High-resolution N$_2$O and CH$_4$ emissions during cabbage and fallow seasons in a paddy soil in Vietnam

Rice paddy soils are a significant source of methane (CH$_4$). Nitrous oxide (N$_2$O) is another potent greenhouse gas (GHG) but its significance to GHG balance in rice paddies remains uncertain. Here, we present high-resolution measurements of CH$_4$ and N$_2$O using automated chambers, combined with subsurface O$_2$ concentrations, over a fallow period and a cabbage-growing season in a subtropical rice-vegetable rotation system. The combination of automated chambers, subsurface oxygen profiling, and LandscapeDNDC modelling calibrated to measured O$_2$ dynamics provides unprecedented mechanistic insight into N$_2$O hot moments in rice–vegetable rotations. Episodic "hot moments" dominated N$_2$O emissions, with 72% of total seasonal flux occurring within 27% of the cabbage season. Mean and peak N$_2$O fluxes during cabbage cultivation reached 104 µg N$_2$O-N m$^{−2}$ h$^{−1}$ and 682 µg N$_2$O-N m$^{−2}$ h$^{−1}$, respectively. High-emission events were linked to fertilisation, changes in water-filled pore space and subsurface oxygen availability, with N$_2$O fluxes initially dominated by denitrification under saturated conditions and later by nitrification as nitrate and oxygen increased and ammonium decreased. ... mehrThis conclusion was supported by LandscapeDNDC. The emission factor for nitrogen applied was 0.52%, and nitrogen use efficiency was 38%. Total N$_2$O emissions during cabbage cultivation were sevenfold higher than fallow, whereas CH$_4$ fluxes remained unchanged. The global warming potential (GWP) in cabbage was driven by N$_2$O rather than CH$_4$. These findings highlight the importance of capturing N$_2$O emission events in paddy-to-vegetable transition systems and the value of process-based modelling to identify dominant biogeochemical pathways.