[1] There have been conjectures that a spatial average could result in a volume-average concentration which is no longer continuous at sharp interfaces between different materials. However, convincing experimental evidence showing the existence of such a discontinuity is not available because of the difficulty associated with measuring solute concentration in the void space within a porous medium. In this paper we used pore-scale simulations to explore the change in macroscopic concentration when solute moves from one material into another. Water flow through the void space was assumed to be laminar, and solute transport consisted of molecular diffusion and advection; both were simulated using the lattice Boltzmann equation methods. To accurately represent the fluid-solid interface, the multiple-relaxation-time lattice Boltzmann equation method was used to simulate fluid flow. We first simulated solute transport in a 3D column with one half packed with fine glass beads and the other half with coarse glass beads. The simulated solute concentration and solute flux at pore scale were then spatially averaged to produce volume-average and flux-average concentration profiles, respectively, in attempts to understand if solute accumulates at the media interface when moving from one medium into another. The results revealed that, when solute migrated from the coarse medium into the fine medium, it did accumulate at the media interface; we also found mass accumulation at the reservoir-column interface. Such accumulations made solute take more time to break through the column when flowing from the coarse medium to the fine medium than from the fine medium to the coarse medium. We also simulated solute movement in an idealized 2D column packed with different rectangular solids and with high porosity; the results indicated that, although the dispersive properties of the two media differed considerably, there was no mass accumulation and the macroscopic concentration was found to be continuous at the media interface. These simulated results suggest that a sharp change in material properties with moderate porosity will likely lead to a mass accumulation, but knowing the transport properties of the two materials alone is not sufficient to determine if a mass accumulation could develop. What causes mass accumulations appears to be some microstructures in the vicinity of the interface, which cannot be accounted for by the macroscopic transport parameters of each of the two media.Citation: Zhang, X., X. Qi, and D. Qiao (2010), Change in macroscopic concentration at the interface between different materials: Continuous or discontinuous, Water Resour. Res., 46, W10540,

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Despite the known influence of nitrogen fertilization and groundwater conditions on soil microbial communities, the effects of their interactions on bacterial composition of denitrifier communities have been rarely quantified. Therefore, a large lysimeter experiment was conducted to examine how and to what extent groundwater table changes and reduced nitrogen application would influence the bacterial composition of nirK-type and nirS-type genes. The bacterial composition of nirK-type and nirS-type genes were compared at two levels of N input and three groundwater table levels. Our results demonstrated that depression of groundwater table, reduced nitrogen application and their interactions would lead to drastic shifts in the bacterial composition of nirS-type and nirK-type genes. Structural equation models (SEMs) indicated that depression of groundwater table and reduced nitrogen application not only directly altered the species composition of denitrifier bacterial communities, but also indirectly influenced them through regulating soil nutrient and salinity. Furthermore, the variation in soil NO3−–N and electrical conductivity caused by depression of groundwater table and reduced nitrogen application played the most important role in altering the community composition of denitrifier bacterial communities. Together, our findings provide first-hand evidence that depression of groundwater table and reduced nitrogen application jointly regulate the species composition of denitrifier bacterial communities in agricultural soil. We highlight that local environmental conditions such as groundwater table and soil attributes should be taken into account to enrich our knowledge of the impact of nitrogen fertilization on soil denitrifier bacterial communities, or even biogeochemical cycles.