Using stable isotopes to trace the sources and fates of nitrate within mixed land-use catchments on the Banks Peninsula, New Zealand
Thesis DisciplineEnvironmental Sciences
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Increasing land-use change and land-use intensification over the last century has resulted in lotic systems being faced by an increasing intensity environmental. Potentially the most pervasive of these stressors is increased nitrogen runoff. However, understanding the affects of excess nitrogen (primarily nitrate) on lotic systems is severely complicated by the fact that multiple interacting stressors are associated with any given land-use and catchments will invariably contain multiple land-uses. Having analytical tools which can trace nitrogen as it is being cycled through the system is thus important for understanding the impacts of various land-uses on stream nitrogen-cycling. Stable isotopic analyses of δ¹⁵N and δ¹⁸O values within nitrate and δ¹⁵N and δ¹³C values within organic matter allow us to compare how nitrate in entering lotic systems and subsequently moving through them across multiple land-uses. In this study, I compared nitrate δ¹⁵N and δ¹⁸O and invertebrate δ¹⁵N and δ¹³C values within 35 streams across six land-uses (regenerating indigenous forest, horticulture, golf courses, dry stock agriculture, dairy agriculture and land invaded by exotic N-fixing legumes gorse (Ulex europaeus) and broom (Cytisus scoparius)) on the Banks Peninsula, New Zealand. Results showed that gorse and broom streams had significantly higher mean nitrate concentrations than all other land‐uses (mean NO₃-N = 1.02 ppm, P < 0.001). Furthermore, nitrate δ¹⁵N and δ¹⁸O values demonstrated that this elevated nitrate was being fixed by the plants themselves as opposed to a land-use legacy effect. Overall, native regeneration sites had significantly lower nitrate δ¹⁵N values than all other land-uses. Across all land-uses, except for regenerating indigenous forest, nitrate δ¹⁵N and δ¹⁸O values displayed positive covariation, indicative of biological fractionation. Results suggested that, at least within gorse and broom systems, this fractionation was primarily being driven by biological uptake as opposed to denitrification. However, the environmental parameters which had the greatest affects on these fractionation relationships differed substantially between land-uses suggesting that the factors controlling nitrate removal were specific to the land-use environment. Lotic invertebrate responses to land-use included a reduced dietary intake of coarse particulate organic matter (CPOM) across all sites relative to regenerating indigenous forest sites and a larger community trophic niche (range in δ¹³C values) in dairy and gorse sites relative to native regeneration and dry-stock sites. Land-use change lead to invertebrates having less CPOM available and subsequently feed on a wider range on trophic channels. However, no clear relationships were observed with these invertebrate trophic responses and individual land-use stressors (i.e. nitrate concentration or light availability), suggesting that trophic responses resulted from complex interactions between these stressors much in the same way these factors interacted to affect nitrate removal. Nonetheless, average δ¹⁵N values for the whole invertebrate communities were lower in regenerating indigenous forested sites than all other sites, indicating that land‐use induced changes to in-stream nitrogen cycling leaves a δ¹⁵N imprint on the invertebrate community. These findings have identified a significant novel source of nitrate within the regional landscape while also providing a uniquely holistic insight into the ways in which land-use impacts nitrogen cycling and community responses within lotic systems. Although these finding have highlighted the complexity associated with relationships between land-use and lotic systems responses, they also demonstrate how multiple stable isotopic proxies can elucidate vital mechanistic information. With more widespread data collection in New Zealand, in the future stable isotopic studies will be able to be a significant management and research tool for tackling the challenges faced by environmental practitioners in the 21st century.