PARENT SESSION
Oral Session #35: Nitrogen cycling: Response to inputs, variation in time and space. Presiding: M. Fenn.
Tuesday, August 7, 2001. 1:00 PM to 5:00 PM. Hall of Ideas G.

Foliar ¹⁵N natural abundance response to clear-cutting disturbance.

Pardo, L¹, Hemond, H², Fahey, T³, Montoya, J⁴, ^{1 2 3 4}

ABSTRACT- Natural abundance of ¹⁵N in leaves and litter can be an indicator of the ¹⁵N content of plant-available nitrogen. A disturbance, such as a clear-cut, which changes the ¹⁵N of inorganic nitrogen, should alter foliar ¹⁵N. In this study, we measured foliar ¹⁵N in a clear-cut and a reference watershed to evaluate whether foliar ¹⁵N would be higher in the clear-cut watershed than in the reference watershed due to elevated nitrification and nitrate loss. There was little change over 11 years in foliar ¹⁵N for the reference watershed; the mean ¹⁵N was -1.2‰. In most years, we observed a pattern of ¹⁵N amongst the species, such that American beech>yellow birch>sugar maple. The clear-cut watershed, in contrast, showed distinct changes with time after clear-cutting. Foliar ¹⁵N was highest (1.7‰) the first two years after clear-cutting, decreased to 0.0‰ at 3-5 years after clear-cutting, and by 9-11 years after clear-cutting to -1.3‰. The pattern of increased foliar ¹⁵N immediately after clear-cutting coincided with the pattern of increased streamwater nitrate concentration. We concluded that the increased nitrification that caused the elevated streamwater nitrate concentration may also have caused enrichment of the plant-available ammonium pool. The data also suggest that the available nitrogen pool is small relative to the total soil nitrogen pool, and that it is similar in size to plant uptake of nitrogen.

KEY WORDS: ¹⁵N natural abundance, foliar ¹⁵N, clear-cutting , plant response to disturbance