Patterns and bioavailability of soil nutrients and carbon across a gradient of inundation frequencies in a lowland river channel, Murray-Darling Basin, Australia

The distribution of both carbon and nutrient stores in the landscape is not homogeneous, and is influenced by soil properties such as texture and organic content as well as spatial gradients such as inundation frequency. In this study the distribution of water extractable soil nutrients and water mobile dissolved organic carbon (DOC) and its bioavailability were investigated at specific levels along a cross-sectional gradient that extended from the baseflow level deep in the channel to the floodplain. The frequency with which these specific levels are inundated was modelled under three flow regimes to determine if flow change is likely to influence nutrient supply to the river. Soil surface litter and soil organic carbon content, DOC and SRP increased along a trajectory from the base of the channel onto the floodplain. Ammonium increased and nitrate decreased as in-channel height decreased. This reflects an increase in soil inundation frequency lower in the channel decreasing nitrification rates and increasing NO3− losses via mobilisation and denitrification. Bioavailable soil DOC (BDOC) was lowest in the most frequently inundated soils; however, overall soil BDOC did not relate to either changing soil properties or inundation frequency. There were no significant differences in nutrient and carbon supply to the river under the three modelled flow regimes (with flows extraction rules, without flow extraction rules and natural flows) explored as flow change was most marked in the channel bed region, where little organic matter was stored.