Belowground competitive suppression of seedling growth by grass in an African savanna

Coexistence of N2-fixing legumes and non-legume trees with grasses in African savannas results in intense competition between these life-forms. We hypothesised that belowground competition might induce different nutritional constraints in N2- versus non-N2-fixing species. A field (Hluhluwe-imFolozi nature reserve, South Africa) competition experiment with two N2-fixing legume species (Acacia burkei and Acacia karroo) and two non-N2-fixing species (Schotia brachypetala and Spirostachys africana) both with clipped grass and without grass was established. Plants were supplied with no fertilizer, or generous amounts of fertilizer (200 kg N ha−1, 100 kg P2O5 ha−1, 7.1 kg K2O ha−1) supplied as either 28–10 (N–K), P or a combination of these fertilizers (NPK). Regularly clipped grass suppressed growth (by more than 90 %) of both N2- and non-N2-fixing seedlings equally. Biomass accumulation of seedlings grown with grass and the grasses themselves responded positively to NK and/or NPK, but not P, although P-fertilization did have effects on foliar [N] and δ15N values of trees and grasses showing that plants accessed the fertilizer. Tree δ15N values and foliar [N] were also modified by NPK, demonstrating access to fertilizer. However, the ameliorative effects of NPK on grass competition-induced biomass suppression were only partial. This may be due to ‘non-resource competition’ (i.e. root gaps) imposed by dense grass roots. The fact that nutrients were able to partially ameliorate the effects of grass competition, however, indicates that such ‘non-resource competition’ may be partially overcome by even more generous supply of fertilizers.