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。由人为活动转化为活性氮的大气氮比所有自然过程的总和还要多。氮沉积的这种历史上的显著增加导致了几个严重的环境问题，如土壤酸化和氮饱和、水道中的硝酸盐浸出和富营养化以及自然生态系统组成的变化。在最极端的情况下，它还会导致植物死亡。增加的氮还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，氮沉降还可以增加森林对碳的固存，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》杂志之前的一篇论文中，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程度的辩论，这场辩论仍在进行中。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能经受住科学的检验：a)一个实验操作，人为地提高一个实验森林的氮沉降水平，我们在过去10年里一直在测量这个森林的碳固存。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，因为当直接在地面施用肥料时，氮与树木冠层相互作用的许多方式完全被消除了（氮沉积显然是从冠层上方发生的，要么溶解在降雨中，要么作为干沉积在叶子上）。b)在英国各地氮沉降的已知区域梯度中树木生长的比较观测。令人惊讶的是，使用第二种方法进行的经验观察很少，这种方法假设气候、土壤肥力等的变化会掩盖氮沉积造成的任何残留效应。然而，现有的文献有力地表明，在20世纪，高水平的氮沉积促进了欧洲大部分地区的树木生长，这支持了我们的理论。最后，我们将使用三个模型来比较我们的观测结果与模型预测。这也将有助于确定响应氮沉降可能促进高碳固存率的机制。

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 obviously 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 mechaism(s) which may promote high rates of carbon sequestration in response to nitrogen deposition.