Flooding and its influence on diazotroph populations and soil nitrogen levels in the Okavango Delta
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Date
2004Author
Omari, K.
Mubyana, T.
Matsheka, M.I.
Bonyongo, M.C.
Veenendaal, E.
Publisher
South African Journal of Botany http://www.elsevier.com/wps/find/journaldescription.cws_home/707238/description#descriptionType
Published ArticleMetadata
Show full item recordAbstract
Effects of flooding on soil nitrogen (N), and asymbiotic
nitrogen fixing bacterial (diazotroph) populations of the
Okavango Delta were investigated. Diazotrophs from
the rhizosphere of dominant annual and perennial
grasses of the Okavango Delta were isolated on N-free
composite media and identified applying morphological
and biochemical criteria and Restriction Fragment
Length Polymorphism (RFLP). Azotobacter species
were found associated mostly with the grasses
Andropogon guyanus and Vetevaria nigritiana (103CFU
g–1 rhizosphere soil). Annual grasses such as Eragrostis
inamoena, Setaria sphacelata as well as perennials
showed insignificant populations (<101CFU g–1 soil). On
the contrary, Azospirillum spp. was found associated
with most of the grasses. Highest population densities
(above 104MPN g–1 soil) were observed in rhizospheres
of E. inamoena, Cympogon excavatus, Sporobolus
acinifolis, Eragrostis lapila and Eragrostis rigidor and
lowest population densities (below 102MPN g–1 soil) in
rhizospheres of Andropogon guyanus and Panicum repens. Flooding increased the Azotobacter populations,
while Azospirillum spp. populations were significantly
reduced. Nitrogen-fixing sulphur reducers were
observed in the rhizosphere of annuals (A. guyanus and
P. repens) and only in perennials growing in flooded
soils. No Beijerinckia species were found associated
with any of the grasses. The highest soil nitrogen levels
were detected in flooded soils (<0.20% N) while no N
was detected in the rhizosphere soil of grasses growing
in very dry soils (<5% moisture content). The results
indicate that in the Okavango Delta, total soil nitrogen
varies with flooding regime. With flooding, the diazotroph
population shifts towards Azotobacter and Nfixing
sulphur reducers while Azospirillum spp. are
widespread in non-flooded soils. RFLP analysis of the 1
450bp amplicon using the restriction endonuclease
Alu1, showed three different banding profiles, suggesting
the occurrence of three different species of
Azospirillum.