A new dynamic for Phosphorus in RIverbed Nitrogen Cycling - PRINCe

人类已经学会了如何操纵和利用维持地球生命的元素（碳- C、氮- N和磷- P）。事实上，我们在这方面已经变得如此熟练，以至于我们几乎将可用于种植作物的固定氮(N)的数量增加了一倍。按照目前的农业实践，没有它，我们根本无法维持人口。然而，这种对氮的利用给环境带来了相当大的代价，特别是河流、河口和沿海海域，影响了它们作为生态系统的质量和价值。例如，高氮负荷可以作为农田径流进入河流，导致藻华、氧气消耗和它们的普遍恶化。河床可以自然地减少这些高氮负荷，从而在全球范围内提供重要的“生态系统服务”，不仅对河流，而且对它们排入的河口和沿海海洋。因此，河床被认为是氮循环的热点，在一个被称为反硝化的过程中，将约40%的氮径流转化回惰性的大气氮气（N2）。在这里，生活在河床无氧地带的特殊细菌（反硝化细菌）通过一些中间体将N转化为硝酸盐，转化为氮气。这一过程的硝酸盐要么来自陆地径流，要么来自另一种称为硝化作用的微生物驱动过程。硝化作用仅在氧合良好的环境中起作用，并通过亚硝酸盐将氨转化为硝酸盐。直到最近，硝化和反硝化之间的耦合一直是关于河流中固定氮如何被去除的共识观点。然而，我们的研究结果表明，另一个过程也可能对整个生态系统服务至关重要。在氮气生产中，这种替代反硝化的过程被称为厌氧氨氧化（anammox），其中亚硝酸盐和氨更简单地转化为氮气。直到最近，厌氧氨氧化才被认为在含氧良好的河流中没有任何重要性。然而，我们的工作已经表明，厌氧氨氧化在透水性河床（砾石和砂床）中最重要，贡献了高达58%的N2产量，而在不透水性粘土中只有7%。这是非常令人惊讶的，完全不符合目前对河流的功能和控制和调节自然界厌氧氨氧化活性的因素的认识。我们现在还可以证明，完全硝化成硝酸盐（生态系统N保护）或氧化成N2气体（生态系统N损失）的铵的比例似乎取决于磷(P)。在磷含量较高的地方，更多的铵被回收为硝酸盐，而在磷缺乏的地方，更大一部分以氮气的形式损失——尤其是通过厌氧氨氧化。最后，尽管我们知道人类产生的氮和磷都导致了全球富营养化问题——基本上是水中植物生长过多——在这里，我们提出了一种新的磷的拮抗作用，并提出以下问题：1。通过支持氨氮完全硝化成硝酸盐，磷的有效性是否积极地帮助保存生物有效氮而不是将其去除到惰性N2气体中？2. 从淡水中去除磷的管理方案是否具有直接和间接的效益，从而降低P积极促进固定N的去除？目前，P在去除或保持固定N方面的作用是未知的，这是我们新的“蓝天”提议的主要推动力。这些可渗透的河床就像天然的生物催化过滤器，承载着微生物群落，它们协同作用，有效地去除固定氮。为了充分理解和利用这一点，我们需要问谁是主要的微生物，它们是如何相互作用的，是什么调节了它们的活动？这些是我们希望在项目中解决的关键问题。这种理解可以转化为更有效的废水处理过程和开发最佳操作实践，以更好地控制过程和水资源的一般管理。

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.