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FACE-Underground is a NERC project funded for 2020 to 2023 investigating nutrient cycling in a mature temperate forest under long-term eCO₂.

This project will directly addresses a global challenge and uncertainty – how future elevated atmospheric CO₂ (eCO₂) concentrations will influence forest productivity and carbon sequestration, and how soil nutrient availability, particularly nitrogen (N) and phosphorus (P) will constrain such a CO₂ fertilisation effect. This uncertainty about the role of nutrient availability in regulating carbon (C) sequestration by temperate deciduous forests under future climates is particularly critical for Earth System Models.

Summary

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 below-ground to increase nitrogen and phosphorus availability and subsequent uptake by trees.