Ecological effects of reduced versus oxidised nitrogen deposition

20世纪下半叶，欧洲大气中氮化合物的沉积急剧增加。在同一时期，各种植被类型的物种组成发生了重要变化，许多特征物种的消失无法用管理变化等因素来解释。从20世纪90年代初开始，有证据表明氮沉降的增加与物种组成的某些变化有关。现在人们普遍认识到，过去50年来大气氮沉降的增加是各种生态系统中许多特色植物物种急剧减少的一个主要因素。氮沉降可以通过增加土壤的酸度来起作用，因为像钙这样的碱性阳离子被释放并从系统中浸出；这可能导致植物中碱性阳离子的缺乏。它还增加了土壤中可用氮的量，这一过程通常被称为富营养化。已知碱阳离子缺乏和富营养化都是特征物种减少的机制。氮(N)沉积主要由两种形式组成：还原性氮，主要来自动物粪便和粪便，以及氧化性氮，主要来自燃料燃烧。重要的是要了解哪些形式的氮具有最大的生态影响，以便控制排放和保护敏感物种的政策是有效的，特别是在整个欧洲的还原和氧化氮沉积速率存在很大的空间差异的情况下。到目前为止，很少有实验或实地研究恰当地比较了还原和氧化N沉降的不同影响，甚至更少考虑到与土壤酸度相互作用的重要性。然而，最近的研究表明，还原而非氧化氮的沉积增加对敏感生态系统中的许多植物物种是有害的，特别是当这些生态系统已经酸化时。本研究的目的是：(i)确定近几十年来北欧植物物种组成的变化是否与大气中还原氮（而非氧化氮）沉积到敏感生态系统的差异有关；（ii）找出导致氮沉积减少和/或还原/氧化氮比率升高的机制，在土壤的碱性阳离子水平较低时产生更大的影响；（iii）利用这些信息确定受氮沉降威胁最大的生境、物种和地点。我们将结合实验室和实地实验，以及来自英国和荷兰实地调查的数据分析来实现这些目标。

Summary Atmospheric deposition of nitrogen compounds has increased dramatically in Europe in the 2nd half of the 20th century. Over the same period, important changes in species composition of various vegetation types, and the loss of many characteristic species, have occurred which cannot be explained by factors such as changes in management. From the early 1990s onwards, evidence was found that increased nitrogen deposition was associated with some of these changes in species composition. It is now widely recognised that the increased atmospheric nitrogen deposition over the past 50 years is a major factor in the sharp decline of many characteristic plant species from a variety of ecosystems. Nitrogen deposition can act by increasing the acidity of soil, because base cations such as calcium are released and leach out of the system; this can lead to base cation deficiencies in plants. It also increases the amount of available nitrogen in the soil, a process which is often referred to as eutrophication. Both base cation deficiency and eutrophication are known to be mechanisms involved in the decline of characteristic species. Nitrogen (N) deposition is largely composed of two forms: reduced N, which is emitted mainly from animal waste and manure, and oxidised N which is emitted mainly from fuel combustion. It is important to understand which of these forms of nitrogen has the greatest ecological impacts, so that policies to control emissions and protect sensitive species are effective, especially as there are large spatial differences in the rates of reduced and oxidised N deposition across Europe. Very few experimental or field studies up to now have properly compared the different effects of reduced and oxidised N deposition, and even fewer have considered the importance of interactions with soil acidity. However, recent research has shown that elevated deposition of reduced, but not oxidised, N is detrimental for many plant species from sensitive ecosystems, especially when these ecosystems have already become acidified. The aims of this study are:- (i) to determine whether differences in the atmospheric deposition of reduced, rather than oxidised, nitrogen to sensitive ecosystems accounts for the changes in plant species composition seen in northern Europe in recent decades; (ii) to identify the mechanisms which lead to reduced nitrogen deposition, and/or high reduced/oxidised N ratios, having a greater impact when soils have low levels of base cations; and (iii) to use this information to identify habitats, species and locations at greatest risk from N deposition. We will use a combination of laboratory and field experiments, and data analysis from field surveys in both the UK and The Netherlands, to meet these aims.