Preliminary research on N₂O sampling time based on isotopomer signature measurement of vegetable fields in the North China Plain

【Objectives】 Emissions of N2O from vegetable soils are extremely variable in time and therefore there are considerable uncertainties in calculating emission of N2O flux and emission coefficients. It is thus important to investigate the optional sampling time and characteristic of diurnal pattern for N2O. More insights into N2O diurnal variation would also be helpful for long term monitoring of N2O release and for understanding the associated microbiological mechanisms.【Methods】 Field experiment in situ was conducted to study N2O emission after urine amendment and subsequent irrigation for a lettuce production system in North China Plain in 2012. During periods of high N2O fluxes in 5-6 days, N2O emission was collected manually by a closed static flux chamber at two intervals, two hours for diurnal variation and 10 minutes for appropriate enclosure period. N2O concentrations were measured by a gas chromatograph equipped with an electron capture detector (GC-ECD), and isotopomers such as δ15N-N2O, δ18O-N2O and site preference (SP) (difference between the center and the end N atom of the N2O asymmetric molecule) were analyzed and obtained by an elemental analyzer-isotope ratio mass spectrometer(EA-IRMS) system.【Results】 1) During the period of high N2O emission,there was a strong diurnal variation in N2O fluxes ranging from 20.41 to 130.45 μg/(m2·h), with the maximum and minimum values being observed at 1 pm in Apr. 27th and 5 am in Apr. 28th, respectively. Flux measured at 9 am could be used to represent the daily average of the whole day. The fluxes of N2O emitted from the soil were correlated significantly(R2=0.82, P＜0.01) with the soil temperature at 5 cm, varied markedly across time, but insignificantly with water filled-pore space (WFPS). 2) Diurnally, the values of δ15Nbulk-N2O and δ18O-N2O decreased first and then increased gradually with time while the values of SP had an opposite trend, suggesting that production processes of N2O were associated with different microbial pathways. Conversely, the stable values of SP of N2O from 9 am through 5 pm indicated that nitrification was the dominant pathway for N2O production. 3) In measuring N2O emission, appropriate enclosure period in the static chamber should be 40 minutes, which was long enough to allow detection of N2O concentration, δ15N-N2O and SP and also short enough to avoid feedback of N2O concentration and microbial pathway change. 4) The calculated emission-weighted site preference was 22.54‰. The N2O emission in this study was dominantly produced from nitrification, contributing 60.9% of the daily totals of N2O emission.【Conclusions】 There were large diurnal variations of N2O concentration and isotopomers from lettuce production system in North China Plain. Considering both values of N2O flux and isotopomers, it was suggested that 9 am was an appropriate time to measure the flux and isotopomer signature of N2O and that enclosure period for one sample event should be 40 minutes.