Gaseous nitrogen losses from acid sulfate sugarcane soils on the coastal lowlands

NITRIFICATION and denitrification are two important microbial processes in the soil nitrogen cycle that result in the production and emission of gaseous N compounds including the odd oxides of nitrogen NO and NO2, known collectively as NOx, and nitrous oxide, N2O. Emissions of these gases to the atmosphere represent a loss of N to agriculture and have important environmental effects. The paper reports investigations of NOx and N2O emissions from fallow acid sulfate soils (ASS) cultivated for sugarcane production on the coastal lowlands of northern NSW. Two series of measurements were made, using chambers and micrometeorological techniques. These allowed emissions to be measured continuously and revealed their daily time-courses. The emission of NOx appeared to be haphazard, with little time dependence, but there was a clear diurnal cycle for N2O, emphasising the need for continuous measurement procedures for both gases. In the first series, conducted in a field where a crop of soybeans had been harvested previously, the water filled pore space, WFPS, ranged from near 60% to near 80%. Emissions of NOx were high, amounting to as much as 1.2 kg N/ha on the first day of measurement when the WFPS was close to 60%. They decreased when the WFPS increased to around 80% following rain. On the other hand, N2O emissions were high when the WFPS was near 80% (as much as 0.8 kg N/ha/d) and declined rapidly as the WFPS fell to near 60%. In the second series, conducted in a nearby field, the WFPS was <60%, and fluxes of both gases were much smaller than in the first series even though urea was applied at 50 kg N/ha halfway through the measurement period. We conclude that emissions of both NOx and N2O from ASS are large when water contents are high. We measured a combined loss of 5 kg N/ha over 13 days when the WFPS was between 60% and 80%. When the WFPS was less than 60%, emissions were markedly less, amounting to 0.9 kg N/ha over 10 days. When the soil was moist, emissions of NOx were larger than those of N2O.