Application of WNMM to Predicting Nitrous Oxide Emission from a Tropical Soil Producing Sugarcane.

The Water and Nitrogen Management Model (WNMM) predicts the production and emission of nitrous oxide from soils.  The accuracy of the prediction depends strongly on the accuracy of the simulation of the important drivers: the soil water, temperature and mineral nitrogen content (nitrate and ammonium) of the topsoil.  We examine an application of WNMM to predicting both the environmental drivers mentioned above and the N₂O emissions from the surface of an N-fertilised soil producing sugarcane in a tropical region of eastern Australia (latitude 149ºE, longitude 21ºS).  The high soil moisture regimes, high soil temperatures and high levels of available carbon that characterise Australian sugarcane culture can be expected to promote high evaporation rates and intensify the normal processes of nitrogen cycling that lead to N₂O production.  The soil is an alluvial non-calcic brown soil.  Relevant predictions by WNMM are compared with field measurements made over 14 months: evapotranspiration by eddy covariance, soil water content and soil temperature by soil moisture probes and soil thermometers and N₂O emissions by 6 automatic chambers.  In the study, the field received annual applications of 150 kg N/ha, mainly as urea.  For the first growing season of 290 days, the accumulated rainfall was 1924 mm, evapotranspiration amounted to 541mm and the average volumetric soil water content and soil temperature at 10 cm were 0.28 and 25ºC, respectively.  N₂O Emissions over the same period showed a pronounced diurnal cycle and totalled 4.7 kg N ha.  WNMM predicted the total N₂O emissions reasonably well although there were differences between predictions and observations in the lags between rainfall events and N₂O emissions.  The paper will examine the overall agreement in some detail.