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

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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.