Direct flux and 15N tracer methods for measuring denitrification in forest soils

Abstract

Estimates of denitrification are one of the key uncertainties in the terrestrial nitrogen (N) cycle, primarily because reliable measurements of this highly variable process—especially the production of its terminal product (N2)—are difficult to obtain. We evaluated the ability of gas-flow soil core and 15N tracer methods to provide reliable estimates of denitrification in forest soils. Our objectives were to: (1) describe and present typical results from new gas-flow soil core and in situ 15N tracer methods for measuring denitrification, (2) discuss factors that affect the relevance of these methods to actual in situ denitrification, and (3) compare denitrification estimates produced by the two methods for a series of sites in a northern hardwood forest ecosystem. Both methods were able to measure accumulations of N2 over relatively short (2–5 h) incubations of either unamended or tracer-amended intact soils. Denitrification rates measured by the direct flux soil core method were very sensitive to incubation oxygen (O2) concentration and decreased with increased O2 levels. Denitrification rates measured by the in situ 15N tracer method were very sensitive to the 15N content of the nitrate (NO3 −) pool undergoing denitrification, which limits the applicability of this method for quantifying denitrification in N-poor ecosystems. While its ability to provide accurate estimates of denitrification was limited, the 15N tracer method provided estimates of the short-term abiotic and biotic transformations of atmospheric N deposition to gas. Furthermore, results suggest that denitrification is higher and that N2O:N2 ratios are lower (<0.02) than previously thought in the northern hardwood forest and that short-term abiotic and biotic transformations of atmospheric N deposition to gas are significant in this ecosystem.