title: Using 15N to determine a budget for effluent-derived nitrogen applied to forest.
abstract: Using stable N isotopes, the fate of effluent-derived N has been determined within a land based municipal effluent irrigation scheme. Over 900 metric tonnes(t) of effluent-derived N have been applied to 192 ha of production conifer forest near Rotorua (NZ) over the past 11 years. The effluent N has a natural isotopic signal, generated by the treatment process, allowing it to be traced into various components of the system. Using this isotopic signal, a realistic approximation of storage capacity of various components of the system has been generated, including a calculation of the contribution of effluent N exiting the catchment via stream flow. Forest storage accounts for 50% of the applied N with a considerable proportion of that immobilized in wood and soil. The wetland, although not intensively sampled, retains 115 t, (13%) of the applied N. Denitrification, including that occurring within the wetland, accounts for 23 t (3%). Nitrogen isotope data confirm that the rise in NO3 concentrations is directly attributable to effluent N. Currently 88% of NO3-N in the stream is effluent-derived. Using current N isotope values for the stream and extrapolating over the discharge period, export of effluent N via the stream is estimated as 263 t (29%) of the applied N. Overall the forest and wetland ecosystem has intercepted or denitrified 65% of applied N, with 29% lost to the stream, and 50 t (5%) unaccounted for. The forest ecosystem is currently over-supplied with N and a number of management implications flows from these findings. In the long term the continued application of effluent N to the current irrigation area is not sustainable.
authors.affiliation: Department of Biological Sciences, University of Waikato, Private Bag 3105, Hamilton, New Zealand. wct@waikato.ac.nz
authors.forename: W C
authors.initials: WC
authors.lastname: Tozer
authors.forename: K J
authors.initials: KJ
authors.lastname: Wilkins
authors.forename: H
authors.initials: H
authors.lastname: Wang
authors.forename: M
authors.initials: M
authors.lastname: van den Heuvel
authors.forename: T
authors.initials: T
authors.lastname: Charleson
authors.forename: W B
authors.initials: WB
authors.lastname: Silvester
mesh_terms: D018533:Biomass; D017753:Ecosystem; D009520:New Zealand; D009566:Nitrates / Q000032:analysis / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism*; D009587:Nitrogen Isotopes / Q000032:analysis*; D045483:Rivers; D014197:Trees / Q000502:physiology*; D014881:Water Supply
chemical_list: D009566:Nitrates; D009587:Nitrogen Isotopes; D009584:Nitrogen