Factors controlling nitrogen cycling in freshwater ecosystems

人类活动严重改变了全球氮循环，增加了循环中的生物活性氮量，导致进入水生生态系统的氮量显著增加。这导致了严重的环境问题，包括饮用水质量下降、鱼类栖息地退化、生物多样性丧失和有毒藻类的繁殖。影响氮进入湖泊数量的因素已经很好地表征了，然而氮一旦进入湖泊的命运却知之甚少。尽管全球氮模型表明湖泊可能是氮保留的重要场所，但令人惊讶的是，人们对保留的氮的命运知之甚少。反硝化作用，单独或与硝化作用相结合，是一种重要的细菌转化，将溶解的无机形式的氮转化为可从水生生态系统释放到大气中的挥发性气体。反硝化的最终产物二氮（N2）是一种有利的转化，而反硝化和硝化的副产物一氧化二氮（N2O）是一种强效温室气体，其变暖潜能值是二氧化碳的300倍，却不是。湖泊中N2O生成的调控因子及N2O: N2的相对比值尚不清楚。本研究将侧重于估计加拿大湖泊N2O气体排放的产生和调节因素。其目的是确定微生物N转化的“热点”和“热点时刻”，如反硝化作用，相对于更有利的转化为N2，产生或多或少的不良温室气体N2O。这项研究将描述湖内发生氮过程的地点，并比较具有不同物理、化学和生物特征的湖泊，以确定氮处理的类型和速率在时间和空间上的模式。工作还将扩展到研究氮循环微生物的多样性，包括固氮蓝藻。这项工作的结果可以用于区域和更大尺度的模型，目的是减少N2O排放和管理景观水平的N保留，从而保持加拿大淡水的质量和完整性。

Human activities have seriously altered the global nitrogen (N) cycle, increasing the amount of biologically reactive N in circulation and leading to a significant increase in the amount of N transported into aquatic ecosystems. This has resulted in serious environmental problems including reduced quality of drinking water, degraded fish habitat, biodiversity loss, and the proliferation of toxic algae. Factors influencing the amount of N entering lakes are fairly well characterized however the fate of N once it enters a lake is poorly understood. Although global N models suggest that lakes may be important sites of N retention, there is surprisingly little understanding as to the fate of that retained N. Denitrification, alone or coupled with nitrification is an important bacterial transformation that converts dissolved inorganic forms of N into volatile gases that can be released into the atmosphere from aquatic ecosystems. Dinitrogen gas (N2), the end product of denitrification, is a favoured transformation while nitrous oxide (N2O) a by-product of both denitrification and nitrification is a potent greenhouse gas with a warming potential 300 times greater than CO2, is not. Factors regulating the production of N2O and the relative ratio of N2O: N2 in lakes are poorly understood. This research will focus on estimating the production of and the factors regulating N2O gas emissions from Canadian lakes. The intention is to identify "hot spots" and "hot moments" of microbial N transformations, like denitrification, that produce a greater or lesser amount of the undesirable greenhouse gas, N2O, relative to the more favorable transformation to N2. This research will characterize where within the lake N processes are taking place and compare lakes with different physical, chemical and biological characteristics to identify patterns in time and space as to the type and rates of N processing. Work will also be extended to look at the diversity of N cycling microorganisms, including N-fixing cyanobacteria. Results from this work could serve in regional and larger scale models with the objective of reducing N2O emissions and managing N retention at the landscape level thus preserving the quality and integrity of our Canadian freshwaters.