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A comparison between the tower-based gradient method and the automated chamber method for measuring N2O fluxes from an agricultural field
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  • Qiurui Zhu,
  • Jacob Hagedorn,
  • Mark Castro,
  • Eric Davidson
Qiurui Zhu
University of Maryland Center for Environmental Science

Corresponding Author:qiurui.zhu@umces.edu

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Jacob Hagedorn
University of Maryland Center for Environmental Science
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Mark Castro
University of Maryland Center for Environmental Science
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Eric Davidson
University of Maryland Center for Environmental Science
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Abstract

Nitrous oxide (N2O) is a potent greenhouse gas and stratospheric ozone-depleting substance. More than half of anthropogenic N2O emissions result from agricultural activities. A broad objective of this on-farm research in eastern Maryland was to investigate whether drainage water management, which reduces nitrate export, would increase greenhouse gas emissions, but here we focus upon comparing chamber and tower measurements of N2O fluxes from a single field. Chamber methods usually suffer from poor spatial and temporal resolution. Automating chambers using in situ fast response analyzers improves temporal but not spatial resolution. Tower-based micrometeorological methods improve both temporal and spatial resolution, but require a high-frequency, high-sensitivity laser instrument. We compared auto-chamber and micrometeorological gradient methods for N2O flux measurement during a period early in the 2019 corn-growing season. A 3 m tall tower was deployed to allow for near-continuous gradient flux measurements using an Aerodyne Quantum Cascade Laser. Four Eosense closed dynamic automated chambers (eocAC) and a multiplexer (eosMX) were installed near the tower and connected to a Picarro G2308 gas analyzer. Both methods captured strong pulses of N2O fluxes after rainfall and fertilization events, demonstrating these major drivers of large emissions. Fluxes from the two methods were linearly correlated (R2 = 0.54), but the slope (1.29 ± 0.08) and y-intercept (48.3 ± 19.2) indicate that the chambers generally estimated higher fluxes. Aggregating over the measurement period, the automated chamber estimate was 2.5 kg N2O-N/ha in 19 days, whereas the tower-based gradient estimate was 1.3 kg N2O-N/ha in 19 days. The tower footprint includes some area (4%) covered by ditches and could extend beyond the field at times, but this is unlikely the only explanation. The small number of chambers may have sampled an area of above average flux, or there could be unknown measurement bias or interpolation error in one or both methods. To our knowledge, this is the first such methodological comparison of N2O fluxes since these sensitive, fast response instruments have become available, and our results demonstrate that additional work is needed to gain more confidence in reported fluxes by either method.