Livestock-generated nitrogen exports from a pastoral wetland

Abstract

When wetlands are disturbed by cattle, pulses of contaminants may be released. We studied nitrogen exports from a small pastoral wetland (1725 m) in the Lake Taupo Catchment, New Zealand, to which cattle and sheep had periodic access. Flow, turbidity, and water quality samples were collected at the wetland outlet over 2 yr. Turbidity was used to trigger sampling during livestock grazing and as a surrogate for organic N (OrgN) and total N (TN) in flux estimation. The wetland flowed throughout the study (median 0.285 L s) and was baseflow dominated (73%) but responded to rainfall (peak storm flow 166 L s). Organic N was the dominant N form exported (median OrgN:TN ratio 0.86). During cattle grazing periods, concentrations and fluxes of all forms of nitrogen at the outlet were elevated compared with storm and baseflow conditions during nongrazed periods. The TN fluxes were nine times greater when cattle grazed the wetland (306 g d) than under nongrazed baseflow conditions (32 g d). Cattle grazing occurred 9% of the time but accounted for 34% of TN export over 11 mo. Excluding cattle from small wetlands is likely to have immediate water quality benefits.