Quinquennial (half-decadal) carbon and nutrient dynamics in temperate forests: Implications for carbon sequestration in a high carbon dioxide world

大气中更多的二氧化碳增加了光合作用的速率，促进了树木的生长和森林中的碳储存。这一过程被称为“二氧化碳施肥”，其结果是每年从大气中去除20 -30亿吨碳，占每年人类活动排放到大气中的碳的25-30%。因此，二氧化碳施肥大大降低了全球变暖的速度。应对气候变化的斗争依赖于持续到未来的二氧化碳施肥；巴黎气候协议强调，需要全球努力将我们释放的碳量限制在树木、土壤和海洋可以自然吸收的范围内。由于二氧化碳施肥，成熟森林中的碳储量增加，这被认为是目前碳吸收的最重要原因。但是，展望未来，这种高吸收率是否会继续是高度不确定的，因为植物生物量的生产也需要从土壤中吸收养分。关键营养物质（特别是氮和磷）的供应可能严重限制成熟森林中树木继续快速生长的能力。在确定养分供应是否可能限制陆地生态系统未来的碳吸收时，研究成熟森林尤为重要。首先，如上所述，成熟的森林可能是陆地上最重要的碳吸收器；第二，成熟森林的养分利用率一般较低，因为成熟树木的根系在寻找关键养分时可能已经充分探索了土壤。如果成熟的森林在未来无法获得更多的营养物质并保持其碳吸收，那么这将对我们的社会产生重大影响。这意味着，如果我们要避免气候变化带来的最严重后果，我们就必须以比目前预期更大的幅度和更快的速度减少二氧化碳排放。目前，温带森林吸收的碳几乎与所有欧盟国家的排放量一样多。虽然热带雨林当然很重要，但据计算，成熟的温带森林吸收碳的效率要高出四倍，因此值得特别关注。为了能够预测成熟的温带森林在未来将如何应对，我们必须确定大气中二氧化碳浓度的增加是否会使温带森林中的成熟树木：1)从土壤中吸收更多的养分，和/或，2)提高它们利用可用养分产生新植物组织的效率。控制整个成熟森林的二氧化碳是具有挑战性和昂贵的，直到现在还没有实验可以让我们解决上面讨论的不确定性。在英格兰中部成熟的橡树林中建立了一个新的实验设施，这一切都改变了。利用1500万英镑的慈善捐赠和伯明翰大学的同等投资，一个全生态系统的自由空气二氧化碳富集（FACE）实验已经建立起来，该实验成功地使森林斑块的二氧化碳浓度比目前的水平高出三分之一以上。在FACE生态系统中，树冠树至少有160年的树龄，该地点已经被森林覆盖了400年。QUINTUS的目标是进行养分循环的详细测量（超过20,000次分析），这需要回答上面概述的两个关键过程，从而确定成熟的温带森林如何应对大气中不断上升的二氧化碳。这一新的实验理解将用于开发和测试下一代计算机模型，这些模型将用于预测未来气候变化的速度。QUINTUS将从根本上改变我们对温带森林和成熟森林未来碳吸收的理解。这种进步是迫切需要的，具有重大的社会意义。

Having more carbon dioxide (CO2) in the atmosphere has increased rates of photosynthesis, promoting greater tree growth and carbon storage in forests. This process is called 'CO2 fertilisation' and results in 2-3 billion tonnes of carbon being removed from the atmosphere each year, which is 25-30% of the carbon put into the atmosphere by human activity annually. CO2 fertilisation, thus, greatly reduces rates of global warming. The fight against climate change relies on CO2 fertilisation continuing into the future; the Paris climate agreement emphasises that global efforts are required to limit the amount of carbon we release to that which trees, soil, and oceans can absorb naturally. Increased carbon storage in mature forests, due to CO2 fertilisation, is considered to be the most important reason for the current carbon uptake. But, looking forward, it is highly uncertain whether such high rates of uptake will continue, because the production of plant biomass also requires the uptake of nutrients from soils. The availability of key nutrients (especially nitrogen and phosphorus) may severely limit the ability of trees in mature forests to continue to grow more rapidly. Studying mature forests is particularly important when determining whether nutrient availability may limit future carbon uptake by land ecosystems. Firstly, as discussed above, mature forests are likely the most important absorbers of carbon on land; secondly, nutrient availability is generally low in mature forests because the roots of mature trees may have already fully explored their soils in their search for key nutrients. If mature forests are unable to access more nutrients in the future and maintain their carbon uptake, then this would have major implications for our society. It would mean that we would have to reduce our carbon dioxide emissions by a greater extent, and more rapidly than currently expected, if we are to avoid the most serious consequences of climate change. Temperate forests currently absorb almost as much carbon as the emissions from all EU nations. While tropical rainforests are, of course, important, mature temperate forests are calculated to be fourfold more efficient at absorbing carbon, and so merit special attention. To be able predict how mature temperate forests will respond in the future, it is critical that we determine whether greater carbon dioxide concentrations in the atmosphere will allow mature trees in temperate forest to:1) take up more nutrients from soils, and/or,2) increase the efficiency with which they use available nutrients to produce new plant tissue.Manipulating CO2 for whole stands of mature forest is challenging and expensive, and until now there has been no experiment that would have allowed us to address the uncertainties discussed above. All this has changed with the establishment of a new experimental facility in mature oak forest in central England. Leveraging a £15m philanthropic gift and an equivalent University of Birmingham investment, a whole-ecosystem free-air carbon dioxide enrichment (FACE) experiment has been set-up, which is successfully forest patches to CO2 concentrations more than one third higher than current levels. In the FACE ecosystem, the canopy trees are at least 160 years old and the site has been forested for the last 400 years. QUINTUS aims to carry out the detailed measurements of nutrient cycling (more than 20,000 analyses) that are required to answer the two key processes outlined above and, thus, determine how a mature temperate forest responds to rising atmospheric CO2. This new experimental understanding will then be used to develop and test the next generation of the computer models which are used to predict future rates of climate change. QUINTUS will deliver a foundational change in our understanding of future C uptake in temperate forests, and in mature forests generally. Such an advance is urgently required and has major societal relevance.