Field Evaluation of Four Methods for Measuring Denitrification

Abstract

AbstractReliable 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 15N 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 15N 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.