Nitrogen transport and retention in sub-tropical riparian zones

Riparian zones of temperate regions often have shallow groundwater and organic carbon rich soils, providing favourable conditions for nitrogen retention, particularly by denitrification.  These conditions are often absent in sub-tropical climates.  In the Maroochy/Mooloolah catchments of southeast Queensland, Australia, the channel morphology and soil profiles of 12 stream reaches with various land uses were assessed in terms of their potential to retain inorganic nitrogen (IN) from overland and groundwater flow.  Stream channels tended to have either high, steep banks or incised channels, promoting deeper groundwater flowpaths and drier, aerobic surface soils.  IN had highest concentrations in riparian surface soils, and overland flow that occurs during intense summer rain events may mobilise and transport large amounts of this IN to stream channels.  At half of the sites, accumulation of ammonium in deep, saturated soil layers suggested retention of nitrogen by dissimilatory nitrate reduction to ammonium (DNRA), which may be taken up by deep-rooted plants.  The findings suggest alternative nitrogen transport and retention processes than those usually described as dominant in temperate regions.  These processes have been developed into a conceptual model of local-scale nitrogen transport for the sub-tropical Maroochy/Mooloolah catchments to be tested in future research.