Temperate Forest of 2050's: carbon and nutrient cycling responses to seven years of elevated CO2 enrichment at BIFoR-FACE
Authors: Ullah Sami , Mayoral Carolina , Rumeau Manon , Norby Richard , Gardner Anna , Pihlblad Johanna , Reay Michaela , Handy Grace , Hamilton Liz , Hart Kris , Smith Andy , Hartley Iain , MacKenzie Robert 
Published: March 2024

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 &#8220;CO2 fertilization effect&#8221;. However, the projections of the land C sink by the end of the 21st 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&#160; availability including nitrogen (N)&#160; 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. &#160;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 &amp; Biochemistry 184. https://doi.org:10.1016/j.soilbio.2023.109072
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