Hot moments drive extreme nitrous oxide emissions from agricultural peatlands

Tyler L. Anthony, Environmental Science, Policy, and Management, University of California, Berkeley, Berkeley, CA and Whendee L. Silver, Environmental Science, Policy, and Management, University of California Berkeley, Berkeley, CA

Background/Question/Methods

Drained peatlands occupy only 1% of agricultural land but are estimated to represent 32% of global cropland CO2-equivalent (CO2e) emissions, with these emissions dominated by large fluxes from aerobic microbial respiration in drained or partially drained organic soils. Nitrogen fertilization is common in peatland agriculture, potentially creating the perfect storm for high denitrification rates and nitrous oxide (N2O) production in these moist, organic-rich systems. However, N2O emissions are often overlooked or underestimated in agricultural peatland greenhouse gas (GHG) budgets. This is largely driven by previous limitations in the ability to conduct continuous, long-term N2O flux measurements under field conditions. We deployed cavity ringdown spectroscopy and automated chambers to collect over 87,000 individual measurements over three plus years of continuous carbon dioxide (CO2), methane (CH4), and N2O fluxes from a drained agricultural maize peatland in California, USA. We used arrays of soil moisture, temperature, and O2 sensors and weekly soil gas and mineral nitrogen (N) measurements in combination with continuous surface flux measurements to explore potential controls of N2O and CH4 fluxes.
Results/Conclusions
We found that N2O fluxes contributed up to 33% of annual CO2e emissions from these agricultural peatlands. N2O fluxes averaged 26 ± 0.5 kg N2O-N ha-1 y-1, with a maximum annual flux of 42 ± 2 kg N2O-N ha-1 y-1. These results suggest that IPCC benchmarks may underestimate agricultural peatland N-N2O emissions by up to 225% and total emissions up to 40 Tg CO2e y-1. Upscaling to similarly managed peat cropland showed that 0.36% of California’s croplands account for up to 33 ± 4% of California’s annual cropland CO2e emissions. Hot moments of N2O and CH4, defined as individual flux measurements more than four standard deviations from the yearly mean, represented 1.1 ± 0.2 and 1.3 ± 0.2% of measurements, respectively, but increased annual N2O fluxes by 45 ± 1% and CH4 fluxes by 140 ± 9%. Periods with elevated soil moisture and soil NO3- concentrations, coupled with low soil O2 concentrations, drove hot moments of N2O emissions. Significant CH4 fluxes were only observed during an extended period of anoxic conditions driven by soil saturation and corresponding decrease in soil O2 concentrations. Our results demonstrate that continuous automated chamber measurements of soil GHG emissions can capture hot moments of N2O and CH4 production that are essential to accurately quantify GHG budgets.