Research papersNatural attenuation of large anthropogenic nitrate loads in a subtropical stream revealed by δ15N and δ18O
Introduction
The global use of fertilisers and local sewage disposal has altered the aquatic nitrogen (N) cycle (Boynton et al., 1995, Van Meter et al., 2016, Vitousek et al., 1997). Changes in land use, principally through agriculture and deforestation, have dramatically increased aquatic nitrogen loads (Martin et al., 2004, Schilling and Libra, 2000, Van Herpe and Troch, 2000). Nitrogen applied via fertilisers accumulates in soils at rates between 25 and 70 kg N ha yr−1 (Van Meter et al., 2016), often during low rainfall periods and escapes to nearby creeks during or after rainfall (Vink et al., 2007). In estuaries and coastal marine systems, nitrogen is often the limiting nutrient (Fabricius, 2005, Redfield, 1934, Smith et al., 2003). Therefore, after heavy rain events when large fluxes of nitrogen enter coastal waters, rapid eutrophication, algal blooms, hypoxia and fish kills can occur (Howarth, 1988, Howarth et al., 1996, Nixon et al., 1996).
The legacy of agricultural nitrate (NO3−) storage in soils (largely from fertilisers), flushing, and leaching in different soil types is beginning to be better understood. Nitrogen stored in horticultural soils can flush to surface waters following rain events via overland runoff and groundwater seepage (Creed and Band, 1998, Follett and Hatfield, 2001). Downstream waters receive elevated nutrient inputs for years to decades after the termination of agricultural practices upstream, due to soil nutrient storage and leaching (Frei et al., 2020, Grimvall et al., 2000, McCrackin et al., 2017). Long-term legacy eutrophication has been reported in northern hemisphere temperate systems (Grimvall et al., 2000, McCrackin et al., 2017). The predicted recovery time of receiving waters from the stresses of anthropogenically-induced algae blooms and eutrophication are variable, ranging from <1 year to >100 years after termination of horticulture in the catchment (McCrackin et al., 2017).
The diminishing availability of water for irrigation during drought periods creates a demand for other sources of irrigation water. The consistent outflow of greywater (recycled treated water from sewage treatment plants) from sewage effluent has become an attractive source of irrigation water in many areas worldwide (Alghobar and Suresha, 2017, Guo et al., 2017, Pereira et al., 2011, Tarchouna et al., 2010). As greywater typically contains high amounts of nitrogen and other nutrients, it is commonly used to reduce fertiliser use on crops (Khajanchi et al., 2015). However, adding greywater in addition to routine fertiliser application practices may produce excess nitrogen in soils, unbalancing the crop demand whilst not increasing production (Gu et al., 2016, Ju et al., 2009). Therefore, there is a need to consider the form and concentrations of nitrogen in greywater related to crop demand. Applying a high NO3− concentration greywater to a crop that preferentially uptakes NH4+, such as blueberries, can enhance excess nitrogen in soils that is not utilised by the target crop (Merhaut and Darnell, 1995).
While land-use change has dramatically altered nitrogen cycling and loads in temperate systems, less is known about how tropical and subtropical systems respond to increasing nitrogen inputs. Warmer waterways are expected to remove larger amounts of reactive nitrogen via denitrification than temperate systems (Hajati et al., 2020, Wadnerkar et al., 2019), but these attenuation rates have not been quantified in detail. Understanding the sources and time scales of nitrogen delivery to creek systems can provide vital information for land use planning and restoration (Kaushal et al., 2011). The dual isotopic composition of NO3− (δ15N-NO3− and δ18O-NO3−) can provide insights into nitrogen sources and fates in receiving waters (Zhang et al., 2019). Each source of nitrogen in a catchment (i.e., sewage, fertiliser, animal manure, atmospheric deposition) often has a distinctive isotopic signature. As denitrification processes preferentially utilise the lighter isotopes 14N and 16O instead of the heavier isotopes 15N and 18O, a predictable kinetic fractionation can reveal nitrogen transformations or attenuation in receiving waters.
Here, we quantify creek NO3−-N loads, rainfall NO3−-N contributions, anthropogenic NO3−-N sources (recycled greywater and inorganic fertilisers) and natural NO3−-N attenuation in an Australian subtropical creek using a combination of high resolution temporal and spatial observations as well as stable isotopes (δ15N-NO3− and δ18O-NO3−). The studied system is experiencing rapid land-use change, providing an opportunity to better understand how modified nitrogen inputs may impact subtropical creeks in contrast to more investigated temperate systems. Observations were performed during a dry period and over subsequent rain events to assess how short-term high-intensity rainfall may affect the efficiency of in-stream NO3−-N attenuation and nitrogen flushing from agricultural soils.
Section snippets
Study site
Our investigations were performed in the subtropical Double Crossing Creek, a catchment draining to Hearnes Lake, Australia. Hearnes Lake (30°08′10.3″S, 153°11′51.0″E) is a culturally important natural asset on Gumbaynggirr Aboriginal Country and forms part of the highly diverse Solitary Islands Marine Park (SIMP). The region is experiencing rapid land-use change. Historic banana farming in the catchment has given way to blueberry farms and hothouse horticulture since the early 2000′s (Bevan,
Nitrate time-series
Our observations covered a dry summer with 244 mm of accumulated rainfall. During the 91 days of observations, two rain events >30 mm in a day significantly altered the flow regime and created spikes of discharge (Fig. 2). The time series dataset was classified into three hydrological stages: (1) Dry period with <30 mm rain per day (7th January to 29th March 2019); (2) First flush, the first rain event >30 mm in a day after the Dry period (30th March to 1st April 2019); and (3) Secondary flush,
Aquatic nitrogen loads versus catchment inputs
We estimated NO3−-N loss rates by comparing bottom-up estimates of nitrogen sources to the catchment and our calculated aquatic NO3−-N loads. The application of fertiliser across all industries (blueberries, tomatoes, cucumbers and bananas) and the specific amount of applied fertiliser by local farmers are poorly understood. We estimate that the catchment receives 15,877 kg N yr−1 delivered as fertiliser, based on the following assumptions: (1) Farmers apply 100 kg N ha yr−1 to bananas (Newley
Conclusions
We revealed effective nitrogen attenuation of large anthropogenic nitrogen loads in a subtropical creek. A clear shift in NO3−-N and flow was noted between the three hydrological events with greater loads and concentrations during wet conditions. A significant relationship between root zone soil moisture and creek NO3−-N concentrations indicated that soil-stored nitrogen was flushed when soil moisture exceeded 55% saturation. Stable isotopes (δ18O-NO3− and δ15N-NO3−) suggest that NO3−-N in the
CRediT authorship contribution statement
Shane A. White: Conceptualization, Methodology, Investigation, Data curation, Formal analysis, Writing - original draft. Stephen R. Conrad: Investigation, Formal analysis, Writing - review & editing. Rebecca L. Woodrow: Investigation, Formal analysis, Writing - review & editing. James P. Tucker: Investigation, Formal analysis, Writing - review & editing. Wei-Wen Wong: Methodology, Formal analysis, Writing - review & editing. Perran M. Cook: Resources, Formal analysis, Writing - review &
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This project was funded by the Coffs Harbour City Council’s Environmental Levy program and the Australian Research Council (FT170100327). This paper evolved from a report entitled “Nutrient transport and sources in headwater streams surrounded by intensive horticulture” submitted to the funding agency and forms part of SW PhD Thesis. Original data is available in supporting information.
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