Root soil Nitrogen acquisition by mature Oak trees exposed to elevated CO2: Nitrogen preference and uptake rate under a future climate

In a future CO₂ rich world, nitrogen (N) limitation is projected to decrease the CO₂ fertilization effect limiting the ability of temperate forests to mitigate climate change. There are limited direct measurements of roots showing if mature trees are able to increase their N uptake in a future climate. Additionally, it is not currently understood if roots of mature trees can change their N form preference under elevated CO₂ to maintain or enhance their N uptake. These are all gaps in knowledge identified as adding uncertainty to modelling efforts assessing ecosystem response to climate change. We quantified the rate of N uptake of living mature oak tree roots (Quercus robur) and their preference of N forms. On two occasions (July and November 2022) we carefully excavated live oak roots of three mature trees in each of the six experimental Free Air Carbon Enrichment (FACE) arrays (three ambient and three +150 ppm CO2) at the BIFoR FACE facility located in Staffordshire (United Kingdom). The live roots were cleaned and pre-incubated in acid washed sand and a nutrient solution for 24 hours to establish an acclimatized baseline condition following excavation. The roots were then exposed to a mix of inorganic and organic N forms where only one form was labelled in each treatment (¹⁵N-nitrate, ¹⁵N-ammonium, a mix of 20 ¹⁵N labelled amino acids and an unlabelled control) to elucidate N preferences and rate of uptake during a two-hour incubation period. By analysing the root tissue for ¹⁵N our findings will investigate the preferences and uptake rates of N by mature trees under elevated CO₂. We hope to shed light on these mechanisms mediating N uptake of mature trees to explain how mature forest stands respond to climate change in a temperate climate.