Contribution of greenhouse gas emissions during cropping and fallow seasons on total global warming potential in mono-rice paddy soils

Temperate rice paddy fields are generally flooded for less than 100 days a year during the rice cropping season and are kept under dried soil conditions during the fallow season of over 200 days. The impacts of rice paddy soil on the global warming potential (GWP) are generally analysed during rice cultivation, without consideration of the fallow season, using only methane (CH4) and nitrous oxide (N2O) fluxes. To compare the impact of greenhouse gas (GHG) emissions during the flooded rice cultivation and the dried fallow seasons on the annual GWP in a mono-rice cultivation system, the emission fluxes of CH4, N2O and carbon dioxide (CO2) were evaluated under two different fertilization systems (NPK and NPK + Cover crop) for two consecutive years. In the NPK + Cover crop treatment, a mixture of barley and hairy vetch were cultivated as a winter cover crop without fertilization during the fallow season. The total above-ground biomass (36 Mg fresh weight ha(-1), moisture content 68.9 %, C/N ratio 20.6) was incorporated as a green manure one week before rice transplanting. The same levels of chemical fertilisers were applied for rice cultivation in the NPK and NPK + Cover crop treatments. The emission rates of CH4, CO2, and N2O gases were simultaneously monitored once a week using the closed-chamber method. However, because the CO2 fluxes included only soil respiration and excluded soil C sequestration through cover cropping and its recycling of biomass, the net ecosystem C budget (NECB), which is defined as the difference between total organic C input and output, was estimated to ascertain pure CO2 emission fluxes. Finally, the net global warming potential (GWP), which was calculated as CO2 equivalents by multiplying the seasonal CH4, CO2, and N2O fluxes by 25, 1, and 298, respectively, was compared between the two treatments and the two seasons. In the NPK treatment, the annual net GWP value was 10.7-11.7 Mg CO2 eq. ha(-1), in which approximately 56-62 % was affected by the seasonal net GWP value during the fallow season. Cover crop cultivation during the fallow season and its biomass addition as a green manure for rice cultivation significantly increased the total net GWP value to 28.2-31.5 Mg ha(-1), in which approximately 73-76 % was weighted by the seasonal net GWP value during rice cultivation. Carbon dioxide was the most influential GHG on increasing the growth scale of total net GWP during the dried fallow season, but CH4 most strongly influenced the annual net GWP scale during the rice cropping season, irrespective of soil management conditions. The contribution of CH4 to the annual net GWP value significantly increased as a result of cover crops biomass addition from 34-39 % in the NPK treatment to 88-91 % in the NPK + Cover crop treatment. The dried fallow season contributed to approximately 30-60 % of the annual net GWP scale through GHG emissions. Therefore, proper soil management strategies should be developed to decrease GHG emissions during the fallow season in mono-rice paddy fields.