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Land use and episodic rainfall as drivers of nitrogen exports in subtropical rivers: Insights from δ15N-NO3−, δ18O-NO3− and 222Rn
Journal article   Peer reviewed

Land use and episodic rainfall as drivers of nitrogen exports in subtropical rivers: Insights from δ15N-NO3−, δ18O-NO3− and 222Rn

Praktan D Wadnerkar, Luke Andrews, Wei Wen Wong, Xiaogang Chen, Rogger E Correa, Shane White, Perran L.M Cook, Christian J Sanders and Isaac R Santos
The Science of the total environment, Vol.758, pp.143669-143669
2020
DOI: https://doi.org/10.1016/j.scitotenv.2020.143669
PMID: 33277015

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UN Sustainable Development Goals (SDGs)

This output has contributed to the advancement of the following goals:

#6 Clean Water and Sanitation
#14 Life Below Water
#15 Life on Land

Source: InCites

Abstract

Coastal nutrients Pollution Groundwater Land-use Eutrophication
Ongoing land-use intensification in subtropical catchments is expected to release more inorganic nitrogen to downstream coastal waters similar to historical changes in temperate ecosystems. Here, we examined spatial and temporal drivers of stream nitrogen loads across a subtropical land-use gradient using the isotopic compositions of nitrate (NO3−-N) and radon (222Rn), a natural groundwater tracer. We investigated eleven subtropical creeks/rivers over contrasting hydrological conditions in Australia. NOx-N (nitrite (NO2−-N) + nitrate (NO3−-N)) accounted for 13.1%, 34.0%, and 42.6% of total dissolved nitrogen (TDN-N) in forest, peri-urban and agricultural creeks, respectively. Following an 80 mm rain event, loads of dissolved inorganic nitrogen (DIN-N) from agriculture catchments reached 368 mg N m−2 catchment area day−1. Forest and peri-urban catchments had aquatic TDN-N loads 17.8% and 31.1% of loads from agricultural catchments. Radon observations suggest that nitrogen and phosphorus loads were driven primarily by surface runoff rather than groundwater discharge. The δ15N-NO3− and δ18O-NO3− values in the agriculture, forest and peri-urban catchments indicate fertilisers and soil nitrogen as the main sources of NO3−-N. However, one of the catchments (Double Crossing Creek) received a mixture of recirculated greywater and chemical nitrogen fertilisers. Isotopic signatures imply significant NO3−-N losses via denitrification during dry conditions. Groundwater discharge played a minor role because regional aquifers were not contaminated by nitrogen. Overall, intensive agricultural land use and episodic rainfall events were the major spatial and temporal drivers of nitrogen loads. Conceptual model of the coastal catchments with land use gradient and total dissolved nitrogen (TDN-N) exports. [Display omitted] •Land use and episodic rainfall events were the major drivers of nitrogen exports in eleven subtropical rivers.•Groundwater was not a major source of total dissolved nitrogen (TDN-N) into surface water.•Forest and peri-urban catchments had aquatic TDN-N loads 17.8% and 31.1% of the loads from agricultural catchments.•Agriculture, forest and peri-urban catchments creeks had chemical fertilisers and soil nitrogen as the main sources of NO3−-N.

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