Effects of hydro-edaphic characteristics on the spatial and temporal variation in sediment-associated nitrogen concentrations across the Wiesdrift floodplain wetland, Cape Agulhas.

Abstract

This thesis explores the mechanisms governing the storage and spatial distribution of sediment-associated nitrogen in the Wiesdrift floodplain wetland, along a sampling transect. Sediment samples were collected during four discrete sampling trips on 10 June 2023, 08 April 2024, 12 August 2024 and 07 February 2025. Data on rainfall, wetland inundation, flow velocity, water depth, elevation, vegetation characteristics, soil characteristics, and sediment total nitrogen, carbon, sulphur, and hydrogen was analysed to assess the influence of hydrological variables and site characteristics on sediment total nitrogen concentrations. Surface sediment samples were collected at the river inlet and along a floodplain transect using a soil auger. Samples were analysed to determine nitrogen, carbon, and sulphur concentrations using a CHNS analyser. Spearman's rank-order correlation was used to investigate relationships between concentrations of nitrogen and other elements analysed, and to investigate effects of local hydro-edaphic properties such as particle size, sediment pH and sediment salinity (1:2 sediment:water slurries). Strong positive correlations were observed between sediment-associated nitrogen and carbon under high inundation conditions, providing insight into wetland conditions suitable for sediment-associated nitrogen assimilation (e.g., binding to organic matter, which accumulates in diffuse flow environments under anaerobic conditions). Stronger positive correlations were observed when inundation levels were low, providing insight into the ammonification process that occurs under aerobic conditions in wetlands where decomposition occurs and nitrogen is released back into the sediments. Sediment-associated nitrogen concentrations were highest when the floodplain was dry, suggesting enhanced nitrogen accumulation under low inundation conditions. Positive relationships between sediment-associated nitrogen and clay content further indicate that fine-textured sediments play an important role in nitrogen retention due to their greater capacity to retain water and organic matter.