EGU2020-15899
https://doi.org/10.5194/egusphere-egu2020-15899
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Stimulation of soil N cycling after two years of Free Air CO2 Enrichment increases nitrous oxide emissions in a temperate forest, UK.

Fotis Sgouridis1, Suparat Cotchim3, and Sami Ullah2
Fotis Sgouridis et al.
  • 1University of Bristol, School of Geographical Sciences, Bristol, United Kingdom of Great Britain and Northern Ireland (f.sgouridis@bristol.ac.uk)
  • 2Birmingham Institute of Forest Research, University of Birmingham, UK.
  • 3Department of Chemistry, Prince of Songkla University, Thailand.

Increasing atmospheric CO2 concentrations in temperate forests may affect soil nitrogen (N) cycling processes due to the increased demand  for nitrogen availability by trees to support CO2 uptake through photosynthesis. This in turn can affect the emission of nitrous oxide (N2O) from the forest soil leading to a potential trade-off between the enhanced canopy CO2 uptake and soil N2O emission. The Birmingham Institute of Forest Research (BIFoR) established a Free-Air CO2 Enrichment (FACE) facility in a mature oak forest in Staffordshire, UK, which became operational in 2017. In April 2018 and again in May 2019, two years after the start of fumigation with 550 ppm CO2, we collected soil samples (0 – 15 cm depth) from the three elevated CO2 (eCO2) and three control plots. Soils were amended in the laboratory with 98 at % 15N-NH4+ and 15N-NO3- . Gross N mineralisation and nitrification were estimated by the isotope dilution technique, while N2O emission from nitrification (15N-NH4+ treatment) and denitrification (15N-NO3- treatment) were estimated by the 15N Gas-Flux method. Additionally, C/N ratio and δ15N and δ13C were measured in unamended eCO2 and control samples via EA-IRMS. Whilst gross N mineralisation and N2O emission were only marginally higher in eCO2 plots compared to controls after one year of fumigation, there was a significant stimulation of N cycling after the second year that led to more pronounced differences. Gross N mineralisation rates doubled in the eCO2 plots (mean: 4.09 μg N g-1 d-1, P < 0.05) compared to the control plots (mean: 2.02 μg N g-1 d-1), while a similar twofold increase was observed for gross nitrification rates (mean eCO2: 1.63 μg N g-1 d-1;  mean control: 0.70 μg N g-1 d-1, P < 0.05). N2O emission from both denitrification (mean: 0.03 ng N g-1 d-1) and nitrification (mean: 0.02 ng N g-1 d-1) were generally low but of similar magnitude and more than double than in the control plots. C/N ratio was conservative between eCO2 and control plots as a result of proportional increase of C and N contents in the eCO2 plots. The observed stimulation in N cycling was further corroborated by the significantly enriched δ15N signal (-0.66 ‰) in eCO2 plots compared to the controls (-1.38‰). Moreover, the eCO2 samples had a more depleted δ13C signal (-28.37 ‰) compared to the controls (-27.99 ‰), as a result of the additional carbon supplied through fumigation (CO2 δ13C ~ -28 ‰). Following the first year of CO2 fumigation, there were indications of soil N limitation despite the high rates of atmospheric N deposition (22 kg N ha-1 y-1). However, after only 2 years of the FACE experiment there is strong evidence of a shift in key soil N processes to sustain the enhanced nutrient demands to support enhanced canopy CO2 uptake. Further research is underway in BiFOR-FACE to elucidate whether carbon quantity and/or quality drives the stimulation of soil N cycling and what are the long-term implications of the trade-off between enhanced CO2 sequestration and a potential increase in N2O emissions.

How to cite: Sgouridis, F., Cotchim, S., and Ullah, S.: Stimulation of soil N cycling after two years of Free Air CO2 Enrichment increases nitrous oxide emissions in a temperate forest, UK., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15899, https://doi.org/10.5194/egusphere-egu2020-15899, 2020

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