Conservation Agriculture practices reduce the global warming potential of rainfed low N input semi-arid agriculture

Conservation tillage and crop rotations improve soil quality. However, the impact of these practices on greenhouse gas (GHG) emissions and crop yields is not well defined, particularly in dry climates. A rainfed 2-year field-experiment was conducted to evaluate the effect of three long-term (17-18 years) tillage systems (Conventional Tillage (CT), Minimum Tillage (MT) and No Tillage (NT)) and two cropping systems (rotational wheat (Triticum aestivum L.) preceded by fallow, and wheat in monoculture), on nitrous oxide (N2O) and methane (CH4) emissions, during two field campaigns. Soil mineral N, water-filled pore space, dissolved organic carbon (C) and grain yield were measured and yield-scaled N2O emissions, N surplus and Global Warming Potentials (GWP) were calculated. No tillage only decreased cumulative N2O losses (compared to MT/CT) during campaign 1 (the driest campaign with least fertilizer N input), while tillage did not affect CH4 oxidation. The GWP demonstrated that the enhancement of C stocks under NT caused this tillage management to decrease overall CO2 equivalent emissions. Monoculture increased N2O fluxes during campaign 2 (normal year and conventional N input) and decreased CH4 uptake, as opposed to rotational wheat. Conversely, wheat in monoculture tended to increase soil organic C stocks and therefore resulted in a lower GWP, but differences were not statistically significant. Grain yields were strongly influenced by climatic variability. The NT and CT treatments yielded most during the dry and the normal campaign, and the yield-scaled N2O emissions followed the same tendency. Minimum tillage was not an adequate tillage management considering the GWP and the yield-scaled N2O emissions (which were 39% lower in NT with respect to MT). Regarding the crop effect, wheat in rotation resulted in a 32% increase in grain yield and 31% mitigation of yield-scaled N2O emissions. Low cumulative N2O fluxes (<250 g N2O-N ha−1 campaign−1) highlighted the relevance of soil organic C and CO2 emissions from inputs and operations in rainfed semi-arid cropping systems. This study suggests that NT and crop rotation can be recommended as good agricultural practices in order to establish an optimal balance between GHGs fluxes, GWP, yield-scaled N2O emissions and N surpluses.