Quantifying the Potential to Reduce Excess Nitrogen Flows from Pasture Catchments: Designing Isotope Tools
Creators: name: Baisden, Troy
name: Clough, Timothy
name: Eyberg, Claire
name: McCrostie, Ray
name: Murray, Rachel
name: Rissmann, Clinton
name: Meti, Yulianti,
Publisher: name: figshare
Year: 2022
Subjects: Environmental Science
Soil Science
Hydrology

description: <b></b>Baisden, W. T., Clough, T. J., Eyberg, C. E., McCrostie, R., Murray, R., Rissmann, C. W., &amp; Yulianti, M. (2021, 30 November – 2 December 2021). Quantifying the Potential to Reduce Excess Nitrogen Flows from Pasture Catchments: Designing Isotope Tools New Zealand Hydrologic Sciences Society Conference Wellington. <br>https://agu.confex.com/agu/fm21/meetingapp.cgi/Paper/930551<br><b><br></b><b>Introduction</b>High producing grazed pastures occupy almost one third of Aotearoa New Zealand (268,000 km<sup>2</sup>) and produce exported protein to feed more than 40 million people. Since 1990, trends within farming systems include increasing rates of urea fertiliser use and greater concentrations of nitrogen (N) via urine deposition, which contribute to increasing N<sub>2</sub>O emissions and NO<sub>3</sub> losses. We used natural isotope tracers to better pinpoint periods or locations of nitrogen excess that can be identified as control points, to target potential mitigation of nitrate losses. Focussing on N-sensitive lake catchments, we ask: how can isotope tools be applied to reduce uncertainties in the sources and magnitude of N losses, to quantify potential reductions in N excess, and to clarify rates of change in N budgets. <b>Results and Discussion</b>We combined widespread monthly, low flow, and targeted monitoring at many sites with operation of an event-based autosampler in the Lake Ōkaro catchment. This allowed us to augment the concentrations of N species and dual-isotope nitrate with water isotopes and Δ<sup>14</sup>C in dissolved inorganic carbon (DIC). Dual-isotope NO<sub>3</sub> measurements continue to support previous interpretations and differentiate urine and urea-derived sources (δ<sup>15</sup>N &lt; 4 ‰) from mineralized soil organic N (δ<sup>15</sup>N of 4–8 ‰). Nitrate in streams draining the Rotorua region's pumice soils and aquifers is dominated by urine and urea sources, compared to streams flowing from finer soils (e.g., Rotomahana mud) that only show these lower δ<sup>15</sup>N values when large runoff events activate surface flow paths. We have confirmed that shifts in stream water δ<sup>2</sup>H and δ<sup>18</sup>O toward the values observed in the major rainfall events coincide with elevated NO<sub>3</sub> concentrations and low δ<sup>15</sup>N representative of urine and urea-derived sources, across an event-size threshold. A combination of δ<sup>2</sup>H and δ<sup>18</sup>O and Δ<sup>14</sup>C in dissolved inorganic carbon (DIC) largely confirmed tritium-based assessments, suggesting lag times of many decades in some aquifers, but rapid responses to recent N inputs elsewhere.In the Southland region, where tile drainage enables effective pasture growth, we explored flow responses in a drainage tile where NO<sub>3</sub> (δ<sup>15</sup>N &gt; 12 ‰) consistently showed an imprint of denitrification. In this location following major rainfall, NO<sub>3</sub> concentrations remained stable but dissolved organic N concentrations increased, at times associated with stormwater δ<sup>2</sup>H and δ<sup>18</sup>O shifts. The Δ<sup>14</sup>C in DIC yielded apparent ages of several hundred years during low-flow periods, suggesting ongoing breakdown of soil organic matter releases N, which should be considered in farm and catchment N budgets.We conclude that monitoring N concentrations and multiple isotope species can resolve control points of N excess, which reveal targets in space and time for potential mitigation. Specifically, N content in clover-ryegrass pastures seasonally exceeds N demand in grazing animals, suggesting alternate species or feeds could reduce animal urinary N excretion, and therefore limit soil-derived NO<sub>3</sub> losses as well as N<sub>2</sub>O emissions. Other management decisions, including timing of fertilisation, grazing and cut-and-carry can all be considered for potential mitigation of excess N and associated N losses.