Nitrogen fixation and cycling in Natal valley bushveld Acacia species.

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.