Mineralization and Leaching of Nitrogen in an Effluent-Irrigated Pine Plantation
1995
Polglase, P. J. | Tompkins, D. | Stewart, L. G. | Falkiner, R. A.
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 (H) 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.
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