Plant competition reshapes nitrogen utilisation in native Vachellia nilotica (Fabaceae)

Context Vachellia nilotica (Benth.) Kyal. & Boatwr is a leguminous species subjected to competition from co-occurring grassland species in mesic grasslands, including Themeda triandra Forssk. In South Africa, Vachellia nilotica enhances soil nitrogen in nutrient-deficient mesic savanna. The addition of phosphorus and competition between the grass and legume may assist in maintaining balance between the two plant groups. Aims Improved P supply in nutrient-poor soils favours growth of legumes over grasses. This study aimed to assess how nutrient deficiency, P fertilisation, and competition with a dominant grass species, particularly Themeda triandra, affects the aboveground and belowground growth, N nutrition, and nodulation potential of Vachellia nilotica in the Ukulinga Grassland nutrient experiment. Methods Soils used in the study were collected from superphosphate-fertilised trials (+P, i.e. P-enriched) and non-superphosphate trials (−P, i.e. no P enrichment). V. nilotica seedlings were grown independently and in competition with T. triandra seedlings. Key results Our findings confirmed that soil in +P treatment had greater P and calcium concentrations than did the −P soils. Competition from T. triandra decreased the P concentration, specific nitrogen assimilation rates and the %NDFA in V. nilotica, but the opposite effect was observed for specific nitrogen utilisation rates. Conclusions Zero nodule formation in soils with low P concentrations and competition affected the N source reliance of V. nilotica. Implications The addition of P might maximise N inputs through increased legume growth performance and weaken the competitive effect of a grass species.