Stimulation of soil gross nitrogen transformations and nitrous oxide emission under Free air CO₂ enrichment in a mature temperate oak forest at BIFoR-FACE

Forest ecosystems are considered globally important sinks for offsetting increasing anthropogenic atmospheric carbon dioxide (CO₂), however, this may be limited by the soil nutrient supply, predominantly nitrogen and phosphorus. Uncertainty remains regarding how soil N cycling in mature forests may respond to changes in carbon availability, arising from enhanced photosynthesis under elevated CO₂ (eCO₂) due to lack of experimental data. Further, potential positive feedbacks of nitrous oxide emissions may offset benefits of additional carbon sequestration under eCO₂. The Birmingham Institute of Forest Research Free Air Carbon Enrichment experiment (BIFoR-FACE) started fumigating a mature temperate deciduous forest in 2017 at +150 ppm CO₂ above ambient. Soil N cycling responses to eCO₂ were investigated using the ¹⁵N pool dilution approaches to assess gross N mineralisation, immobilisation and nitrification rates, in combination with the ¹⁵N-gas flux method to quantify and source partition N₂O production from 2018 to 2020 (2nd to 4th year of fumigation). Soil gross N mineralisation increased by 20% under eCO₂ (6.6 μg N g⁻¹ d⁻¹) compared to the control treatment (5.3 μg N g⁻¹ d⁻¹) and despite the trends being consistent over the three years (2018–2020), the high variability between arrays reduced statistical significance except in 2019. Ammonium immobilisation by microbes increased by 20% under eCO₂ (3.5 μg N g⁻¹ d⁻¹) as well. Overall, gross mineralisation was 4 times higher than nitrification, indicating a much higher ammonium turnover rate compared to nitrate (1.5 vs. 12 days mean residence time). N₂O emission from denitrification (0.18 ng N g⁻¹ h⁻¹) was significantly higher under eCO₂. After four years of CO₂ fumigation, there are modest indications of enhanced soil N transformation rates and N availability to support the observed enhanced canopy CO₂ uptake. Increased N₂O fluxes under eCO₂ indicated the potential for positive feedbacks on C sequestration under rising atmospheric CO₂. The overall implications for C sequestration will depend on how long upregulation of soil N transformations and N bioavailability will last to meet plant demands before manifestation of N limitation, if any.