Denitrification dominates nitrate attenuation and nitrous oxide effluxes under water table fluctuations

Agricultural soils in riparian zones near rivers often experience frequent water table fluctuations, which can lead to increased nitrogen losses and greenhouse gas emissions via the nitrogen biogeochemical processes. However, the influence of water table fluctuations on the multiple nitrogen transformation processes that dominate nitrate attenuation and nitrous oxide (N₂O) effluxes remains poorly understood. In this study, the dynamic changes in depth-dependent nitrate attenuation and soil N₂O effluxes, and the responses of microbial communities influenced by water table fluctuations were studied using a series of large column experiments. Our results revealed that dissolved oxygen (DO) concentrations at a depth of -10 cm in sand columns with three different grain sizes (fine→medium→coarse) oscillated, producing oxidizing conditions during drainage and reducing conditions during imbibition periods. DO micro-sensors installed in a layered (sand and sandy loam) column as well as in two sandy loam columns with different regimes in induced water table changes all revealed steady hypoxic conditions. The diversity of the microbial community was significantly correlated with total nitrogen, total organic carbon, and nitrate concentrations, as well as potential denitrification rates. The dominant microbial populations related to the nrfA gene were Methanothrix and Sedimentibacte, whereas those related to denitrification (nirK, nirS, and nosZ) were Pseudomonas and Sulfuricaulis. These findings improve our understanding of the effects of water table fluctuations on groundwater nitrate loss in riparian corridors.