Temperate Forest of 2050's: carbon and nutrient cycling responses to seven years of elevated CO2 enrichment at BIFoR-FACE

Land ecosystems absorb ~29% of the total CO₂ emissions from anthropogenic sources. Global forests contributes ~62% to the total land ecosystem atmospheric CO₂ sinks. The carbon (C) sink in forests is predicted to increase with increasing atmospheric CO₂ concentration, called the “CO₂ fertilization effect”. However, 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 relatively uncertain where a 25 to 50% reduction in the C sink is predicted when nutrient  availability including nitrogen (N)  is accounted for. This uncertainty emanates from poor representation of key ecosystem types, particularly mature forests, to changing nutrient supplies under eCO₂.

To elucidate the feedbacks between elevated CO₂ (eCO₂), C capture and nutrient availability, the Birmingham Institute of Forest Research (BIFoR) established a Free-Air CO₂ Enrichment (FACE) facility in a mature temperate forest in the UK, where three FACE arrays (30 m dia) are exposed to elevated CO₂ (+150 ppm above the ambient) during the growing season.¹ The FACE enrichment started in 2017 and continues to date. In response to the CO₂ enrichment, photosynthetic CO₂ uptake increased by an average of 23% in the first three years and this enhanced uptake was sustained by the seventh year of CO₂ enrichment.² The enhanced CO₂ uptake resulted in an overall significant increase in tree dry matter (+10.5%) and a 28% increase in tree basal area increments.  Belowground C allocation via litter fall (+9.5 %), root exudates (+40%) and fine root biomass and specific root length in organic and mineral soil layers were increased as well. The overall net primary productivity calculated for years 2021 and 2022 was higher by ~2 tons of dry matter under eCO₂ compared to ambient arrays confirming and quantifying the extent of the CO₂ fertilization effect.

Whilst the litter fall increased under elevated CO₂, the N content of the litter decreased significantly pointing towards N conservation via resorption by trees before senescence. Similarly, root C exudation increased; however, exudation of N was not affected, thus leading to a shift in the C:N ratio from an average of 13 to 18 under eCO₂. Thus N was conserved via resorption and low root N exudation by trees to sustain enhanced photosynthesis and growth. Gross N mineralization rates were 20% higher under eCO₂.³ Enhanced N cycling processes sustained larger soil mineral N supply (~25 kg N ha^-1 y^-1) under eCO₂. Root uptake of N increased by 26% and potential uptake rates of amino acids was larger than mineral N. Tree N conservation and faster N cycling in soils appear to have sustained enhanced tree N uptake and demands. The implications of nutrient availability for C sequestration will depend on how long upregulation of soil N availability via soil organic matter decomposition will last before manifestation of nutrient limitation, if any.

References

¹ Hart, K. M. et al. 2020. Global Change Biology 26, 1023-1037. https://doi.org:10.1111/gcb.14786

² Gardner, A., et al. 2022. Tree Physiology 42, 130-144. https://doi.org:10.1093/treephys/tpab090

³ Sgouridis, F. et al. 2023. Soil Biology & Biochemistry 184. https://doi.org:10.1016/j.soilbio.2023.109072