Assessment of VA mycorrhizal inoculum potential in relation to the establishment of cactus seedlings under mesquite nurse-trees in the Sonoran Desert

A commonly observed preferential association was quantified between mature native mesquite (Prosopis articulata) trees and the seedlings of six cactus species (Pachycereus pringlei, Opuntia cholla, Lophocereus schottii, Macha erocereus gummosus, Lemaireocereus thurberi, Mammilaria sp.) in a previousl y-disturbed area of the Sonoran Desert of Baja California, Mexico. We hypot hesized that, in addition to more favorable edaphic factors, the inoculum p otential of beneficial vesicular-arbuscular mycorrhizal (VAM) fungi was hig her, and therefore, more favorable for cactus seedling establishment under the mesquite tree canopy (UC) compared to adjacent barren areas (BAs) away from the trees. In the greenhouse inoculum potential assays, VAM fungi were detected in onion (Allium cepa) trap plants from all soil samples regardle ss of collection site, but cardon cactus (P. pringlei) trap seedlings forme d no VAM even after 6.5 months. Test soils were further used to preinoculat e new onion seedlings transplanted into pots, to serve as nurse plants to i noculate adjacent cardon seedlings by vegetative transfer. After 15 months, cardon seedlings did develop slight VAM colonization, confined exclusively to the outermost cortical layers. Examination of test soils for spores or root fragments revealed very few to none, and spore production on onion tra p plant roots was also sparse even though colonization was high. Analysis o f UC and BA soils revealed that the water holding capacity, nutrient conten t, cation exchange capacity, total carbon, and total nitrogen contents of t he UC soils were all higher than those of the BA soils. Since the VAM inocu lum density in this study was not different between sites under and away fr om the mesquite tree canopy, we concluded that VAM inoculum density is not the primary factor for the establishment of cactus seedlings and that edaph ic factors probably play a more important role. Our results suggest, howeve r, that VAM inoculum potential in these hot desert soils, although relative ly low, is probably maintained in the upper layers by means of hyphal fragm ents rather than spores. (C) 2000 Elsevier Science B.V. All rights reserved .