Logo image
Back
Anthropogenic nitrogen fate from headwaters to estuaries
Thesis   Open access

Anthropogenic nitrogen fate from headwaters to estuaries

Rebecca Woodrow
Southern Cross University
Doctor of Philosophy (PhD), Southern Cross University
2025
DOI:
https://doi.org/10.25918/thesis.585
Appears in  Recent Southern Cross PhD Theses
pdf
Woodrow R PhD 202510.28 MBDownloadView
Open

Metrics

1 Record Views

Abstract

Nitrogen Nitrogen cycling Nitrous oxide Nitrogen remediation
Anthropogenic nitrogen (N) pollution impacts water, air, and soil quality. River systems are important conduits of nutrients between terrestrial, aquatic, and atmospheric environments. Currently, there is a global effort underway to better understand sources, pathways and sinks of N in coastal waters, particularly in regions with increasing horticultural activity. Here, I examine how N is processed from coastal headwaters to estuaries. I also investigate woodchip bioreactors as a solution to manage N pollution in regions with environmental impact from intensive horticulture. I quantify N2O in a headwater stream, investigate drivers including 222Rn as a tracer of groundwater, and contextualise the data through conversion to CO2 equivalents and emissions factors. I utilise Plutonium (239+240Pu) dating methods to quantify historical N retention, identify sources using stable isotopes (δ13C and δ15N), and contextualise N burial in relation to the N budget. I examine the N removal efficiency of two distinct woodchip bioreactor designs, the drivers of denitrification, and whether N removal is significantly offset by pollution swapping to N2O and thus negating the environmental benefit as an aquatic N pollutant mitigation strategy. My first data chapter assesses N2O over diel cycles and during flood events over 62 days of high-frequency continuous observations in a horticulturally impacted sub-tropical headwater stream. Overall nocturnal N2O emissions were 32% greater than daytime. Failing to capture emissions at night and during flooding would underestimate N2O by 21%. My analysis has implications for global estimates of greenhouse gas emissions from impacted headwater streams. The second data chapter provide the first estimates of mangrove N burial rates across a latitudinal gradient. I contrast burial rates to other previously quantified N pathways. From a budget perspective, N burial was the equivalent of ~22 and ~35% of the porewater and surface water exports respectively. N2O uptake was the equivalent of between 0.03 and 1.1% of N burial. My analysis fills gaps in literature on mangrove nutrient burial. My final data chapter investigates NO3--N removal within two woodchip bioreactor configurations. The NO3--N removal efficiency was ~34% within the inline pipe bioreactor, and ~85% within the bed bioreactor. The percentage of NO3--N converted to N2O in both bioreactors was well below the IPCC default EF5r, indicating that neither bioreactor was significantly pollution swapping. Overall, my thesis adds data towards improving global budgets for N2O and N burial rates and also informs sustainable N management in Australia’s emerging protected cropping industry.

Details

Logo image