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Phosphorus and iron-oxide transport from a hydrologically isolated grassland hillslope

journal contribution
posted on 2024-06-21, 03:51 authored by Genna Smith, Rich McDowellRich McDowell, Leo Condron, Karen Daly, Daire Ó Huallacháin, Owen Fenton
  • Losses of N, P and Fe in drainage responded differently to waterlogging.
  • Moderate nitrate concentration prevented Fe-P dissolution and loss in drainage.
  • Low nitrate concentrations corresponded with high iron and P losses in drainage.
  • Timing of N inputs were important for P losses in drainage.
Dissolved reactive phosphorus (DRP) loss from agricultural soils can negatively affect water quality. Shallow subsurface pathways can dominate P losses in grassland soils, especially in wetter months when waterlogging is common. This study investigated the processes controlling intra- and inter-event and seasonal DRP losses from poorly drained permanent grassland hillslope plots. Temporal flow-related water samples were taken from surface runoff and subsurface (in-field pipe) discharge, analysed, and related to the likelihood of anaerobic conditions and redoximorphic species including nitrate (NO3) over time. Subsurface drainage accounted for 89% of total losses. Simple linear regression and correlation matrices showed positive relationships between DRP and iron and soil moisture deficit; and negative relationships between these three factors and NO3 concentrations in drainage. These data indicate that waterlogging and low NO3 concentrations control the release of P in drainage, potentially via reductive dissolution. The relationship between DRP and metal release was less obvious in surface runoff, as nutrients gathered from P-rich topsoil camoflaged redox reactions. The data suggest a threshold in NO3 concentrations that could exacerbate P losses, even in low P soils. Knowledge of how nutrients interact with soil drainage throughout the year can be used to better time soil N and P inputs via, for example, fertiliser or grazing to avoid to excessive P loss that could harm water quality.


Funded by the New Zealand Ministry for Business, Innovation and Employment's Our Land and Water National Science Challenge (Toitū te Whenua, Toiora te Wai) as part of project Linking Legacies to Wai


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  • English

Does this contain Māori information or data?

  • No

Journal title

Journal of Environmental Management

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