Reactive oxides of nitrogen (N) in the atmosphere (NOx: NO and NO2) take part in the production and/or consumption of atmospheric oxidants such as ozone and hydroxyl radicals and are removed from the atmosphere through the formation and deposition of nitric acid. It has been suggested that soil emissions of NO produced by microbial nitrification may comprise a significant fraction of the unaccounted losses in soil N balance sheets, and that the return of NOx from the atmosphere to the land results in substantial redistribution of N over the landscape and may lead to the formation of the greenhouse gas, N2O. There is a trend in the southwest of Victoria, Australia for the conversion of pasture land to cropping. The N balance sheet associated with this land use change is poorly constrained.
In collaboration with a long-term project measuring N2O emissions from pasture, and pasture converted to cropping, we are measuring parallel NOx emissions from automated chambers in two pasture treatments (mixed pasture and high clover; both with high phosphorus (P) fertility) and two crop (wheat) treatments (one high P fertility and one low P). Preliminary estimates of soil NO emissions measured from mid-June 2007 to end of February 2008 show no difference in emissions between the crop treatments (0.09 kg NO-N ha-1). The mixed pasture treatment had slightly higher emissions of 0.14 kg NO-N ha-1 for the same period, while those of the high clover pasture treatment were considerably higher at 0.82 kg NO-N ha-1.
Using information on soil type and agronomic practices, as well as ancillary measurements of soil variables (mineral N, pH, temperature and moisture) and atmospheric data, we will use the Water and Nitrogen Management Model (WNMM) to model the NOx emissions in order to estimate the effects of land use change on these gases.