No 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, 156 RAO ET AL. 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.

Low phosphorus (P) supply is a major limitation to pasture establishment and production in highly weathered acid soils. Previous research showed that the forage legume, Arachis pintoi, was more efficient in acquiring P from infertile acid soils than the forage grass, Brachiaria dictyoneura. The main objective of the present study was to determine whether the higher P-uptake efficiency (defined as "milligrams of P uptake in shoot biomass per unit root length") of the legume results from its greater ability to mobilize P from less available forms of phosphate (aluminum phosphate and organic P). The grass, Brachiaria dictyoneura CIAT 6133, and the legume, Arachis pintoi CIAT 17434, were grown in the greenhouse for 75 days in monoculture or in association. They were planted in large plastic containers holding either one of two acid soils of contrasting texture (sandy or clay loam) and amended with 1176 RAO ET AL. different sources of P. Four sources of P were used: di-calcium phosphate (Ca-P), aluminum phosphate (Al-P), phytic acid (organic-P), and cow manure (dung-P). Except for Al-P, which was applied at two levels (20 and 100 kg P ha -1 ), the other sources were applied at 20 kg P ha -1 . We found greater differences in P acquisition between the grass and legume than between the two types of acid soil. Acquisition of P by the legume was markedly greater than that of the grass, regardless of the source of P. The legume accessed 205 to 220% more sparingly soluble, inorganic P from soil compared to the grass. The relationship between shoot P uptake and root length showed that the legume roots acquired more P per unit root length than did the grass roots. The superior ability of the legume roots to acquire P from different P sources was associated with higher levels of inorganic P in roots. These results indicate that one reason for the success of Arachis pintoi in competing with aggressive grasses was its ability to acquire P from less available forms present in acid soils. This study also suggests that the measurement of inorganic-P levels in roots may serve as a selection method to evaluate tropical forage legumes for their adaptation to less available P sources in P-deficient acid soils.

No abstract

No abstract