Impacts of nitrogen deposition on the forest carbon cycle: from ecosystem manipulations to national scale predictions

化石燃料的燃烧和农业中化肥的使用增加了大气和生物圈中氮化合物的含量。更多的大气氮被人类活动转化为活性氮，而不是所有自然过程的总和。氮沉降的这种历史性的惊人增加造成了若干严重的环境问题，例如土壤酸化和氮饱和、水道中的硝酸盐沥滤、富营养化和自然生态系统组成的变化。在其最极端的形式，它也可以导致植物死亡。增加的氮也有助于在对流层形成活跃的臭氧（在那里它是有害的），并产生温室气体甲烷和一氧化二氮。为了部分抵消所有这些负面影响，氮沉降也被认为可以增加森林的碳固存，这有助于减缓气候变化。然而，污染物提供的这种间接积极服务的大小是不确定的，许多估计表明，这种影响相当小。然而，在《自然》杂志上的一篇论文中，我们报告了新的数据，表明这种影响对北方和温带地区的森林非常大。这项工作在社区中提出了几个问题，并引发了一场关于这种影响的真实的程度的辩论。在这个项目中，我们建议通过两个互补的测试来测试我们的发现是否经得起科学的审查：a）一个实验性的操作，人为地提高了一个实验森林的氮沉积水平，我们在过去的10年里一直在测量碳封存。过去已经进行了许多施肥实验，但我们认为，它们并没有真实地模拟氮沉积的过程，因为当肥料直接施在地面上时，氮与树冠相互作用的许多方式都被完全消除了（氮沉积发生在树冠上方，要么溶解在降雨中，要么作为树叶上的干沉积）。B）在英国各地已知的氮沉降区域梯度上树木生长的比较观察。令人惊讶的是，使用第二种方法进行的经验性观察很少，假设气候，土壤肥力等的变化会掩盖氮沉降的任何残余影响。然而，现有的文献强烈表明，高水平的氮沉降增加了树木生长在世纪在欧洲大部分地区，在支持我们的理论。最后，我们将使用三个模型来比较我们的观察结果与模型预测。这也将有助于确定可能促进高速率碳封存以响应氮沉积的机制。

Combustion of fossil fuels and use of fertilisers in agriculture has increased the amount of nitrogen compounds present in the atmosphere and the biosphere. More atmospheric nitrogen is converted into reactive nitrogen by anthropogenic activities than by all natural processes combined. This phenomenal historical increase in nitrogen deposition is responsible for several serious environmental problems, such as soil acidification and nitrogen saturation, nitrate leaching in water courses and eutrophication and changes in the composition of natural ecosystems. In its most extreme forms, it can also cause plant mortality. The added nitrogen also contributes to forming active ozone in the troposphere (where it is damaging) and to producing the greenhouse gases methane and nitrous oxide. To partially counterbalance all these negative effects, nitrogen deposition is also known to increase the sequestration of carbon by forests, which helps to slow down climate change. However, the magnitude of this indirect positive service provided by a pollutant is uncertain, with many estimates suggesting that the effect if rather small. In a previous paper in Nature however, we reported new data showing the effect to be very large for forests of the boreal and temperate regions. This work raised several questions in the community and stimulated a debate which is still ongoing on the real magnitude of this effect. In this project we propose to test whether our findings stand to scientific scrutiny by using two complentary tests: a) an experimental manipulation which artificially elevates the levels of nitrogen deposition over one experimental forest, where we have been measuring carbon sequestration for the past 10 years. Many fertilisation experiments have been carried out in the past, but we contend that they did not realistically simulate the process of nitrogen deposition, as the many ways that nitrogen can interact with the canopy of trees are entirely eliminated when the fertiliser is applied directly on the ground (nitrogen deposition occurs from above the canopy either dissolved in rainfall or as dry deposition over the foliage). b) comparative observations of tree growth across known regional gradients in nitrogen deposition across the UK. Surprisingly few empirical observations have been carried out using this second method, on the assumption that changes in climate, soil fertility, etc. would mask any residual effect due to nitrogen deposition. However, the available literature strongly suggests that high levels of nitrogen deposition have increased tree growth during the 20th century across much of Europe, in support of our theory. Finally, we will use three models to compare our observations against the model predictions. This will also help to determine the mechanism(s) which may promote high rates of carbon sequestration in response to nitrogen deposition.