Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/4/2007
Publication Date: 7/1/2008
Citation: Parkin, T.B. 2008. Effect of Sampling Frequency on Estimates of Cumulative N20 Emissions. Journal of Environmental Quality. 37:1390-1395. Interpretive Summary: Nitrous oxide (N2O) is a major greenhouse gas that contributes approximately 6% to the total radiative forcing of the earth's atmosphere, and agricultural soils are a significant source of atmospheric N2O. Nitrous oxide emissions from soil exhibit high temporal variability, with transient peak fluxes occurring after rainfall and fertilization events. This variability can cause uncertainty in estimates of total N2O emissions when measurements are only made periodically. This study was conducted to determine the errors in estimates of cumulative N2O emissions as a function of sampling frequency. It was found that as the time between sampling events increases precision associated with estimates of cumulative N2O flux decreases. Total N2O emissions determined by sampling at 7 d intervals have an error of +- 20%, but total emissions determined by sampling at 14 d intervals can have an error of +- 60%. This information should help scientists design better sampling procedures, and will help policy makers assess the value of published N2O flux estimates.
Technical Abstract: It is generally recognized that soil N2O emissions can exhibit pronounced day-to-day variations; however, measurements of soil N2O flux with soil chambers typically are done only at discrete points in time. This study evaluated the impact of sampling frequency on the precision of cumulative N2O flux estimates calculated from field measurements. Automated chambers were deployed in a corn/soybean field and used to measure soil N2O fluxes every 6 h from 25 February 2006 through 11 October 2006. The field had been fertilized with anhydrous ammonia in November 2005, and automated chambers were located in two positions relative to the fertilizer bands; directly over a band or between fertilizer bands. Sampling frequency effects on cumulative N2O-N flux estimation were assessed using a jackknife technique where populations N2O fluxes were constructed from the average daily fluxes from each chamber. As sampling interval increased from 7 d to 21 d, variances associated with cumulative flux estimates increased. At relatively frequent sampling intensities (i.e. once every 3 d) N2O-N flux estimates were within + 10% of the expected value at both sites. As the time interval between sampling was increased, the potential deviation in estimated cumulative N2O flux increased, such that sampling once every 21d yielded potential estimates within +60% and -40% of the actual cumulative N2O flux. The spread of potential fluxes associated with the between-band positions was less than the over-band position, indicating that the underlying temporal variability impacts sampling efficacy.