A new dynamic for Phosphorus in RIverbed Nitrogen Cycling - PRINCe

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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.