Abandoned agricultural land could potentially accumulate soil organic carbon (SOC) when it is no longer used for cultivation and is allowed to revert to natural vegetation. In Japan, no tillage with weed mulching will be adopted in marginal farmland as a new organic farming system because this system minimizes the disturbance of the soil ecosystem and reduces the cost for crop production. The present study aimed to compare the effects of two organic farming systems, namely no-tillage with weed cover mulching and conventional tillage (CT), and two organic fertilizer application modes, namely no fertilizer (N-) and organic fertilizer (N+; 50kgNha-1 during 2010 and 2011 and 80kgNha-1 during 2012) on greenhouse gas (GHG) fluxes, soil carbon sequestration, net global warming potential (GWP), and nitrate leaching. Pumpkin (Cucurbita spp.) was cultivated as the main crop in 2010 and 2011, whereas mixed cropping of okra (Abelmoschus esculentus L.), bell pepper (Capsicum annuum L.), and eggplant (Solanum melongena L.) was implemented in 2012. Tillage management increased CH4 uptake immediately after the tillage; however, the effects did not continue in the long term. On the contrary, NTW increased CH4 uptake, and the soil carbon content at the soil surface linearly increased every year after conversion to NTW indicating that improving soil physics by continuing NTW contributed to enhanced CH4 uptake. N2O emissions in NTW were higher only immediately after a weed mowing; however, NTW did not increase the annual N2O emission. In addition, the difference between initial and final SOC (dSOC) was greater in NTW than in CT, which significantly decreased net GWP in NTW in comparison with CT. Nitrate leaching was 48.6% and 47.3% lower in NTW than in CT at soil depths of 30-60 and 60-90cm, respectively. These results show that no-tillage with weed cover mulching contributed to conserve the regional and global environment by reducing nitrate leaching and net GWP from the agro-ecosystem by increasing the annual CH4 uptake and soil carbon sequestration. This system will be adopted for abandoned agricultural land because it reduces net GWP shortly after conversion to this management.
