut watershed than in a reference watershed due t[delta]elevated nitrification and nitrate loss; and (2) that foliar [delta]1 5 N in a clear-cut watershed will track the rapid changes in streamwater nitrate after clear-cutting. Increased foliar [delta]15 N coincided with increased streamwater nitrate concentration, suggesting that the increased nitrification that caused elevated streamwater nitrate concentration als[delta]caused enrichment of the plant-available ammonium pool. Finally, I tested the hypotheses: (1) that [delta]15 N in soil and litter increases across a spatial gradient of nitrate loss, and (2) that [delta]15 N in soil and litter are elevated when nitrification potential is elevated. The enrichment factor, defined as [delta]15 N1foliar - [delta]15Nbs is a method of comparing [delta]15 N values from different sites by normalizing for the spatial heterogeneity in mineral soil [delta]15 N values. When net nitrification potential was high, the enrichment factor was higher, when nitrification potential was low, the enrichment factor was lower. The enrichment factor may prove valuable for comparing sites with different nitrogen cycling patterns.