Missing components of nitrogen cycling budgets across the United States

The problem

Human activities such as inefficient nitrogen fertiliser application have led to excess nitrogen concentrations and the continued degradation of coastal and fresh water around the globe (Figure 1). The effects of human activity on the nitrogen cycle are particularly strong across the United States of America, with the production of nitrogen-based fertiliser a major cause of the nutrient pollution that persists across the country. Although the associated environmental degradation is thought to cost the country billions of dollars a year and reducing human-derived nitrogen inputs is vital for restoring a functioning ecosystem, efforts such as those introduced by the Clean Water Act have often resulted in slower than anticipated water-quality improvements. Correctly identifying and estimating all processes that can act as sources or sinks of nitrogen is thought to be an important step in reducing inputs.

The science

The USA has one of the largest freshwater abstraction volumes per capita in the world, with major uses for fresh water including irrigation, thermoelectric power, and public water supply. For this project, we used publicly available datasets on countrywide withdrawal volumes and nitrate (NO₃) concentrations for both surface water and groundwater. We found that freshwater abstraction will temporarily retain any associated NO₃ in the abstracted water from the aquatic environment.

We estimated the abstraction NO₃ flux for the contiguous United States to be 417 kilotons of nitrate nitrogen per year (kt NO₃-N yr^-1) and found large disparities between county-level abstraction flux estimates (Figure 2). We assessed the significance of the national-level abstraction NO₃ flux estimate in the context of pre-existing US nitrogen budgets through comparison to other nitrogen budget components and found our estimate to be equivalent to 57 per cent of total denitrification estimates.

The results

Our research indicates that freshwater abstraction can act as a significant temporary retention mechanism, meaning that it temporarily delays the delivery of nitrogen from the land to the oceans, hence it should be considered when developing nitrogen budgets.

When considering this mechanism, it’s worth noting that leaking US water mains cause an average of 16 per cent of the water initially entering the distribution network to be lost the environment. We used publicly available data to estimate the release of NO₃ to the environment in association with this leakage at around 7 kt NO₃-N yr^-1 (across the contiguous United States). Although this estimate is insignificant to national-level nitrogen cycling, county-level fluxes vary greatly (Figure 3), with the magnitude of the flux having a positive correlation with urbanisation.

The localised significance of leakage-derived NO₃ is highlighted by the exceedance of these fluxes over agricultural fertiliser nitrogen inputs across some counties (Figure 4) and suggests that these fluxes should be incorporated alongside abstraction NO₃ fluxes within nutrient budgets and considered when developing nutrient-management strategies. Future work should aim to further resolve these fluxes, both across the United States and around the world.

My PhD research

As part of my PhD thesis, I recently published a paper on investigating the effects of freshwater abstractions and mains water supply leakage upon nitrogen cycling across the United States (Flint et al., 2022).

My ongoing research will investigate the potential for both mains water leakage and the use of phosphate-dosed water outdoors at domestic residences to act as sources of phosphorus across the United States. I will also be investigating the potential for the stable oxygen isotope composition of phosphate to identify phosphate-dosed drinking water as a source of phosphorus within the environment and to assess the processes affecting phosphate-dosed drinking water within water and waste-water networks.

Author

Elizabeth Flint (ORCID 0000-0002-5781-2523)

With thanks to my supervisors Matthew Ascott, Daren Gooddy, Ben Surridge and Mason Stahl.