The Impact of Scoria-Filled Aeration Trenches on the N-cycle and Greenhouse Gases Emissions from a Clayey Soil

Abstract. Treated wastewater (TWW, i.e., treated effluents) is a growing water source. However, irrigation with TWW irrigation exacerbates oxygen deficiencies in the root zone, particularly in clayey soils. Coarse-textured filled trenches are used to ameliorate soil oxygen deficiencies in agriculture. This study aimed to investigate the impact of scoria-filled soil aeration trenches on nitrous oxide (N₂O) and carbon dioxide (CO₂) emissions from clayey soil irrigated with TWW. N₂O and CO₂ fluxes were measured in the field for three years, along with soil water content monitoring (10 and 35 cm depth) and porewater (30 cm depth) sampling. Irrigation and intense rain events led to transient (hours-long) near-saturation conditions in the clay soil. Concomitantly, the soil at the bottom of the trench remained saturated for prolonged periods, extending to days and even weeks. Nitrate was the dominant N-form and showed a seasonal trend with high concentrations (>50 mgL^-1) between June and October. N₂O fluxes were positively correlated with fertilizer applications, and fluxes from the trenches were higher throughout the year, with maximal differences during the winter. CO₂ fluxes were higher from the trenches during the fertigation seasons yet lower during the winter. Simulation results of N₂O fluxes showed higher fluctuation in the scoria-filled trenches following fertigation events. Further, it showed that filling the trench with finer medium, aimed to maximize the rate of water uptake by the trees' roots, increased the emissions maxima, dampening its minima. Overall, our study shows that aeration trenches may serve as N₂O hotspots and that, during winter, they might be counterproductive. Further study is needed to find the optimal filling material that would maximize aeration yet minimize water build-up at the trench bottom.