SOIL AMELIORATION AND ROOT SYMBIOSES OF PARKIA-BIGLOBOSA (JACQ) BENTHIN WEST-AFRICA

Parkia biglebosa is an important multipurpose tree from the savanna zo ne of West Africa. It has been reported to increase soil fertility and crop yields beneath its crown. However, no work has been conducted to determine the role of root symbioses in soil amelioration by this spe cies. The existing reports of nodulation in Parkia biglobosa are contr adictory and the presence of mycorrhizae is not documented in the lite rature. Fieldwork was conducted at six sites in Burkina Faso and Niger ia, to investigate the amelioratory effect of Parkia biglobosa on soil fertility and to ascertain the occurence of root symbioses. Soil samp les were collected from points close to the trunk, the mid-crown, crow n edge and the open areas away from individual trees. The samples were analysed chemically to assess pH, organic matter, nitrogen, phosphoru s and potassium content. Root samples were also collected from the fie ld and examined for nodules and for evidence of mycorrhizal infection. The results of the soil analysis showed a significant amelioration fo r total nitrogen and available potassium with proximity to the tree, b ut organic matter, available phosphorus and soil pH showed no signific ant trend. This pattern was similar for all sites. The results also in dicated that amelioration of nitrogen and potassium with proximity to the tree increased with tree size. Root nodules were absent from the r oots of trees examined in the field, both in Burkina Faso and Nigeria. A pot experiment was set up using the soil collected from beneath the trees and a laboratory grown cowpea miscellany culture as rhizobial i noculum for Parkia biglobosa seedlings. The roots of these seedlings a lso showed no evidence of nodulation. Ectomycorrhizae were not present in roots collected from the field, but endomycorrhizal infection was prolific. A study of root distribution in Burkina Faso suggested that one mechanism for increased soil fertility beneath Parkia biglobosa is the widespread lateral root system, which redistributes nutrients fro m an extensive radius to a concentrated area beneath the crown. The ro ot system is aided in the efficient uptake of nutrients by endomycorrh izae. Other possible mechanisms include interception of dust by the cr own and deposition by throughfall and stemflow, defacation by animals beneath the tree and decomposition of leaf litter.