Landscape-scale effects of nitrogen on lake ecosystems: Accelerating a new paradigm for freshwater eutrophication.

我们假设，到2050年，为了养活30亿人口，需要增加农业施肥，这将使土壤中磷(P)饱和，并为随后流入的氮(N)调节发达地区的水质创造条件。然而，与绿色革命时期不同的是，未来的氮肥将使用尿素。尿素在1960年之前很少使用，现在占全球氮肥的50%，在世界各地的湖泊中无处不在，对水生生态系统有潜在的有害影响，但人们对其知之甚少。鉴于目前约有10亿人缺乏清洁水，我们寻求建立一个严谨的理论基础，以了解这些全球变化对人类和淡水的影响。

We hypothesize that increases in agricultural fertilization needed to feed 3 billion more people by 2050 will saturate soils with phosphorus (P) and create conditions in which subsequent influx of nitrogen (N) regulates water quality in developed regions. However, unlike during the Green Revolution, future N fertilization will be conducted with urea. Rarely used prior to 1960, urea now accounts for >50% of global N fertilizer, is ubiquitous in lakes worldwide, and has potentially detrimental but poorly understood effects on aquatic ecosystems. Given that ~1 billion people currently lack clean water, we seek to develop a rigorous theoretical basis to understand the consequences of these global changes for humans and freshwaters. Our program will develop a novel paradigm for future water quality degradation by conducting 3 integrated projects. First, we will combine explicit land-use models with detailed measurements of lake and stream parameters to produce a new biogeochemical model of how urea is created, destroyed and transported in aquatic ecosystems. Second, we will use ~60,000-L experiments and paleoecological (fossil) analyses as a surrogate for whole-ecosystem fertilization to identify chemical thresholds for N effects on P-rich lakes. Here, very large open- and closed-bottom enclosures in lakes of contrasting P content will be fertilized with N to define thresholds for N damage and quantify the role of sediments in regulating the response of surface waters. The relevance of thresholds to whole lakes will be tested by replicated fossil analysis of the past relationship between N influx and algal abundance in 20 lakes along a gradient of P content. Third, we will take advantage of a whole-catchment N diversion to demonstrate experimentally how elimination of N pollution from P-rich lakes regulates ecosystem production and toxicity. Here we will combine landscape-scale mass balances (N, P, C) with a unique 20-year ecological research program to determine definitively whether N diversion can return P-rich lakes to baseline conditions. These projects should resolve a 40-yr long scientific debate and provide managers, scientists, governments, and the public with a means to avoid future water-quality loss.