Understanding denitrification in permeable sands

Sandy sediments cover the majority of the continental shelf, and yet are poorly understood compared with the cohesive, muddy sediments often observed in rivers, lakes and estuaries. A major difference between the two types of benthos lies in solute transport – slow, homogenous diffusion in muds, compared with rapid, two-dimensional advection through sands. This difference massively affects the biogeochemistry of permeable sediments, but its effects on denitrification – the major process for N removal from most systems – are poorly understood. In this thesis, denitrification in permeable sediments is investigated under a variety of scenarios. First, a combined laboratory and modelling approach is used to demonstrate that the rapid transport in permeable sediments limits the effectiveness of coupled nitrification-denitrification, significantly limiting overall denitrification rates in permeable sediments. Second, the effect of these transport phenomena on 15N isotope fractionation during denitrification is investigated. The apparent effect observed is consistent with low denitrification rates and supports the low isotope fractionation observed in global oceans. Third, a modelling investigation seeks to test a hypothesis that carbonate sands are capable of higher denitrification rates compared with quartz sands, due to pores within the grains themselves facilitating high rates of coupled nitrification-denitrification. The models indicate that this sort of intragranular reaction is plausible, and could indeed lead to higher denitrification rates in a real system. Finally, a further computational study investigates the effect that migrating ripples may have on denitrification in permeable sediments. While moving ripples drive very different transport and solute distribution, the rates of denitrification in sediments with moving ripples is similar to those with stationary ripples.