CH4 and N2O fluxes during paddy rice crop development, post-harvest, and fallow

Paddy fields are major sources of greenhouse gases, mainly methane (CH4) and nitrous oxide (N2O). Defining the sampling times for determining the average diurnal emission rates is an important step in optimizing field measurement, avoiding the influence of possible peaks. With this purpose, diurnal gas measurements (CH4 and N2O) were taken using the static chamber method during five 24 h-periods (campaigns), every 2 h, at three rice crop development stages (R2, C1 campaign; R5, C2 campaign, and R8, C3 campaign), and in post-harvest (PH, C4 campaign) and in fallow (FP, C5 campaign) periods. The CH4 fluxes remained close to the average flux both at C1 (9.4 ± 1.0 mg CH4 m-2 h-1) and C2 (10.2 ± 1.4 mg CH4 m-2 h-1), allowing the gas sampling at any time of the day, except at 5:00 p.m. when a peak was observed at C1. As the CH4 fluxes for C3, C4, and C5 were close to zero, no average value was identified. The average N2O fluxes were low at C1 (1.0 ± 5.7 μg N2O m-2 h-1) and at C4 (6.7 ± 2.6 μg N2O m-2 h-1), increasing at C2 (26.9 ± 9.3 μg N2O m-2 h-1) and C3 (21.2 ± 7.2 μg N2O m-2 h-1) and reaching higher values during the C5 campaign (73.7 ± 33.3 μg N2O m-2 h-1). In general, considering the average flux values recorded in this study, the most appropriate times for sampling N2O during the C1, C2, C3, and C4 campaigns would be from 9 p.m. to 1 a.m. and also around 11:00 a.m. Average N2O flows in fallow would be more likely around 11:00 p.m. and 11 a.m.