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

我们假设，到2050年，为养活30亿人而增加的农业化肥将使土壤中的磷(P)饱和，并创造条件，使随后流入的氮(N)调节发达地区的水质。然而，与绿色革命不同的是，未来的氮肥将使用尿素进行。1960年以前很少使用尿素，现在尿素占全球氮肥的50%，在世界各地的湖泊中随处可见，对水生生态系统有潜在的有害影响，但人们对此知之甚少。鉴于目前约10亿人缺乏清洁水，我们寻求建立一个严格的理论基础，以了解这些全球变化对人类和淡水的后果。我们的计划将通过实施3个综合项目，为未来的水质退化开发一个新的范例。首先，我们将把明确的土地利用模型与湖泊和河流参数的详细测量相结合，以产生一个新的生物地球化学模型，说明尿素是如何在水生生态系统中产生、销毁和运输的。其次，我们将使用~60,000-L实验和古生态(化石)分析作为全生态系统施肥的替代，以确定富P湖泊N效应的化学阈值。在这里，在磷含量不同的湖泊中，非常大的开放式和封闭式围栏将被施以N肥，以确定N损害的阈值，并量化沉积物在调节地表水反应中的作用。通过对20个湖泊中沿磷含量梯度的氮流入和藻类丰度之间的过去关系的重复化石分析，将检验阈值与整个湖泊的相关性。第三，我们将利用全流域N分流的优势，通过实验演示消除富磷湖泊的N污染如何调节生态系统的生产和毒性。在这里，我们将结合景观尺度的质量平衡(N、P、C)和一个独特的20年生态研究计划，最终确定N分流是否能使富P湖泊恢复到基线条件。这些项目应该解决长达40年的科学争论，并为管理者、科学家、政府和公众提供一种避免未来水质损失的手段。

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.