Spatial and temporal variations in nitrogen export from a New Zealand pastoral catchment revealed by stream water nitrate isotopic composition

Abstract

Viable indicators of nitrogen (N) attenuation at the catchment scale are needed in order to sustainably manage global agricultural intensification. We hypothesized that the dominance of a single land use (pasture production) and strong ground-to-surface water connectivity would combine to create a system in which surface water nitrate isotopes (δ15N and δ18O of NO3-) could be used to monitor variations in catchment-scale attenuation. Nitrate isotopes were measured monthly over a 2 year period in four reaches along a spring-fed, gaining stream (mean NO3 - N of 6 mg L-1) in Canterbury, New Zealand. The stream water NO3- pool indicated that the highest degree of denitrification occurred in the shallow upper reaches. Moving downstream through increasingly sandy soils, the isotopic signature of denitrification became progressively weaker. The lowest reaches fell into the expected range for NO3- produced from the nitrification of pasture N sources (urine and fertilizers), implying that the attenuation capacity of the groundwater and riparian systems was lower than the rate of N inputs. After excluding months affected by effluent spills or extreme weather (n = 4), variations in the degree of denitrification over stream distance were combined with the measured NO3- discharge to estimate N attenuation over time in the subcatchment. Attenuation was highly responsive to rainfall: 93% of calculated attenuation (20 kg NO3 - N ha-1 yr-1) occurred within 48 h of rainfall. These findings demonstrate the potential for detailed NO3- stable isotope data to provide integrative measures of catchment NO3- loss pathways.