A comparison between the tower-based gradient method and the automated
chamber method for measuring N2O fluxes from an agricultural field
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.