Determinants of community organization of a South African mesic grassland

. Question: What is the long-term influence of nutrient availability, productivity and soil pH on grassland community organization?Location: Ukulinga research farm, KwaZulu-Natal, South Africa.Methods: The influence of fertilization on soil pH, nitrogen (N) and phosphorus (P) on variation in plant traits, community composition and species richness were examined in a 50-year grassland fertilization experiment.Results: Averaged over 30 years, above-ground net primary production (ANPP) was 337, 428 and 518 g.m-2 in sites not fertilized, fertilized with N, and fertilized with N plus P respectively. ANPP depended directly on N-fertilization but not on P-fertilization or liming, and responded positively to the interaction of N (first limiting nutrient) and P (second limiting nutrient). Short narrow-leaved grass species —Themeda triandra, Tristachya leucothrix and Setaria nigrirostris— dominated sites of lowest ANPP where N was limiting (unfertilized, P-fertilized or limed sites). A tall narrow-leaved species, Eragrostis curvula, dominated sites of intermediate ANPP where P was limiting (N-fertilized sites). By contrast, a tall broad-leaved species, Panicum maximum, dominated the most productive sites where neither N nor P were limiting (N- and P-fertilized sites). Certain species responded to liming and type of N-fertilizer apparently because of their effects on soil pH. N-fertilization reduced the density of herbaceous dicots (forbs) from 14 (unfertilized) to two (high N, no P, no lime) and five species per m2 (high N, no P, limed). This effect was attributed to increased ANPP and a decrease in soil pH from 4.6 (KCl) in unfertilized sites to 3.49 (high N, no lime) and 4.65 (high N and lime). Soil acidification had no effect on grass species richness but influenced the abundance of certain species.Conclusions: Grassland community organization is determined not only by the influence of N availability, but also by the hierarchical interaction of N and P availability, in part through their compounded effect on ANPP, and by individualistic species responses to soil pH.