MINERALIZATION AND LEACHING OF NITROGEN IN AN EFFLUENT-IRRIGATED PINE PLANTATION

In Australia, irrigating tree plantations with domestic sewerage effluent is becoming an increasingly common and necessary alternative to disposal of effluent in rivers. This study estimated field rates of soil N mineralization and concentrations of N in soil solution under a newly established stand of radiata pine (Pinus radiata D. Don) to which secondary-treated sewerage effluent was applied by spray-irrigation at various rates. These rates nominally were: medium treatment (M) irrigated at the rate at which water was used by the plantation (allowing for inputs in rain); high treatment N irrigated at twice the rate M; low treatment (L) irrigated at half the rate M; and a bore-water control (W, groundwater containing no N or P) irrigated at the rate of water use (less rainfall). Annual rates of in situ N mineralization were large in all treatments, ranging from 290 kg ha(-1) in the L treatment to 410 kg ha(-1) in the H treatment. The amount of soil N decreased, on average, by about 190 kg ha(-1) during the first two irrigation seasons in the H, M, and W treatments. A total of 374 kg of N was added in effluent during the first three irrigation seasons in the H treatment. Despite this addition and the large amount of N mineralized, most of which was nitrified, leaching of NO3-N (and of total N) was limited. During a 2-yr sampling period, for example, the concentrations of NO3-N leaching at 100 cm averaged only 1.1 mg L(-1) across all treatments. Weed growth and weed management, in combination with irrigation and its effect on Soil moisture availability can greatly influence soil N turnover and leaching in young tree plantations.