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

摘要泥炭和其他有机土壤提供了全球主要的碳(C)储存，不断生长的泥炭为大气中的二氧化碳提供了持续的汇，通过光合作用固定并作为缓慢分解的有机物质储存。生长在泥炭生态系统中的植物具有对低水平基本营养氮的耐受性，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极大地增加了向大气中排放的污染物氮(N)，并随后向这些生态系统沉积。这种增加的氮素供应已被证明导致适应低氮条件的物种被其他能够更好地利用增加的养分可获得性的物种所取代，降低了生物多样性，并可能威胁到泥炭地作为碳汇的功能。长期而言，“氮饱和”(氮素供应超过生物需求)也可能发生，过量的硝酸盐被淋溶到地表水中，从而导致酸化和富营养化。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)以及由此从酸化中恢复，许多生态系统也正在经历戏剧性的化学变化。事实上，我们对氮沉积升高下的氮循环的全部了解，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，这些生态系统也受到酸化硫沉积的影响。我们认为，这些体系的C和N循环都受到酸度变化的强烈影响，因此目前观察到的行为可能在未来发生变化。具体地说，我们认为，由于生物产量的增加和DOC的溶解性增加，pH的上升将增加碳和氮作为溶解有机物(DOM)从系统中的损失。氮素的这种损失，再加上不断增长的微生物生物量对氮素的需求增加，以及植物生产力的提高，将导致氮循环收紧，NO3的泄漏减少。氮素循环的这种紧缩性可能会减少高氮沉积土壤中硝酸盐的损失，甚至可能完全阻止低氮沉积地区的硝酸盐损失。我们将在英国泥炭生态系统中进行一系列现实的、基于田间的实验来验证我们的假设，这些生态系统暴露在历史上相对较低和较高的氮污染水平。在每个实验中，我们将在两年的时间里控制土壤的pH值，在此期间，我们将测量酸度的变化对气态和溶解状态下C和N损失的影响。我们还将利用先进的同位素示踪技术和酶测量相结合的方法，测量酸度变化对一系列关键生态系统过程的影响，例如N在有机和无机形态之间的转化、分解以及大气中的二氧化碳被吸收到不同的有机质储存中。我们将利用这些结果来支持碳和氮循环模型的开发，该模型与大多数现有模型不同，完全纳入了酸度变化的影响，该模型将用于预测生态系统碳和氮平衡的未来变化。我们相信，这项研究可以极大地完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，随着生态系统变得不那么酸性，它们将变得不那么容易受到氮饱和(以及由此造成的环境破坏)的影响。同时，随着pH的升高(以及氮的升高)，增长速度的提高可能会为这些重要生态系统内的大气二氧化碳封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.