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Leaching and uptake of nitrogen, and pasture production, associated with irrigation of treated wastewater, Taupō, New Zealand

Secondary-treated wastewater from Taupō Township is irrigated onto perennial ryegrass (Lolium perenne) which is harvested and removed from the View Road wastewater treatment site. To determine the fate of the applied wastewater nitrogen, 48 undisturbed barrel lysimeters (30 cm diameter x 43 cm depth) were installed throughout 29 hectares. Centre pivot travelling irrigators were programmed to vary in speed to provide target wastewater application rates of about 0, 450, 550 and 650 kg N ha⁻¹yr⁻¹. The targeted application treatments were not achieved, with the achieved nitrogen loading rates grouped into a low treatment (286 - 380 kg N ha⁻¹yr⁻¹), a medium treatment (380 - 445 kg N ha⁻¹yr⁻¹), and a high treatment (445 - 567 kg N ha⁻¹yr⁻¹). Nitrogen input in control sectors was assumed to be 5 kg N/ha/yr from atmospheric nitrogen deposition. A mean of 4 to 6% of the applied nitrogen was leached. The mean amount of nitrogen leached from the high treatment (28.6 ± 10.1 kg N ha⁻¹yr⁻¹) was higher (P < 0.05) than from the low treatment (12.7 ± 4.2 kg N ha⁻¹yr⁻¹). The medium treatment (16.0 ± 7.2 kg N ha⁻¹yr⁻¹) was not significantly different than the low treatment or the high treatment. The mean amount of nitrogen leached from the control treatment was 2.8 ± 0.6 kg N ha⁻¹yr⁻¹. Nitrogen leaching that occurred in the high application treatment was below the consented limit of 30 kg N ha⁻¹yr⁻¹. A mean of 79 to 100% of the applied nitrogen was removed by pasture. There were no significant differences in pasture dry matter production, or nitrogen removal, between the low treatment (13 922 ± 1196 kg DM ha⁻¹yr⁻¹and 341 ± 25 kg N ha⁻¹yr⁻¹), the medium treatment (13 543 ± 1475 kg DM ha⁻¹yr⁻¹ and 360 ± 51 kg N ha⁻¹yr⁻¹), and the high treatment (15 285 ± 1919 kg DM ha⁻¹yr⁻¹ and 385 ± 43 kg N ha⁻¹yr⁻¹). Pasture production was higher (P < 0.001) in irrigated pastures (mean of all irrigated treatments, 14 250 ± 349 kg DM ha⁻¹yr⁻¹) than unirrigated controls (5300 ± 839 kg DM ha⁻¹yr⁻¹). A mean of -4 to 16% of the applied wastewater nitrogen was unaccounted for. The majority of unrecovered nitrogen was presumed to be volatilised with lower potential for denitrification and soil storage. A second experiment was undertaken to determine whether a five week or a ten week harvesting frequency would result in greater pasture production and nitrogen removal. During the ten month trial, 265 kg N ha⁻¹ of wastewater nitrogen was irrigated and pasture plots (1 m x 1 m) were cut with a mower. Nitrogen removal and pasture production were higher (P < 0.05) with a five week harvesting interval (8231 ± 186 kg DM ha⁻¹ and 250 ± 5 kg N ha⁻¹) than a ten week harvesting interval (7354 ± 67 kg DM ha⁻¹ and 191 ± 3 kg N ha⁻¹). Harvesting at five week intervals during late-November to May (late-spring to late-autumn) and ten week intervals from May until early-November (late-autumn to mid-spring) is recommended to maximise both pasture production and nitrogen removal, while minimising harvesting costs.
Type of thesis
Telfer, E. I. C. (2013). Leaching and uptake of nitrogen, and pasture production, associated with irrigation of treated wastewater, Taupō, New Zealand (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/7938
University of Waikato
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