Acidity controls on organic matter cycling and nitrogen saturation in organic soils.

泥炭和其他有机土壤提供了全球主要的碳（C）储存，而不断生长的泥炭为大气中的CO2提供了持续的汇，通过光合作用固定并储存为缓慢分解的有机物。在泥炭生态系统中生长的植物的特点是对低水平的必需营养素氮的耐受性，但在世界许多地区，包括欧洲和北美，集约化农业和化石燃料燃烧大大增加了污染物氮（N）的排放到大气中，并随后沉积到这些生态系统中。这种氮供应的增加已被证明会导致适应低氮条件的物种被其他更能利用增加的养分供应的物种所取代，从而使生物多样性退化，并可能威胁到泥炭地作为碳汇的功能。在较长的一段时间内，“氮饱和”（氮供应超过生物需求）也可能发生，过量的硝酸盐沥滤到地表沃茨，在那里它可能有助于酸化和富营养化。然而，虽然有机土壤生态系统目前正在从大气中积累氮，但由于硫沉积减少（由于控制硫排放的立法），许多生态系统也正在经历巨大的化学变化，并从酸化中恢复过来。事实上，我们对氮沉降升高下的氮循环的全部理解，以及我们对碳循环的理解的很大一部分，都是基于生态系统内的工作，这些工作也受到酸化硫沉降的影响。我们建议，这些系统的C和N循环正在强烈改变酸度的变化，因此，目前观察到的行为可能会改变在未来。具体来说，我们认为，pH值的上升将增加碳和氮的损失，溶解有机物（DOM）从系统中由于生物生产的增加，并增加这种DOC的溶解度。这种氮的损失，加上不断增长的微生物生物量对氮的需求增加，以及植物生产力的提高，将导致氮循环的收紧和NO3的泄漏减少。这种氮循环的收紧可能会减少氮沉降量高的土壤中硝酸盐的损失，甚至可能完全停止氮沉降量低的地区的硝酸盐损失。我们将测试我们的假设与一组现实的，实地为基础的实验，在英国泥炭生态系统已暴露于相对较低和较高的历史水平的N污染。在每个实验中，我们将在两年的时间内控制土壤pH值，在此期间，我们将测量酸度变化对气态和溶解态C和N损失的影响。我们还将测量酸度变化对一系列关键生态系统过程的影响，例如有机和无机形式之间的氮转化，分解以及大气CO2同化成不同的有机物质储存，使用尖端同位素示踪技术和酶测量相结合。我们将使用的结果，以支持C和N循环模型的发展，与大多数现有的模型，充分结合了酸度变化的影响，这个模型将被用来预测未来的生态系统碳和氮平衡的变化。我们相信，这项研究可以大大改善我们目前对泥炭生态系统储存大气中碳和氮的能力的理解。如果得到证实，我们的假设将意味着生态系统将变得不那么容易受到氮饱和（以及由此产生的环境破坏），因为它们变得不那么酸。与此同时，随着pH值的上升（以及氮的升高），生长率的增加可以为这些重要的生态系统中的大气CO2固存提供一个增强的汇。

Summary Peats and other organic soils provide a major global store of carbon (C), and growing peats provide a continuing sink for CO2 from the atmosphere, fixed through photosynthesis and stored as slowly-decomposing organic matter. The plants that grow in peaty ecosystems are characterised by a tolerance for low levels of the essential nutrient, nitrogen, but in many areas of world, including Europe and North America, intensive agriculture and fossil fuel burning have greatly increased the emissions of pollutant nitrogen (N) to the atmosphere, and subsequent deposition to these ecosystems. This increased nitrogen supply has been shown to lead to the displacement of species adapted for low-nitrogen conditions by other species better able to exploit increased nutrient availability, degrading biodiversity and potentially threatening the function of peatlands as a carbon sink. Over a prolonged period, 'nitrogen saturation' (nitrogen supply exceeding biological demand) can also occur, with excess nitrate leached to surface waters, where it can contribute to acidification and eutrophication. However, while organic soil ecosystems are currently accumulating nitrogen from the atmosphere, many are also undergoing dramatic chemical changes due to decreasing sulphur deposition (due to legislation to control sulphur emissions), and resulting recovery from acidification. Virtually our entire understanding of nitrogen cycling under elevated N deposition, and a significant part of our understanding of C cycling, is based on work within ecosystems that have also been impacted by acidifying sulphur deposition. We propose that both the C and N cycles of these systems are being strongly altered by acidity change, and therefore that currently observed behaviour may change in future. Specifically, we believe that rising pH will increase the loss of carbon and nitrogen as dissolved organic matter (DOM) from the system due to increases in biological production, and an increasing solubility of this DOC. This loss of N, coupled with increased demand for N by a growing microbial biomass, and increased plant productivity, will result in a tightening of the N cycle and less leakage of NO3. This tightening of the N cycle is likely reduce nitrate loss in soils subjected to high N deposition, and may even halt nitrate loss entirely from areas of lower N deposition. We will test our hypotheses with a set of realistic, field-based experiments in UK peaty ecosystems that have been exposed to relatively low and high historic levels of N pollution. In each experiment, we will manipulate soil pH over a two year period, during which time we will measure the effect of changing acidity on losses of C and N in gaseous and dissolved forms. We will also measure the effects of acidity change on a range of key ecosystem processes, such as the transformation of N between organic and inorganic forms, decomposition, and the assimilation of atmosperic CO2 into different organic matter stores, using a combination of cutting edge isotopic tracing techniques and enzyme measurements. We will use the results to support the development of a model of C and N cycling which, unlike most existing models, fully incorporates the effects of changing acidity, and this model will be used to predict future change in ecosystem carbon and nitrogen balances. We believe that this study could significantly refine our current understanding of the capacity of peaty ecosystems to store carbon and nitrogen from the atmosphere. If proven, our hypotheses would imply that ecosystems will become less susceptible to nitrogen saturation (and resulting environmental damage) as they become less acidic. At the same time, increased growth rates with rising pH (together with elevated nitrogen) could provide an enhanced sink for atmospheric CO2 sequestration within these important ecosystems.