Optimum sampling time and frequency for measuring N 2 O emissions from a rain-fed cereal cropping system

Abstract

Annual cumulative nitrous oxide (N2O) emissions from soil have historically been calculated from intermittent data measured manually via the static chamber method. The temporal variability in emissions, both diurnally and between days, introduces uncertainty into the up-scaling of static chamber data. This study assessed the most appropriate time of the day to sample and the best sampling frequency to ensure reliable estimates of annual cumulative emissions. Sub-daily N2O emissions were measured using automatic gas sampling chambers over three years in a sub-tropical cereal crop system. The sub-daily dataset was divided into eight time periods per day to assess the best sampling time of the day. Daily mean N2O emissions were subsampled from the dataset to simulate different sampling frequencies, including pre-set and rainfall-based scenarios. Annual cumulative N2O emissions were calculated for these scenarios and compared to the 'actual' annual cumulative emissions. The results demonstrated that manual sampling between mid-morning (09:00) and midday (12:00), and late evening (21:00) and midnight (24:00) best approximated the daily mean N2O emission. Factoring in the need to sample during daylight hours, gas sampling from mid-morning to midday was the most appropriate sampling time. Overall, triweekly sampling provided the most accurate estimate (± 4% error) of annual cumulative N2O emissions, but was undesirable due to its labour intensive high sampling frequency. Weekly sampling with triweekly sampling in the two weeks following rainfall events was the most efficient sampling schedule, as it had similar accuracy (± 5% error) to the triweekly sampling, the smallest variability in outcomes and approximately half the sampling times of triweekly sampling. Inter-annual rainfall variability affected the accuracy and variability of estimations of annual cumulative emissions, but did not affect the overall trends in sampling frequency accuracy. This study demonstrated that intermittent samplings are capable of estimating the annual cumulative N2O emissions satisfactorily when timed appropriately.

Reducing nitrous oxide emissions by changing N fertiliser use from calcium ammonium nitrate (CAN) to urea based formulations

“…Gas sampling was undertaken between 10.00 and 14.00 hours, as this was reported to best represent the average daily flux (Smith and Dobbie, 2001;van der Weerden et al, 2013). Reeves and Wang, (2015) refined the optimum sampling time to between mid-morning (09:00) and midday (12:00) as sampling conducted in the early afternoon was observed to overestimate daily emissions due to higher soil temperatures. Headspace samples (20 ml at HB, 10 ml at JC and MP) were taken after a 40 min chamber closure period on four occasions per week during the first and second week after N application, reducing to twice per week for the next two weeks and then once per week until the next N application.…”

Biochar Reduced Nitrous Oxide and Carbon Dioxide Emissions from Soil with Different Water and Temperature Cycles

“…The containers had a radius of 8 cm and a height of 10 cm and volume of 2 L. The soil depth was 7 cm and each treatment was replicated four times. At the completion of the experiment, 20 g of soil was mixed with 20 mL of water, centrifuged, and analyzed for dissolved organic C (Rice et al, 2012).…”

“…These findings suggest that GHG emissions and soil moisture cycles were offset from each other, and that gas samples collected at 700 h would underestimate GHG emissions, while samples collected at 1400 h would overestimate emissions. Others have noted that the sampling protocols can impact calculated GHG emissions (Parkin, 2008;Reeves and Wang, 2015).…”

Non-linear response of soil N2O emissions to nitrogen fertiliser in a cotton–fallow rotation in sub-tropical Australia

“…The number of chambers was decreased to one replicated chamber within each treatment per experimental plot (n = 3 per treatment) over the fallow period when only small fluxes (<5.0 g N 2 O-N ha -1 day -1 ) were observed in all treatments. Sampling frequency was optimised according to the recommendations of Reeves and Wang (2015). Specifically, measurements were conducted three times a week immediately after fertilisation followed by heavy rainfall events and weekly over the remaining period, which was expected to provide a highly accurate estimate (AE10% error) compared with measurements with a sub-daily temporal resolution (Reeves and Wang 2015).…”

“…Sampling frequency was optimised according to the recommendations of Reeves and Wang (2015). Specifically, measurements were conducted three times a week immediately after fertilisation followed by heavy rainfall events and weekly over the remaining period, which was expected to provide a highly accurate estimate (AE10% error) compared with measurements with a sub-daily temporal resolution (Reeves and Wang 2015). Fluxes were measured once on each sampling day during 0900-1100 hours, which has been shown to best approximate the daily mean N 2 O flux (Reeves and Wang 2015).…”