The effects of drying and re-flooding on the sediment and soil nutrient dynamics of lowland river-floodplain systems: A synthesis

La Trobe University Faculty of Science, Technology and Engineering Murray Darling Freshwater Research Centre

MDFRC item.

Lowland river-floodplain systems are characterized by a high degree of variability in both the frequency and period of inundation of various parts of the floodplain. Such variation should profoundly affect the processes underlying nutrient transformations in these systems. This paper explores the effect of various hydrologic regimes on nutrient cycles. Partial drying of wet (previously inundated) sediments will result in an increased sediment affinity for phosphorus and will produce a zone for nitrification coupled with denitrification. Hence, partial drying may reduce the availability of nitrogen (N) and phosphorus (P). Conversely, complete desiccation of sediments may lead to the death of bacteria (and subsequent mineralization of N and P), a decrease in the affinity of P for iron minerals, a decrease in microbial activity and a cessation of all anaerobic bacterial processes (e.g. denitrification). Colonization of exposed sediments by terrestrial plants may lead to N and P moving from the sediments to plant biomass. Re-wetting of desiccated soils and sediments will result in an initial flush of available N and P (which can be incorporated into bacterial or macrophyte biomass), coupled with an increase in bacterial activity, particularly nitrification. Inundation of floodplain soils will result in the liberation of C, N and P from leaf litter and floodplain soils. This will result in an increase in productivity, which ultimately may lead to the onset of anoxia in floodplain soils and, consequently, an increase in anoxic bacterial processes such as P release and denitrification.