FACE underground:can trees in mature forests gain greater access to soil nutrients under elevated atmospheric CO2?

包括森林在内的陆地生态系统捕获了人类活动释放的约30%的二氧化碳。这种吸收主要归因于大气CO2浓度（eCO2）增加对光合作用速率的有益影响（“CO2施肥效应”）。基于目前的二氧化碳吸收率和预测的大气二氧化碳浓度的增加，已经尝试使用不同的大尺度模型来预测森林未来的碳(C)吸收率。然而，该模型的估计是高度不确定的，因为我们对有限的土壤养分，特别是氮(N)和磷(P)的可用性是如何调节二氧化碳施肥效应的缺乏清晰的认识。例如，将氮有效性纳入模型后，预测的二氧化碳吸收率降低了约50%，这表明以前的估计可能过于乐观。此外，以前的实验集中在幼林上，迄今为止还没有在成熟的温带森林中进行大规模的二氧化碳富集实验。这一点很重要，因为：1)北温带地区的成熟森林目前负责全球近一半（~40%）的净碳吸收率；2)在eCO2下，幼林可能能够通过促进根系生长来增加对养分的获取，以探索更多的土壤空间，而成熟森林已经拥有发达的根系，因此向根系及其相关真菌伙伴（菌根）分配更多的碳可能会减少增加养分获取的潜力。因此，对养分有效性在控制成熟温带森林对eCO2的响应中的作用进行现实评估是至关重要的。考虑到温带森林的全球重要性，伯明翰森林研究所（BIFoR）于2017年在斯塔福德郡的一个160岁的落叶林场建立了一个二氧化碳施肥实验（投资1500万英镑）。这是在成熟的温带森林中进行的第一次这样的实验，因此提供了一个独特的机会来测试一个关键问题：成熟的树木能否在eCO2下获得更多的有限营养，如果是这样，它们采用什么策略来做到这一点？因此，我们的目标是验证一个广泛的假设，即在eCO2下，成熟的温带落叶森林将把额外的碳转移到地下，以增加氮和磷的有效性，并随后被树木吸收。研究将在eCO2下的三个地块和三个对照地块进行。我们将测量根和菌根菌丝的产量，以及根部释放的基质（渗出物）。我们还将进行一系列实验，以确定根与菌根真菌在控制分解速率和养分循环方面的相对作用，以及这些受到eCO2影响的程度。这些分解实验将涉及根和/或菌根真菌的排除，以及模拟根渗出的新方法。结果将使我们能够确定树木是否以及通过何种机制促进eCO2下的分解和养分动员。最后，我们将确定根系吸收的含氮化合物的类型在eCO2下是否发生了变化，以及这与它们在土壤中的可用性有何关系。总之，我们将在成熟的温带森林中使用第一个FACE实验，以确定成熟的温带森林树木是否能够在eCO2下获得更多的土壤养分，从而确定这些生态系统中是否可能存在一个大而持续的碳汇，从而解决碳循环模型中的一个主要不确定性。如果我们的研究结果表明，森林对养分的吸收将在未来变得越来越有限，那么它将产生重大的社会影响，因为需要更大程度地减少温室气体排放，以避免气候变化最危险的后果。

Land ecosystems including forests capture about 30% of the carbon dioxide (CO2) released by human activities. This uptake is mainly attributed to the beneficial effects of increasing atmospheric CO2 concentrations (eCO2) on rates of photosynthesis (the "CO2 fertilisation effect"). Based on current CO2 uptake rates and the predicted increases in atmospheric CO2 concentrations, an attempt has been made to predict future Carbon (C) uptake by forests using different large-scale models. However, the model estimates are highly uncertain because we lack a clear understanding of how the limited availability of soil nutrients, particularly nitrogen (N) and phosphorus (P), regulate the CO2 fertilisation effect. For example, incorporating nitrogen availability into models reduced predicted uptake rates of CO2 by ~50%, which shows that previous estimates may have been optimistic. Furthermore, previous experiments have focused on young forests and to date there are no large-scale CO2 enrichment experiments in mature temperate forests. This is important because: 1) mature forests in northern temperate regions are currently responsible for almost half (~40%) of the global net C uptake and 2) young forests may be able to increase access to nutrients by increasing root growth under eCO2 to explore more of the soil space, whereas mature forests already have well-developed root systems, so greater carbon allocation to roots and their associated fungal partners (mycorrhizas) may have less potential to increase access to nutrients. Therefore, a realistic assessment of the role of nutrient availability in controlling the responses of mature temperate forests to eCO2 is essential. Given the global significance of temperate forests, the Birmingham Institute of Forest Research (BIFoR), established a CO2 fertilization experiment (>£15 million investment) in a >160 year old deciduous forest stand in Staffordshire in 2017. This is the first such experiment in a mature temperate forests and thus provides a unique opportunity to test a key question: can mature trees gain greater access to limiting nutrients under eCO2 and, if so, which strategies do they employ to do so? Thus, we aim to test the broad hypothesis that under eCO2 a mature, deciduous temperate forest will transfer additional carbon belowground to increase nitrogen and phosphorus availability and subsequent uptake by trees. The research will be undertaken in three plots under eCO2 and three control plots. We will measure root and mycorrhizal hyphal production, and the release of substrates (exudates) from roots throughout the year. We will also carry out a series of experiments to determine the relative roles of roots versus mycorrhizal fungi in controlling rates of decomposition and nutrient cycling, and the extent to which these are affected by eCO2. These decomposition experiments will involve root and/or mycorrhizal fungi exclusion, as well as a novel approach for simulating root exudation. The results will enable us to determine whether, and through which mechanisms, trees can stimulate decomposition and nutrient mobilization under eCO2. Finally, we will determine if the types of nitrogen containing compounds that roots take up changes under eCO2 and how this relates to their availability in the soil. In summary, we will use the first FACE experiment in a mature temperate forest to determine whether mature temperate forest trees will be able to access more soil nutrients under eCO2, and therefore, whether there is likely to be a large and sustained carbon sink in these ecosystems, addressing a major uncertainty in carbon cycle modelling. If our results suggest that the forest uptake will become increasingly nutrient limited in the future then it would have major societal implications as greater cuts in greenhouse gas emissions would be needed to avoid the most dangerous consequences of climate change.