Effect of competition from a C₄ grass on the phosphorus response of a subtropical legume

Tropical pasture systems are typically dominated by C₄ grasses growing on nitrogen (N) deficient soils. Under these conditions, N₂-fixing legumes should have a competitive advantage, yet low legume contents are often reported in these systems. This work investigates whether below-ground competition for phosphorus (P) is limiting the ability of legumes to compete in swards of C₄ grasses when grown in a sand matrix. The external P requirement of a subtropical legume (butterfly pea, 'Clitoria ternatea' L.) and a C₄ grass (buffel grass, 'Cenchrus ciliaris' L.) were initially determined in a P-response experiment. Four rates of P (4.6-78.2 mg P kg-¹ of Colwell P) were subsequently selected to investigate the growth response of the butterfly pea when grown with and without competition from a sward of N-deficient buffel grass. Shoot dry matter was determined over successive cuts and P uptake determined at the final harvest at 72 days. Buffel grass dominated the mixed swards and reduced the shoot dry matter production of the butterfly pea by >50% relative to the pure swards. A significant difference in the soil P response curve and shoot P uptake of butterfly pea was not detected between pure swards and those with competition from buffel grass. The ability of C₄ grasses to acquire and convert resources (i.e. light, water and nutrients) more efficiently into shoot dry matter is likely to be a major factor resulting in grass-dominated pastures in tropical systems.