BIFoR FACE: Responses of a temperate forest to seven years of elevated CO2.

Forests sequester ~18% of the global carbon dioxide (CO 2 ) emissions from anthropogenic sources (Canadell et al. 2021, Jia et al. 2019). This “CO 2 fertilization effect” results in a predicted 67% increase in the land carbon (C) sink through enhanced CO 2 capture (Friedlingstein et al. 2020, Terrer et al. 2020). The projections of the land C sink by the end of the 21 st Century based on simulations of state-of-the-art Earth System Models (ESM) is highly uncertain, with a 25 to 50% reduction in the predicted land C sink when nutrient availability including nitrogen (N) is accounted for (Fleischer et al. 2019). This uncertainty emanates from poor representation of key ecosystem types, particularly mature forests, to changing nutrient supplies under eCO 2 . The Birmingham Institute of Forest Research (BIFoR) established a Free-Air CO 2 Enrichment (FACE) facility in a mature temperate forest in the UK, where fumigation of three forest plots with elevated CO 2 (+150 ppm above the ambient) was started in 2017 and continues to date. In response to CO 2 fumigation, trees CO 2 uptake increased by an average of 23%, and the tree basal area increments increased by ~ 28%. Belowground C allocation via litterfall (+14%), root exudates (+40%) and fine root biomass increased. However, the litterfall N content decreased by 12% pointing towards N resorption by trees before senescence. Similarly, the soluble N to C ratios in exudates decreased corroborating the observation that trees conserved N for sustaining the growth response. Soil gross N mineralization rates were 20% higher under eCO 2 . These processes sustained larger soil mineral N supply (~25 kg N ha -1 y -1 ) under eCO 2 showing that the trees invested captured C for N acquisition. Whilst microbial biomass phosphorus increased in soils, a consistent treatment effect was not observed. Overall, the forest sustained high C capture and allocation into biomass and soils. The net implications of nutrient availability for C sequestration will depend on how long upregulation of soil nutrient availability will last in meeting plant nutrient demands before manifestation of nutrient limitation, if any. The ongoing eCO 2 fumigation lasting at least until 2030 will help resolve the feedbacks between C capture and nutrient availability to enable a realistic assessment of the role of forests in climate change mitigation.