Influence of seasonal flooding on soil total nitrogen, organic phosphorus and microbial populations in the Okavango Delta, Botswana

The effect of flooding on soil total nitrogen, phosphorus and microbial population in different vegetation zones (floodplain, island and woodland) and profile depth (0•1, 0•5, 2•0, 3•0, 4•0 and 5•0 m) of the Okavango Delta was studied from February to July 1999. Total nitrogen significantly differed with soil profile depth, moisture regime and months. In the woodlands, insignificant total nitrogen was detected at all depths except at 0•1 m, where 0•03% and 1•17% were detected in February and March, respectively. In the island samples, only 0•05% was detected at 4 m in February. Nitrogen in the floodplain samples was concentrated in the A1 horizon where 0•12%, 0•61% and 0•03% were detected in February, March, and May, respectively. Organic phosphorus significantly differed with vegetation zone but not with months and depth. Although organic phosphorus was low (0•02–0•52%) at all sites, it was liberally distributed throughout the profiles. On the island, actinomycetes were only detected up to 2 m in February and up to 3 m in July. Fungi concentrated in the top 0•5 m (103–105). In both the floodplain and island samples, bacteria concentrated in the upper 3 m. However, after May, populations decreased significantly. In the floodplain, significant actinomycetes populations were only detected in the upper 0•5 m. Generally, organic phosphorus showed positive correlations with fungal populations. This study indicates that these soils are low in total nitrogen and phosphorus.