Informing Policy and Management in New Zealand Agricultural Regions Using Nitrogen and Oxygen Isotopes to Quantify Hot Spots and Hot Moments of Nitrate Loss

Nitrate losses from agriculture have significant impacts on freshwater, and occur when runoff generation coincides with nitrogen excess in soil. Analysis of δ15N and δ18O in NO3 is well suited to characterize hot spots and hot moments when NO3 losses occur, by working back from impacted water to soil sources. New Zealand's lack of high δ18O atmospheric and fertilizer NO3 sources and the nation's intensive pastoral agriculture enable δ15N and δ18O in NO3 to differentiate soil and effluent sources, as well as processes linked to flow in different soil physiographic zones. This presentation reviews δ15N-NO3 and δ18O-NO3 results gathered across multiple catchments in three regions (Manawatu, Wairarapa, and Southland) with at least 100 measurements per region. River monitoring sites provide an integrated measure of sources, while fractionation associated with removal processes such as denitrification is largely lost. Within rivers, predominantly pastoral regions show a tight pattern along a 1:1 line where δ15N ranges between 4 and 8 ‰ and δ18O ranges between 0 and 4 ‰, in proportion to agricultural intensity. The δ15N of NO3 appears linked to the δ15N of soil organic matter. Exploration of catchments with higher proportions of crop-based agricultural land use, as well as highly intensive grazing with a decreased reliance on pastoral cover, show migration to lower δ15N values with no change in δ18O. This result was in contrast to expectations of greater evidence for effluents and denitrification, and is interpreted as breakthrough of urea fertilizer or animal urine inputs which have δ15N values close to 0 ‰. Drilling into the landscape using springs, drains and groundwater wells shows greater variability both in space and time. These data provide evidence for a range of sources, including urine, fertilizer and effluents, which breakthrough according to soil and physiographic attributes of landscapes that explain the magnitude and timing of processes controlling the coincidence of nitrogen excess and runoff generation. There appears to be considerable potential for dual-isotope NO3 measurements to be used in conjunction with a simplified classification of landscape physiographic units to support environmental policy and agricultural management.