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

大气中有更多的二氧化碳（CO2）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。这个过程被称为“二氧化碳施肥”，每年从大气中去除20 - 30亿吨碳，占人类活动每年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二氧化碳施肥持续到未来;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CO2施肥，成熟森林的碳储存增加，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持续下去是非常不确定的，因为植物生物量的生产也需要从土壤中吸收养分。关键养分（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，成熟森林可能是陆地上最重要的碳吸收器;其次，成熟森林的养分供应量一般较低，因为成熟树木的根部可能已经在寻找关键养分时充分探索了土壤。如果成熟的森林在未来无法获得更多的养分并维持其碳吸收，那么这将对我们的社会产生重大影响。这将意味着，如果我们要避免气候变化的最严重后果，我们就必须比目前预期的更大程度、更快地减少二氧化碳排放。温带森林目前吸收的碳几乎与所有欧盟国家的排放量一样多。热带雨林当然很重要，但成熟的温带森林吸收碳的效率是其他森林的四倍，因此值得特别关注。为了能够预测成熟的温带森林在未来的反应，我们必须确定大气中更高的二氧化碳浓度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。控制成熟森林整个林分的二氧化碳是具有挑战性和昂贵的，到目前为止还没有实验可以让我们解决上面讨论的不确定性。随着在英格兰中部成熟的橡树林建立一个新的实验设施，这一切都发生了变化。利用1500万英镑的慈善捐赠和伯明翰大学的同等投资，建立了一个全生态系统自由空气二氧化碳富集（FACE）实验，该实验成功地将森林斑块的二氧化碳浓度提高到比目前水平高出三分之一以上。在FACE生态系统中，树冠树木至少有160年的历史，该网站在过去的400年里一直被森林覆盖。QUINTUS旨在对养分循环进行详细测量（超过20，000次分析），以回答上述两个关键过程，从而确定成熟的温带森林如何应对大气CO2上升。这一新的实验性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将为我们对温带森林和成熟森林未来碳吸收的理解带来根本性的变化。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.