Growth response of <i>Pterocarpus soyauxii</i> and <i>Lophira alata</i> seedlings to host soil mycorrhizal inocula in relation to land use types

Abstract

Deficiency in mycorrhizal inoculum in soils due to land use types (LUT) can be alleviated by inoculum addition. Inoculum effects may depend both on quantity and on quality of inoculum applied. A greenhouse bioassay was carried out to determine the effect of host soil mycorrhizal inoculum on mycorrhizal colonization, carbon allocation and partitioning of seedlings of two native timber species of Cameroon humid forest, raised on surface soils (0-20 cm) collected from early secondary forests, LUT derived from slash-and-burn agriculture and selective logging. Host soil mycorrhizal inoculum was collected from the root zones of con-specific tree species. Mycorrhizal inoculation effect (MIE) was estimated as percent difference of average total biomass between seedlings grown on inoculated and non-inoculated soils. Six months-old seedlings of Pterocarpus soyauxii and Lophira alata were mainly colonized by members of the Glomaceae and Gigasporaceae, respectively, as shown by molecular typing. They generally performed poorly in soils with indigenous inoculum. But addition of soil inoculum from P. soyauxii trees favored nodulation, significantly increased mycorrhizal colonization and total biomass but decreased root-to-shoot ratios, resulting in large and positive MIE, irrespective of LUT. ln contrast, host soil inoculum of L. alata did not affect fractional mycorrhizal colonization but significantly increased total biomass, resulted in high carbon allocation to roots, in low and sometimes negative MIE. Therefore, seedlings' responses to mycorrhizal inoculum depend on host soil inoculum and that could be critical for successful rejuvenation of tropical trees.

Key words. Arbuscular mycorrhiza-host soil inoculum-Iand use types-Pterocarpus soyauxii-Lophira alata-Cameroon