Determination of the effect of C4 pastures to reduce nitrate-N leaching.

There is a strong presumption that perennial pastures will have a positive impact on catchment hydrology by reducing salinity risk associated with rising groundwater, as well as other benefits such as nutrient recycling. Water balance study is concers the effect of pastures on the nitrate-N behaviour in deep-sand soils. To evaluate these presumptions so that future adoption of perennials can be based on actual grounds and not speculation. Soil samples were collected monthly from paddocks of the annual pasture, perennial grass pasture and tagasastes at Mingenew, Western Australia. Water balance parameters calculations were made for each pasture type and along with soil NO3 sampling was used to predict water drainage and nitrogen leaching. The perennial grasses have less soil water throughout the early parts of the year and less deeper in the profile. Tagasastes also has lower soil water content than annuals and less than the perennial grasses at depth. The annuals have significantly more water near the surface (~0.04 cm3/cm3). The additional drying of the soil in the summer months under the perennial grasses and tagasaste created a buffer to soak up more of the winter rain before drainage commenced. All of the pasture types had large amount of water in the profile during September 2007 following rainfall events that significantly exceeded ETp. So even under perennials there was water moving down the profile following heavy rainfall that is common during winter. In 2007 and 2008 the total leached NO3-N below 1.5 m was 51 kg/ha N for the annuals, 17 kg/ha N for the perennial grasses and 26 kg/ha N for tagasaste. Interestingly there was less leaching of NO3 under the tagasaste despite it being a legume. The annual pastures lost the most NO3 through leaching.