Environmental drivers of CO2 fluxes from 'historical' soil organic matter and the implications for European carbon stocks

“历史”土壤有机质二氧化碳通量的环境驱动因素及其对欧洲碳储量的影响

• 批准号: NE/G018944/1
• 负责人: David Robinson
• 金额: $ 67.75万
• 依托单位: University of Aberdeen
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2009
• 资助国家: 英国
• 起止时间: 2009 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FG018944%2F1
• 关键词: Environmental; drivers; CO2; fluxes; historical

Predictions about how Earth's climate will change depend on good information about how climate influences, and is influenced by, the amount of CO2 in the atmosphere. The atmosphere's CO2 content is the balance between CO2 production (mainly from respiration and from biomass and fossil fuel burning) and CO2 consumption (mainly by photosynthesis). Photosynthesis and respiration rates vary with climate: they slow down when it becomes too dry or wet, or too warm or cold. But models must be more explicit than this. They require robust mechanistic relations between CO2 fluxes and the main climatic factors - temperature and moisture - that govern them. Models also need to simulate how these relations vary among ecosystems. The problem is that some of the most important CO2 fluxes are difficult to measure; arguably, the most difficult of all is that resulting from the decomposition of soil organic matter (SOM). This flux is particularly important because it, along with photosynthesis, determines if an ecosystem is a net source or sink for CO2. If ecosystems are net sources, the CO2 produced contributes to the rise in atmospheric CO2 concentration that has been occurring for the past 150 years. But if an ecosystem is a net sink, it partly moderates atmospheric CO2 increases. It is important to know the mechanisms that cause an ecosystem to become a source or sink to predict the contributions of different ecosystems to future climate forcing. SOM contains most of the carbon (C) in land ecosystems. Most of the C in SOM is strongly resistant to decomposition. It takes centuries or millennia for this C to fully decompose (hence its description as 'historical' C). Yet it is the CO2 from 'historical' C that, with photosynthesis, determines an ecosystem's source-sink balance. To measure this flux, it must be distinguished from the CO2 arising the respiration of living roots, fungal hyphae and microbes, as all contribute to the soil flux. Conventional methods involve killing roots and hyphae, removing litter from the soil surface, or introducing SOM whose C is distinct from that of native soil C. All of these invasive procedures can, worryingly, produce estimates of CO2 fluxes that do not necessarily reflect those of undisturbed systems. Knowing how the 'historical' CO2 flux varies between different ecosystems and in relation to temperature and moisture would be a major step towards improving climate models, most of which use information about CO2 fluxes. Our solution to this long-standing problem has been to develop a method that measures 'historical' CO2 fluxes directly, but with minimal disturbance. We do this by accurately measuring the natural isotopic (13C) composition of CO2 derived from the decomposition of SOM and from the respiration of living roots. These sources of CO2 usually differ in their 13C content by a small, but measurable, amount. By measuring the CO2 flux at the soil surface and its 13C content, we can estimate directly how much of it derives from 'historical' C. We will use this technique to measure the 'historical' soil CO2 flux in three similar forests across a European climatic gradient at three times of year in two consecutive years. This is to obtain a wide a range of temperatures and soil moistures to which we can relate the 'historical' soil CO2 flux. The C balances of the sites that we will use (in Italy, Germany and Finland) have been measured for many years, allowing us to compare our measurements with other ecosystem C fluxes. The relationships we obtain between 'historical' soil CO2 flux, temperature and moisture will be used to improve the description of temperature and moisture sensitivity of SOM decomposition in existing models that predict interactions between climate and the C cycle, and which forecast resulting changes in the C stocks of ecosystems. We will then use these models to assess with greater reliability the effects that future climate is likely to have on the sustainability of Europe's soil C.

对地球气候将如何变化的预测取决于气候如何影响大气中二氧化碳含量以及受其影响的良好信息。大气中的CO2含量是CO2产生（主要来自呼吸作用以及生物质和化石燃料燃烧）和CO2消耗（主要来自光合作用）之间的平衡。光合作用和呼吸速率随气候变化而变化：当天气变得太干或太湿，或太热或太冷时，它们会减慢。但模型必须比这更明确。它们需要在CO2通量和控制它们的主要气候因素--温度和湿度--之间建立强有力的机械关系。模型还需要模拟这些关系如何在生态系统之间变化。问题是，一些最重要的CO2通量很难测量;可以说，最困难的是土壤有机质（SOM）分解产生的。这种通量特别重要，因为它与光合作用沿着决定了生态系统是二氧化碳的净源还是净汇。如果生态系统是净排放源，那么产生的二氧化碳会导致过去150年来大气中二氧化碳浓度的上升。但是，如果一个生态系统是一个净汇，它在一定程度上减缓了大气中二氧化碳的增加。了解导致生态系统成为源或汇的机制对于预测不同生态系统对未来气候强迫的贡献是很重要的。土壤有机质中含有陆地生态系统中大部分的碳（C）。有机质中的大多数碳具有很强的抗分解性。这个C需要几个世纪或几千年才能完全分解（因此它被描述为“历史”C）。然而，正是来自“历史”C的CO2与光合作用一起决定了生态系统的源-汇平衡。为了测量这种通量，必须将其与由活根、真菌菌丝和微生物呼吸产生的CO2区分开来，因为所有这些都有助于土壤通量。传统的方法包括杀死根和菌丝，去除土壤表面的凋落物，或引入有机质，其C与天然土壤C不同。令人担忧的是，所有这些侵入性程序都可能产生对CO2通量的估计，而这些估计不一定反映未受干扰的系统。了解不同生态系统之间的“历史”二氧化碳通量如何变化以及与温度和湿度的关系将是改进气候模型的重要一步，其中大多数模型都使用有关二氧化碳通量的信息。我们对这个长期存在的问题的解决方案是开发一种直接测量“历史”CO2通量的方法，但干扰最小。我们通过精确测量来自SOM分解和活根呼吸的CO2的天然同位素（13 C）组成来做到这一点。这些CO2来源的13 C含量通常相差很小，但可以测量。通过测量土壤表面的CO2通量及其13 C含量，我们可以直接估计其中有多少来自“历史”C。我们将使用这种技术来测量“历史”土壤CO2通量在三个类似的森林在欧洲气候梯度在三个时间在一年中连续两年。这是为了获得广泛的温度和土壤湿度，我们可以与“历史”土壤CO2通量。我们将使用的网站（在意大利，德国和芬兰）的碳平衡已经测量了很多年，使我们能够比较我们的测量与其他生态系统的碳通量。我们获得的“历史”土壤CO2通量，温度和湿度之间的关系将被用来改善现有的模型，预测气候和C循环之间的相互作用，并预测由此产生的变化，在生态系统的C股票的SOM分解的温度和湿度的敏感性的描述。然后，我们将使用这些模型来评估更可靠的影响，未来的气候可能对欧洲的土壤C的可持续性。