Managing nitrate losses and nitrous oxide emissions from streams in heavily fertilised catchments: from problem to solution

Shane White

doi:10.25918/thesis.218

Agricultural nitrogen runoff is a worldwide issue. On Australia’s East Coast, heavily fertilised intensive horticulture has expanded quickly. Here, I quantify fertiliser losses to creeks and related emissions of the potent greenhouse gas nitrous oxide (N2O). I examine nutrient sources using stable isotopes, highlight hot spots and hot moments of nitrate and nitrous oxide, explore nitrogen attenuation in natural and constructed systems, and evaluate N2O emissions from blueberry farms. My first data chapter assesses how episodic rain events release nitrate and N2O from a horticultural catchment. Nitrate loads were equivalent to ~15% of applied fertiliser. Aquatic N2O emissions were amongst the highest reported from a natural waterway globally, representing ~10% of the total (N2O + CO2 + CH4) CO2-equivalent emissions, ~60% higher than the IPCC global modelled average. Groundwater tracing using radon (222Rn) revealed that surface water flushed during episodic rain events, rather than groundwater discharge, was the major driver of nitrogen losses. In my second data chapter, I use high-resolution sampling to quantify horticultural stream nitrate loads, sources and natural attenuation downstream of intensive horticultural sources. My results exceeded bottom-up estimates of nitrogen input at upstream sites. Stable isotopes (δ15N and δ18O) and Bayesian analysis revealed nitrate sources were recycled greywater (~55%), fertilisers (~29%), and rainfall (~16%). The creek attenuated 52–84% of upstream nitrate load per km depending on hydrological conditions. My third data chapter examines whether a denitrifying woodchip bioreactor can be a solution to mitigate nitrate exports whilst avoiding pollution swapping to N2O. The nitrate removal efficiency ranged from ~73% in ideal hydrological conditions to ~18% in non-ideal conditions. The fraction of nitrate converted to N2O was ~3.3-fold lower than the IPCC default emission factor, indicating the bioreactor was not significantly pollution swapping nitrate to N2O. Hence, edge-of-field surface-flow bioreactors are a cost-effective solution to reduce nitrate runoff. Finally, in my fourth data chapter, I built a rain simulator to create three on-farm nitrogen budgets. Significant nitrate losses to groundwater occurred on-farm, likely reaching the shallow aquifer and creating a legacy nitrate leaching problem. Soil N2O emissions were highly variable, representing between 1.9 and 29.2% of fertiliser lost depending on soil moisture conditions; This is far higher than the expected IPCC default values (1.3 to 1.9%). Overall, this thesis expands nitrogen cycling knowledge in subtropical Australia, revealing direct (soil) and indirect (waterways) N2O emissions, groundwater and surface water nitrogen losses, as well as nitrogen attenuation in both natural and constructed systems. My work encourages scientists, managers, farmers, and government to collaborate to optimise fertiliser use. The clear evidence for large nitrogen losses highlights opportunities to reduce nitrogen use to create both economic and environmental outcomes without reducing farm productivity.

Managing nitrate losses and nitrous oxide emissions from streams in heavily fertilised catchments: from problem to solution

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Managing nitrate losses and nitrous oxide emissions from streams in heavily fertilised catchments: from problem to solution

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