Biomodulation of nitrogen cycle in suspended sediment

Biogeochemical experiments and modeling were coupled to investigate how nutrient leaching to aquatic ecosystems changes the dynamics of microbial community on suspended sediment and how these changes modulate the nitrogen cycle. Mineral suspensions amended with inorganic nitrogen (NH⁺₄ and NO^-₃) and inoculated with native sedimentary microbial strains were tested in a settling column under continuous water quality measurements. Experiments were used to calibrate and validate a kinetic model that explicitly described the chemical adsorption on minerals, aqueous complexation, gas dissolution, microbial metabolism, necromass dynamics, and microbial competition for limiting substrates. Modeling revealed that the interactions between microbial functional groups were highly nonlinear and highly sensitive to changes in nutrient and dissolved oxygen concentrations, showing shifts in regimes where a functional group became prevalent over the others. Our results suggested that necromass dynamics played a major role in sustaining microbial growth in low nutrient conditions and had an important control over the N cycle. The reaction network and model structure presented in this study provide a tool to analyze and predict the long-term nutrient dynamics of both natural and engineered aquatic ecosystems.