FIELD-EVALUATION OF 4 METHODS FOR MEASURING DENITRIFICATION

Reliable and direct monitoring of in situ denitrification N loss will aid understanding of the N cycle and development of efficient agricultural management systems. The objective of this research was to evaluate four approaches for measuring denitrification in the field. A method using coated CaC2 as a source of acetylene (C2H2) to block N2O reduction was compared with two existing C2H2 methods and a N-15 method. Use of CaC2 granules in soil around gas-measuring chambers produced soil atmospheric C2H2 concentrations comparable to those observed with continuous C2H2 flow through diffusion tubes, which were sufficient to inhibit N2O reduction for a 48-h period. Acetylene concentrations in the soil atmosphere exceeded 1.0 mL L-1 and completely inhibited the reduction of N2O to N2, giving N2O-N recoveries of 105 +/- 7% of total denitrification. Daily denitrification rates (two gas samplings each day) and total N emissions over 4 d were comparable for N-15 chamber, C2H2-inhibition chamber, and CaC2-C2H2-inhibition chamber methods but were five-to sevenfold higher for the C2H2 inhibition method using intact soil cores. Where N-gas flux was measured four times daily in chambers and intact soil cores, denitrification rates were associated with daily changes in soil temperature and decreases in soil water-filled pore space (WFPS) in chamber soils due to evaporation/percolation. Either of three chamber methods may be used for measuring field denitrification, provided daily variation in gas-N flux is considered in choosing sampling times. To avoid overestimation of denitrification using the acetylene-inhibition soil-core method, fresh intact soil cores should be taken daily to simulate field soil physical conditions, especially soil WFPS.