Adaptive attributes of tropical forage species to acid soils. III. Differences in phosphorus acquisition and utilization as influenced by varying phosphorus supply and soil type

Abstract

Low phosphorus (P) supply is a major limitation of forage production in acid soils of the tropics. A glasshouse experiment was conducted to determine the differences in P acquisition and utilization among three legumes (Arachis pintoi, Stylosanthes capitata and Centrosema acutifolium) and one grass (Brachiaria dictyoneura). The plants were grown under monoculture or in grass + legume associations in two acid Oxisols of contrasting texture (sandy loam or clay loam). The soils were amended with soluble phosphate at rates ranging from 0 to 50 kg‐ha^‐1. After 80 days of growth, total P and nitrogen (N) content in different plant parts, inorganic P content and acid phosphatase activity in roots and leaves, vesicular‐arbuscular mycorrhizal root infection, total P acquisition from soil, P acquisition efficiency, P transport index, and P use efficiency were determined. Total P acquisition (per unit soil surface) of A. pintoi was higher than that of the other two legumes at 50 kg‐ha^‐l of P supply. Phosphorus acquisition efficiency (mg of P uptake in shoot biomass per unit root weight) of the legumes in both soil types was several times higher than that of the grass at 50 kg‐ha^‐1 of P supply. Acid phosphatase activity in roots of the grass decreased markedly as the supply of P increased in the soil, whereas enzyme activity was maintained in the legumes at 50 kg‐ha^‐1 of P supply. Phosphorus use efficiency (g of forage produced per g of total P acquisition) of the grass in both soil types was markedly higher than that of the three legumes at all levels of P supply. This study highlighted the fact that inherent differences in efficiencies of P acquisition and utilization exist between tropical forage grass and legume species. These differences may play a major role in legume persistence in tropical pastures.

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