Effect of Sampling Frequency on Estimates of Cumulative Nitrous Oxide Emissions

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 Feb. 2006 through 11 Oct. 2006. The 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 of N₂O fluxes were constructed from the average daily fluxes measured in each chamber. These test populations were generated by selecting measured flux values at regular time intervals ranging from 1 to 21 d. It was observed that as sampling interval increased from 7 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 sampling positions. As the time interval between sampling was increased, the deviation in estimated cumulative N₂O flux increased, such that sampling once every 21 d yielded estimates within +60% and −40% of the actual cumulative N₂O flux. The variance of potential fluxes associated with the between-band positions was less than the over-band position, indicating that the underlying temporal variability impacts the efficacy of a given sampling protocol.