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

在过去的两个世纪里，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。最近二氧化碳的排放量每年高达90亿吨碳-几乎相当于地球上每个活着的人1吨碳。这一上升令人担忧，因为大气中二氧化碳的增加导致了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来帮助减少大气中二氧化碳的上升。但陆地生态系统也在呼吸过程中排放二氧化碳，每年释放的二氧化碳几乎是燃烧化石燃料释放的二氧化碳的20倍！然而，二氧化碳的光合作用吸收可以补偿呼吸损失，因此森林中生物量和土壤汇中的碳储量通常会随着时间的推移而增加。要了解不断变化的气候中陆地生态系统对二氧化碳浓度的影响，需要更好地了解光合作用和呼吸作用之间的平衡--例如，在2003年夏天，有人认为森林从二氧化碳的汇变成了二氧化碳的来源。在这项提案中，我们试图了解树木上方空气和土壤上方空气中二氧化碳的“进出”情况。为此，我们利用了这样一个事实，即当二氧化碳与树叶和土壤交换时，它会与周围水中的氧交换一个氧原子(O)。有两种类型的氧原子，重同位素和轻同位素(18O和16O)，土壤水和叶水在这些同位素的比例上不同，这是由于在叶片蒸发蒸腾过程中较轻的同位素优先蒸发所致。因此，二氧化碳和水之间的氧气交换提供了一个独特的视角来了解光合作用和呼吸作用之间的平衡，并允许我们在土壤、茎和叶之间划分呼吸源。目前，我的工作已经表明，不同地区的土壤氧同位素交换的速度不同，但不同的土壤和不同气候条件下的叶水同位素特征如何相互作用，提供在大气中大量二氧化碳中可识别的信号，仍存在很大的不确定性。在这项提议中，我将更准确地定义水和二氧化碳的氧气在时间和空间上的变化，以便改进不同气候情况下土壤和生物量呼吸之间相互作用的区域预测模型。归根结底，如果我们要为持续的碳封存制定地球系统政策，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.