Mitigating greenhouse gas emissions from a subtropical Ultisol by using long-term no-tillage in combination with legume cover crops

Greenhouse gas (GHG) emissions can be partially mitigated with conservation agriculture. In this study, we assessed the effects of conventional tillage (CT) and no-tillage (NT), as well as five NT cover crop-based cropping systems, on yield-scaled GHG emissions in two long-term experiments (18 and 19 years) on a subtropical Paleudult. Air samples collected in static closed chambers were used to measure nitrous oxide (N2O) and methane (CH4) fluxes. The annual rate of change in soil organic C from the beginning of the experiments was used as proxy for net CO2 flux. Cumulative annual emissions of the three GHG and the CO2 costs of agricultural inputs and operations were taken in full account when estimating the global warming potential (GWP). Under legume cover crops, NT soil exhibited increased N2O emissions relative to CT soil (531 vs 217 kg CO2eq ha−1 yr−1); however, emissions of this gas from NT soil were fully offset by CO2 retention in soil organic matter (−2063 to −3940 kg CO2 ha−1 yr−1). Soil CH4 fluxes were very low with all management systems (−1.5 to 30.5 kg CO2eq ha−1 yr−1). NT soil under legume cover crops behaved as a net sink for GHG (GWP ranged from −971 to −2818 kg CO2eq ha−1 yr−1); by contrast, CT soil and NT soil with a low biomass input were net sources of GHG (GWP ranged from 857 to 2133 kg CO2eq ha−1 yr−1). The legume cover crops increased maize yield and further reduced yield-scaled GHG emissions. This result suggests that conservation management practices involving no-till in combination with legume cover crops provide an effective approach to sustainable low-C footprint food production in subtropical regions.