A new dynamic for Phosphorus in RIverbed Nitrogen Cycling - PRINCe

Lead Research Organisation: University of Essex
Department Name: Life Sciences

Abstract

Humans have learnt how to manipulate and harness the elements that sustain life on Earth (Carbon - C; nitrogen - N and phosphorus - P). Indeed, we have become so skilled at this that we have practically doubled the amount of fixed nitrogen (N) available to us to grow crops and, with current farming practices, we simply couldn't sustain the human population without it. This harnessing of N has come at a considerable cost to the environment, however, particularly rivers, estuaries and coastal seas, where it affects their quality and value as ecosystems. For example, high N loads can enter rivers as run-off from agricultural land to cause algal blooms, oxygen depletion and their general deterioration. Riverbeds can naturally reduce these high N loads and thus provide an important "ecosystem service" globally, not only for the rivers - but also for the estuaries and coastal seas into which they drain. Consequently, riverbeds are recognised hotspots of N cycling, converting ~40% of N-runoff back to inert, atmospheric nitrogen gas (N2) in a process known as denitrification. Here, specialized bacteria (denitrifying bacteria), living in oxygen-free zones of the riverbed, convert N as nitrate, via a number of intermediates, to N2 gas. The nitrate for this process is provided either from terrestrial run-off or from another microbial driven process called nitrification. Nitrification is only active in well-oxygenated environments and converts ammonia to nitrate - via nitrite. This coupling between nitrification and denitrification was, until recently, the consensus view on how fixed N was removed in rivers. However, our findings suggest that another process may also be essential for this overall ecosystem service.

This alternative process to denitrification in N2 production is known as anaerobic ammonium oxidation (anammox), whereby nitrite and ammonia are converted more simply to N2 gas. Up until recently, anammox was not considered to be of any importance in well oxygenated rivers. However, our work has already shown that anammox is of greatest significance in permeable riverbeds (gravel and sand-beds), contributing up to 58% of N2 production, and compared to only 7% in impermeable clays. This is very surprising and completely at odds with present knowledge on the function of rivers and factors governing and regulating anammox activity in nature. We can also now demonstrate that the fraction of ammonium that is either fully nitrified to nitrate (ecosystem N conservation) or oxidised to N2 gas (ecosystem N loss) appears to be dependent on phosphorus (P). Where P is higher, more ammonium is recovered as nitrate and where P is scarce a greater fraction is lost as N2 gas - particularly through anammox.

Finally, whereas we know that both human derived N and P contribute to the global problem of eutrophication - basically too much plant growth in water - here we are proposing a new antagonistic effect of P and ask whether: 1. By supporting complete nitrification of ammonium to nitrate, does the availability of P actively help to conserve bioavailable N over its removal to inert N2 gas? 2. Could management schemes aimed at removing P from freshwater have both direct and indirect benefits, whereby lowering P actively promotes the removal of fixed N? Currently the role of P in relation to the removal or conservation of fixed N is unknown and that is the main thrust of our new, 'blue-skies' proposal. These permeable riverbeds function as natural biocatalytic filters, hosting microbial communities that, in concert, efficiently remove fixed N. To fully understand and exploit this we need to ask who the main microbes are, how they interact and what regulates their activity? These are the key questions we wish to address in our project. Such understanding could be translated into more efficient wastewater treatment processes and the development of operational best practice for better process control and general management of water resources.

Planned Impact

Who will benefit?
This Blue-Skies science is seeking to understand the fundamental, microbiological interaction between P and the final fate of fixed N in riverbeds. Ultimately, the project will benefit many end-users in both private & public sectors e.g. Defra, Environment Agency (EA), CEFAS, Natural England, wastewater companies, water authorities, landfill management, landowners, agricultural and farming sector. Learning how changes in P ultimately influence the attenuation of fixed N will enable these organisations to save resources by a targeted management of catchments. It will be a source of information for policy advisors and scientific researchers (e.g. biogeochemists, microbial ecologists). The project will produce two trained PDRAs with molecular ecology and biogeochemical skills who can enter private/public sector.

How?
Rivers convert ~40% of terrestrial N-runoff per year to atmospheric N2. Yet, this ecosystem service is modelled and conceptualised as N2 production through text-book denitrification. Anammox alters this perception and our supporting data suggest even greater unknowns in relation to phosphate. This novel research will determine linkages between riverbed nitrification, anammox and denitrification to provide a completely new paradigm for riverine ecosystem services.

Enhances quality of life, health & environment and the effectiveness of public services & policy:
1. Data on N transformations and affiliated microbes under perturbed phosphate will inform Defra's policies on the impact of nutrient pollution on the environment (e.g. Nitrates Directive, Water Framework Directive, National Emissions Ceilings Directive).This will also benefit the Environment Agency (EA) regions with agricultural catchments and inform policy of Urban Wastewater Treatment Directive, Habitats Directive. This will help to meet the Government's goals for protecting & sustaining natural resources.
2. Data on how different agricultural management practices influence N transformations will give added value to Defra's Demonstration Test Catchments (DTC) program by generating nutrient data from sites in the catchment not currently monitored by Defra.
3. Data on spatial & temporal scales of NH4+, NO2-, NO3- transformations & whether interactions with P increase denitrification will inform Defra's strategy on N2O emissions enabling improved mitigation strategies for reducing pollution & greenhouse gas emissions (Low C Transition Plan, Climate Change Act and environmental air quality.
4. Data on N transport will benefit Cefas via the Marine Strategy Framework which manages sustainable marine resources.
5. Microbial diversity and transcriptomics data will inform Defra's policy on how diffuse pollution impacts on biodiversity and activity of microbes driving N cycling.
6. There is strong public engagement in the UK with all environmental matters.

Benefits to commercial/private sector:
1. Our new knowledge will benefit wastewater industries by paving the way to more efficient waste water reactors - such as those already in place in Europe e.g. the CANON process or Completely Autotrophic Nitrogen removal Over Nitrite. Understanding the role of P in the highly efficient, riverbed coupling between nitrification and the production of N2 gas could be translated into more efficient reactors for wastewater treatment.
2. Both sewage treatment companies and the farming industry cover many of the costs of mitigating diffuse nutrient loads in rivers. Learning how changes in phosphate influences riverine N cycling will enable these organisations to save resources by targeting their actions specifically on those aspects of N cycling that will have the greatest benefit.

Economic benefits: Ultimately, IP from the project will foster industrial collaborators and enhance economic competitiveness through more efficient waste water treatment and targeted management.

Timescale to realization: In the realm of 10-15 years.

Publications

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