USDA LTAR Common Experiment Measurement: Chamber-based fluxes of nitrous oxide (N2O) and methane (CH4) from soil v1

Emissions of nitrous oxide (N2O) and methane (CH4) from soil are dynamic and spatially variable. Chamber based methods have been used since the 1980s to document fluxes and are widely used in both managed and natural ecosystems (Clough et al., 2020; Holland et al., 1999; Parkin and Venterea, 2010). They are well-tested, relatively inexpensive, and can be deployed extensively. They are also suitable for emissions of other gases, notably CO2 and NOx. Chambers can also be automated (e.g, Grace et al., 2020) to capture temporal dynamics. Here we provide hands-on, best-practice guidelines for the minimum and preferred chamber sampling plans sufficient to compare Common Experiment flux differences across all sites in the Long-Term Agroecosystem Research (LTAR) Network. Although each site is different, and exceptions will likely be necessary, following these guidelines will maximize the potential for making cross-site comparisons and provides those new to soil greenhouse gas sampling a straightforward path for successful measurement campaigns. In general, chambers will need to be of a sufficient size and placed to capture representative fluxes, keeping in mind plant spacing, fertilizer placement, and other features of the site that affect spatial variability such as topography. Once placed, chambers will need to be sampled at intervals that capture episodic emissions that result from management events such as tillage and nitrogen fertilization and environmental events such as rainfall following dry periods. Fluxes are typically low during dry periods and in winter if freeze-thaw events are infrequent, and during these periods sampling frequencies can be relaxed. In general, we recommend a minimum of weekly sampling following events known to stimulate fluxes (tillage, N fertilization, rainfall following drought) and biweekly sampling otherwise except when soil is very dry or frozen, when sampling can be monthly. By convention, fluxes should be expressed as μg N2O-N m-2 h-1 or μg CH4-C m-2 h-1.