Soil nutrient ecology associated with Acacia sieberana at different tree densities in a South African savanna

Abstract

Three aspects of soil nutrient ecology in an Acacia sieberana savanna were studied. First, information was collected about the effects of the savanna tree species Acacia sieberana on soil fertility, in terms of isolated tree effects and in terms of tree density effects. Second, the influence of savanna trees on arbuscular mycorrhizal abundance was investigated, and third, arbuscular mycorrhizal dependence of the indigenous African savanna grass species Panicum maximum was tested. Organic matter content, pH, mineral nitrogen, total nitrogen, mineralisable nitrogen, resin extractable phosphorus, total phosphorus and mineralisable phosphorus were determined in soils from open and subcanopy positions and in soils from sites with increasing tree density. In bioassays Panicum maximum growth, phosphorus content, mycorrhizal infection levels and mycorrhizal dependence were studied. A positive influence of isolated trees, in this case Acacia sieberana, on soil fertility and mineralisation was found, causing increased plant growth independent of possible differences in microclimate between open and subcanopy positions. At higher tree densities, differences in soil nutrient levels were not related to position under tree canopies and levels of nutrients appear to increase. No influence of isolated trees on arbuscular mycorrhizal abundance was found. Mycorrhizas enhanced growth and phosphorus uptake of Panicum maximum.

Additional index words:

• Fertile islands
• mycorrhizas
• Panicum maximum
• patchiness