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

Abstract

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.