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

Abstract

Abstract 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 (N 2 O) and methane (CH 4 ) emissions, during two field campaigns. Soil mineral N, water-filled pore space, dissolved organic carbon (C) and grain yield were measured and yield-scaled N 2 O emissions, N surplus and Global Warming Potentials (GWP) were calculated. No tillage only decreased cumulative N 2 O losses (compared to MT/CT) during campaign 1 (the driest campaign with least fertilizer N input), while tillage did not affect CH 4 oxidation. The GWP demonstrated that the enhancement of C stocks under NT caused this tillage management to decrease overall CO 2 equivalent emissions. Monoculture increased N 2 O fluxes during campaign 2 (normal year and conventional N input) and decreased CH 4 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 N 2 O emissions followed the same tendency. Minimum tillage was not an adequate tillage management considering the GWP and the yield-scaled N 2 O 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 N 2 O emissions. Low cumulative N 2 O fluxes ( 2 O-N ha −1 campaign −1 ) highlighted the relevance of soil organic C and CO 2 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 N 2 O emissions and N surpluses.