Bradyrhizobium Populations Occur in Deep Soil under the Leguminous Tree Acacia albida

Abstract

Soil cores were drilled under the leguminous tree Acacia albida growing in two different ecoclimatic zones of West Africa: the Sahelian area (100 to 500 mm of annual rainfall) and the Sudano-Guinean area (1,000 to 1,500 mm of annual rainfall). Soil samples were collected at different depths from the surface down to the water table level and analyzed for the presence of rhizobia able to nodulate A. albida. In both areas, population densities of rhizobia were substantially greater near the water table than near the surface. In the Sahelian area, rhizobia were present as deep as 34 m at a concentration of 1.3 × 10 3 /g of soil. In the Sudano-Guinean area, population densities at 0.5 to 4.5 m depth were higher than in the Sahelian area and, at several depths, comparable to that of temperate soils supporting legume crops (10 4 rhizobia per g of soil). Surface and deep soil isolates from all four sites were found to be slow-growing rhizobia ( Bradyrhizobium sp.). The proportion of effective isolates was almost the same within surface and deep soils.

Groundwater in the Earth's critical zone: Relevance to large‐scale patterns and processes

“…Outside of the seasonal Amazon, evidence of groundwater support for vegetation appears widespread in Mediterranean‐type climates characterized by ample winter rains and warm‐dry summers [e.g., Walter , ; Kimber , ; Dawson and Pate , ; Barbeta et al ]; here deep roots terminating in the capillary fringe above the water table are found in woody plants [ Kimber , ; Dawson and Pate , ]. It is also observed in the seasonal tropics of the Campos Cerrados of southern Brazil [ Rawitscher , ; Christina et al ., ], the Sahelian western African [ Dupuy and Dreyfus , ], and the Gangetic Plains of northern India with its highly seasonal monsoon rains [ Howard , ].…”

“…This dry gap narrows at the lower position‐2, and the roots may sense the capillary rise from the groundwater that is further aided by the occasional deep rainfall infiltration and the water table rise and fall that can “lead” the roots into deeper soils [ Naumburg et al ., ; Canham , ]. In a climate with seasonal rain, dimorphic root systems, with a shallow root cluster utilizing rain in the wet season and a deep cluster utilizing groundwater in the dry season, are often observed [e.g., Howard , ; Kimber , ; Dupuy and Dreyfus , ; Dawson and Pate , ; Rood et al ., ]. At the still lower position‐3, rainfall infiltration meets groundwater capillary rise (no dry gap), and soil water profile is probably not the primary driver of rooting depths.…”

“…Below‐ground niche separation may hold the key to the more readily observable aboveground separation or coexistence [ Weaver , ; Walter , ; Terradas et al ., ; Bartelheimer et al ., ; Ward et al ., ]. As Weaver [] wrote, “One cannot fully understand why a particular plant is found in a particular place without the knowledge of its root systems.” As roots penetrate deeper and venture into the previously sterile sediments, they also invite along a whole ecosystem of organisms either as symbiotic associations such as nitrogen‐fixing rhizobium in leguminous root nodules [ Dupuy and Dreyfus , ; Bornyasz et al ., ] or mycorrhiza fungi that extend root surface area for water and nutrient acquisitions particularly the less mobile nutrients [e.g., Richter et al ., ], or simply as fodder at the base of the trophic web in soils [see Schulze , for review]. In all cases, roots act as pioneers of biological activities in soils [ Richter and Markewitz , ; Maeght et al ., ], the latter a definitive line between sterile sediments and living soils.…”

Pulsed-field gel electrophoresis for genotypic comparison of Rhizobium bacteria that nodulate leguminous trees

“…Leguminous woody plants, which are able to utilise atmospheric nitrogen by forming a symbiosis with nitrogen-fixing bacteria, play a major role in the sustainable management of arid areas around the world [1][2][3]. In recent years the importance of finding efficient nitrogen-fixing, rootand stem-nodulating bacteria for use in tropical countries has promoted research on newly discovered rhizobial species that infect leguminous trees * Corresponding author.…”

Indigenous Arbuscular Mycorrhizal Fungi Associated with Acacia albida Del. in Different Areas of Senegal

“…In these semiarid zones, most indigenous and exotic Acacia species are associated with rhizobia and with ectomycorrhizal and AM fungi (6,10). But only AM fungi and slow-growth strains of rhizobia simultaneously colonize A. albida roots (7,10).…”