Global Warming Potential, Soil Ecological Diversity and Productivity in No Tillage with Weed Cover Mulch As Alternative Organic Farming System in Marginal Environments in Japan.

Soil organic carbon (SOC) can accumulate by processes that essentially reverse some effects responsible for SOC losses when abandoned agricultural land is no longer used for cultivation and is allowed to revert to natural vegetation. In Japan, no tillage with weed cover mulching (NT) is adopted in marginal farmland because this farming system can be easily adopted on abandoned land and allows crops to grow with natural vegetation. This system also improves access to farming for new farmers because it eliminates the cost for crop production. However, the effect of this farming system on net global warming potential (GWP) and nitrogen leaching remains unclear. Therefore, the present study aimed to compare the effects of two organic farming systems, namely NT and conventional tillage with weed removal (CT), and two organic fertilizer application modes, namely no fertilizer (N?) and organic fertilizer (N+; 50 kg N ha?1 for 2010 and 2011 and 80 kg N ha?1 for 2012) on global warming gas fluxes, soil carbon sequestration, net GWP, and nitrate leaching. CT increased CH4 uptake immediately after the tillage operation only in 2010; however, no difference in annual uptake was observed compared with NT. N2O emissions in NT were higher only immediately after a weed mowing operation; however, NT did not increase the annual N2O emission. Fertilizer application increased N2O flux in 2012. Soil carbon content in the soil surface linearly increased every year after converting to NT and contributed to increase CH4 uptake, indicating that improving soil physics by continuing NT contributed to enhance CH4 uptake. In addition, the ratio of soil carbon sequestration (ƒĒSOC) was greater in NT than in CT, which significantly decreased net GWP in NT than in CT. Nitrate leaching was 48.6% lower in NT than in CT and 47.3% lower in NT than in CT at soil depths of 30-60 and 60-90 cm, respectively. A nitrogen tracer experiment showed that leaching loss of applied fertilizer in August was significantly lower in NT (11.4% at 30- 60 cm and 9.0% at 60-90 cm) than in CT (42.0% at 30-60 cm and 47.8% at 60-90 cm). These results show that the NT organic farming system contributed to conserve the regional and global environment by reducing nitrate leaching and net GWP from the agroecosystems while increasing annual CH4 uptake and soil carbon sequestration.