|dc.description.abstract||Five species, Acacia karroo, A. robusta, A. nilotica, A.
sieberana and A. tortilis, were inoculated with Rhizobium and
grown in potted sand in a temperature controlled greenhouse.
After six months, results showed a higher percentage plant
nitrogen for all five species when inoculated plants were
compared to uninoculated controls. Inoculated treatments of
A. karroo and A. sieberana had the greatest growth in shoot
length and biomass. Acacia robusta showed the highest
nitrogenase activity when nodules were tested using acetylene
reduction methods. Inoculants of A. tortilis showed the
poorest growth for all parameters measured.
A. karroo and A. nilotica were studied at a field site at
Ashburton, 15km east of Pietermaritzburg. Acacia karroo and
A. nilotica had similar mean percentage leaf nitrogen, but A.
karroo had a significantly higher mean percentage stern
nitrogen than A. nilotica. Rainfall, canopy throughfall and
stemflow from A. karroo and A. nilotica were collected in late
spring and examined for inorganic nitrogen content. Acacia
nilotica yielded the highest nitrate levels in both
throughfall and stemflow samples. Acacia karroo produced
lower nitrate concentrations in samples of both throughfall
and stemflow, than was found in rainfall. Both A. nilotica
and A. karroo exhibited higher concentrations of ammonium in
samples of throughfall and stemflow as compared to levels. Soil analyses yielded highest levels of organic nitrogen at
the surface (0 - 5 cm) but this decreased significantly at 20
cm deep. Surface organic nitrogen was highest under A. karroo
canopies and lowest in open grassland. At 20 cm, there was
little difference in organic nitrogen content between soils
sampled from open patches and those under canopies of A.
nilotica or A. karroo. Nitrate showed little variation with
species, but highest levels were found in the top five
centimetres and levels were higher under grasslands than under
canopies. Ammonium showed no significant differences between
different depths but was higher in open grassland sites than
under canopies. No pattern could be found to relate tree size
to soil organic nitrogen content.||en