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

关于地球气候将如何变化的预测取决于有关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。大气中的二氧化碳含量是二氧化碳产生（主要来自呼吸作用、生物质和化石燃料燃烧）和二氧化碳消耗（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。它们要求二氧化碳通量与控制它们的主要气候因素——温度和湿度——之间存在可靠的机制关系。模型还需要模拟这些关系在不同生态系统之间的变化。问题是一些最重要的二氧化碳通量很难测量；可以说，其中最困难的是土壤有机质（SOM）的分解。这种通量特别重要，因为它与光合作用一起决定了生态系统是二氧化碳的净来源还是净汇。如果生态系统是二氧化碳的净来源，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。但如果一个生态系统是一个净汇，它会在一定程度上减缓大气中二氧化碳的增加。了解使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。土壤有机质含有陆地生态系统中大部分的碳(C)。SOM中的大部分C具有很强的抗分解性。这种C需要几个世纪或几千年才能完全分解（因此它被称为“历史”C）。然而，正是来自“历史”C的二氧化碳，通过光合作用，决定了生态系统的源库平衡。为了测量这种通量，必须将其与活根、真菌菌丝和微生物呼吸产生的二氧化碳区分开来，因为它们都有助于土壤通量。传统的方法包括杀死根和菌丝，清除土壤表面的凋落物，或引入碳含量与天然土壤不同的SOM。所有这些侵入性的方法都可能令人担忧地产生不一定反映未受干扰系统的二氧化碳通量的估计。了解“历史上”二氧化碳通量在不同生态系统之间以及与温度和湿度的关系是如何变化的，将是朝着改进气候模型迈出的重要一步，大多数气候模型都使用有关二氧化碳通量的信息。对于这个长期存在的问题，我们的解决方案是开发一种直接测量“历史”二氧化碳通量的方法，但干扰最小。我们通过精确测量SOM分解和活根呼吸产生的二氧化碳的自然同位素（13C）组成来做到这一点。这些CO2源的13C含量通常相差很小，但可测量。通过测量土壤表面的二氧化碳通量及其13C含量，我们可以直接估计有多少来自“历史”c。我们将使用这种技术在连续两年的三个时间里测量欧洲气候梯度上三个类似森林的“历史”土壤二氧化碳通量。这是为了获得一个广泛的温度和土壤湿度范围，我们可以将“历史”土壤二氧化碳通量与之联系起来。我们将使用的地点（在意大利、德国和芬兰）的碳平衡已经测量了多年，使我们能够将我们的测量结果与其他生态系统的碳通量进行比较。我们获得的“历史”土壤CO2通量、温度和湿度之间的关系将用于改善现有模型对SOM分解的温度和湿度敏感性的描述，这些模型预测气候与碳循环之间的相互作用，并预测生态系统中碳储量的最终变化。然后，我们将使用这些模型更可靠地评估未来气候可能对欧洲土壤C的可持续性产生的影响。

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.