Precipitation gradient and crop management affect N2O emissions: Simulation of mitigation strategies in rainfed Mediterranean conditions

In Mediterranean areas high precipitation variability and crop dependence on soil water availability make the interaction between climate and agricultural management a key issue for mitigating N₂O emissions. In this study we used the STICS model to capture the effect of a water deficit gradient and precipitation variability on N₂O emissions and mitigation strategies (i.e. N fertilizer type, grain legumes introduction in crop rotations and crop residues management) in a rainfed Mediterranean transect (HWD-Senés, MWD-Selvanera and LWD-Auzeville, i.e. high, medium and low water deficit, respectively). The model was first tested against a database of daily N₂O fluxes measured during twelve growing seasons of winter crops at the LWD site. Several scenarios were then run on each site, always over 9 successive growing seasons to take into account precipitation variability. STICS showed a good ability to simulate the driving variables of N₂O fluxes at the daily time scale. The mean observed and simulated cumulative emissions during the growing season were 0.71 and 0.82kgN₂O–Nha⁻¹, respectively. The simulated N₂O emissions (mean of all scenarios) decreased with increasing water deficit being 2.51, 0.65 and 0.26kgN₂O–Nha⁻¹yr⁻¹ for LWD-Auzeville, MWD-Selvanera and HWD-Senés, respectively, which is consistent with published results. The lower N₂O emissions in the driest sites were not only related to lower fertilization rates but also to other factors associated with the Mediterranean characteristics, particularly, the drier water regime. Simulated N₂O emissions were highly sensitive to the interannual variability of the climatic conditions. According to the simulations, urea fertilizer would lead to slightly higher N₂O emissions (+6 and +8%) than ammonium- and calcium nitrate, respectively. The incorporation of winter pea in the traditional cereal-based Mediterranean rotations would reduce by ca. 22% the N₂O emissions in HWD-Senés without changing wheat yields. Differently, in MWD-Selvanera and LWD-Auzeville, N₂O emissions would remain unchanged since the emissions associated to the decomposition of low C:N ratio pea residues would counteract the lower application of N fertilizer. The systematic removal of crop residues at LWD-Auzeville would decrease the N₂O emissions by 20%. However, this practice seems not recommendable if tillage is practiced due to the concomitant decrease of soil organic matter, fact that would worsen the C footprint of the system and increase the susceptibility to soil erosion. Our work highlights the interest of combining experimental and modelling approaches to account for climatic variability and evaluate long-term effects of N₂O mitigation practices under Mediterranean conditions.