Linking the atmosphere and terrestrial biosphere carbon and water cycles using oxygen isotopes.

在过去两个世纪里，大气中二氧化碳（CO2）浓度的上升主要是由于化石燃料的燃烧。最近的二氧化碳排放量每年高达90亿吨，几乎相当于地球上每个人一吨。这种上升是令人震惊的，因为大气中二氧化碳的增加会导致全球变暖和气候变化。陆地生态系统，特别是森林，可以通过将碳储存在木材和土壤中，帮助减少大气中二氧化碳的增加。但是陆地生态系统在呼吸过程中也会排放二氧化碳，每年释放的二氧化碳几乎是燃烧化石燃料释放的20倍！然而，光合作用吸收的CO2弥补了呼吸作用的损失，因此，随着时间的推移，森林生物量和土壤汇中的碳储存通常会增加。要了解气候变化中陆地生态系统对CO2浓度的影响，就需要更好地了解光合作用和呼吸作用之间的平衡-例如，2003年夏天，有人提出森林从CO2的汇变成了源。在这个提议中，我们试图了解树木上方空气中和土壤上方空气中的二氧化碳的“来龙去脉”。为此，我们使用的事实是，当二氧化碳与树叶和土壤交换时，它会将一个氧原子（O）与周围水中的氧气交换。有两种类型的氧原子，重和轻同位素（18 O和16 O），土壤水和叶片水在这些同位素的比例不同，由于在叶片蒸散过程中较轻的同位素优先蒸发。因此，CO2和水之间的氧气交换为光合作用和呼吸之间的平衡提供了独特的见解，并使我们能够在土壤，茎和叶之间分配呼吸源。目前，我的工作已经表明，不同地区的土壤在氧同位素交换的速度上有所不同，但在不同的气候条件下，不同的土壤和叶水同位素特征如何相互作用，以提供在大量大气CO2中可辨别的信号，这是很大的不确定性。在这个建议中，我将更精确地定义如何在时间和空间上的水和CO2的氧气变化，以提高不同气候条件下土壤和生物量呼吸之间的相互作用的区域预测模型。最终，如果我们要制定持续固碳的地球系统政策，我们必须了解陆地生态系统，特别是森林中光合作用和呼吸作用之间的平衡。

The rise in the concentration of carbon dioxide (CO2) in the atmosphere over the last two centuries results mainly from the burning of fossil fuel. Recent emissions of CO2 are as much as 9 billion tonnes of carbon annually - almost equivalent to one tonne for every human alive on the planet. This rise is alarming, because increased CO2 in the atmosphere contributes to global warming and climate change. Terrestrial ecosystems, especially forests, can help to reduce the rise of CO2 in the atmosphere by storing carbon in wood and soil. But terrestrial ecosystems also emit CO2 during respiration, releasing almost 20 times more CO2 annually than that released by burning fossil fuels! However, photosynthetic uptake of CO2 compensates for respiration losses, so that the storage of carbon in biomass and soil sinks usually increases in forests over time. Understanding the impact of terrestrial ecosystems on CO2 concentrations in a changing climate requires a better knowledge of the balance between photosynthesis and respiration - for instance, in the summer of 2003, it has been suggested that forests changed from sinks to sources of CO2. In this proposal we are trying to understand the 'ins and outs' of CO2 in the air above the trees, and in the air above the soil. For that, we use the fact that when CO2 exchanges with leaves and soils it swaps one oxygen atom (O) with oxygen in the surrounding water. There are two types of oxygen atoms, a heavy and a light isotope (18O and 16O), and that soil water and leaf water differ in the proportion of these isotopes, due to preferential evaporation of the lighter isotope during evapotranspiration from leaves. Thus, the oxygen exchange between CO2 and water provides a unique insight into the balance between photosynthesis and respiration, and allows us to partition respiratory sources between soil, stems and leaves. Presently, my work has already shown soils from different regions vary in the rapidity of oxygen isotopic exchange, but there is great uncertainty how different soils, and leaf water isotopic signatures in different climatic situations interact to provide the signal discernable in bulk atmospheric CO2. In this proposal I will define more precisely how the oxygen of water and CO2 changes temporally and spatially, so as to improve regional predictive modelling of the interactions between soil and biomass respiration under different climatic situations. Ultimately, we must understand the balance between photosynthesis and respiration in terrestrial ecosystems, and especially forests, if we are to develop earth system policies for continued carbon sequestration.