Chambers, micrometeorological measurements, and the New Zealand Denitrification-Decomposition model for nitrous oxide emission estimates from an irrigated dairy-grazed pasture

Nitrous oxide (N2O) emissions from soils are notoriously variable in space and time. Measuring and understanding variance in these emissions is imperative for improving the accuracy of the greenhouse gas inventory and assessing the viability of mitigation options; but data for N2O emissions are rather limited. Farm-scale emissions data are also required for developing and verifying predictive model estimates. A measurement campaign was undertaken from 12 October to 1 November 2006 at a highly productive grass-clover irrigated dairy farm on a stony silt loam soil in North Canterbury, South Island, New Zealand. The similar to 7 ha experimental field, grazed in two morning 6-h grazing sessions (21-22 October 2006) by 718 milking dairy cattle, received two irrigations during the measurements, one before the grazing event and the other during grazing period. We first compare the emission measurements using a chamber technique against those made using a micrometeorological technique with tuneable diode-laser technology. We then compare the measured emissions against emissions predicted by a process-based model (New Zealand Denitrification-Decomposition (NZ-DNDC)). Daily averaged micrometeorological measurements gave a pre-grazing emission of 35 g N2O N/ha/day that increased to >60 g N2O N/ha/day following grazing by the dairy herd. The average pre-grazing emission of 10 g N2O N/ha/day from the chambers increased to 25 g N2O N ha(-1) day(-1) following grazing. The emissions were simulated with NZ-DNDC model, which gave average daily emissions of 15 +/- 9 g N2O N ha(-1) day(-1) for the pre-grazing period and 22 +/- 6 g N2O N ha(-1) day(-1) for the post-grazing period. Here we describe these measurement approaches, compare their emission estimates and discuss the advantages of combining them for verification of emissions.