The bioavailability of particulate nitrogen in eroded sediment: Catchment sources and processes

Abstract

PurposeAnthropogenic land use change has caused an increase in particulate nutrient loads from catchments draining to the Great Barrier Reef (GBR). The research in GBR catchments has indicated that particulate nutrients are bioavailable to both freshwater and marine phytoplankton, but relative importance of this source of nutrients to the GBR is unknown. We quantified the contribution of this source of bioavailable nitrogen in a dry-tropics grazing and a wet-tropics fertilized mixed land use catchment of the GBR.Materials and methodsThe different bioavailable nitrogen pools and associated processes through which dissolved inorganic nitrogen (DIN) is generated from eroded sediment (mass of DIN generated per mass of sediment) were identified. These pools and processes were quantified from a range of representative sediment sources (e.g. surface and subsurface soil and different land uses). We collected 17 sediment source samples in the wet tropics and 41 in the dry tropics. We combined the N pool concentration data with spatial and hydrological fine sediment modelling to estimate the contribution from different sources and processes/pools to the end-of-catchment DIN load.Results and discussionThe modelled load of DIN generated from sediment accounted for all the monitored DIN load in the grazing-dominated catchment but was insignificant in the fertilized mixed land use catchment. Sediment from surface erosion (hillslope erosion) and some soil types contributed disproportionally to the modelled DIN generation. Fast solubilisation of DIN was the main process in the catchments studied. The importance of mineralisation of the organic fraction increased with the time the sediment was in suspension.ConclusionParticulate nutrients in sediment are a significant source of bioavailable nitrogen in eroding grazing catchments. The processes that drive this bioavailability are complex, vary with sediment source and operate at different timeframes and spatial scales.