Single and multi-strain rhizobial inoculation of African acacias in nursery conditions

Abstract

Inoculation experiments were conducted in Kenya on seven African Acacia species/subspecies (Faidherbia albida, Acacia karroo, A. arenaria, A. nilotica ssp. kraussiana, A. tortilis ssp. spirocarpa, A. tortilis ssp. heteracantha, A. senegal) in sterilised and untreated soil. The untreated soil contained 103 rhizobia g−1. In six of seven species in untreated soil the multi-strain inoculated plants contained significantly more total nitrogen than control plants. The exception was A. arenaria in which significant increase in total nitrogen was achieved only with the single strain inoculum in sterile soil. In A. tortilis ssp. spirocarpa the single strain was better than the multi-strain inoculum. Significant increases over controls in dry weight ranged from 19 to 75% and in total nitrogen from 11 to 89%. Nitrogen derived from fixation (Ndff) was determined for three species/subspecies using the natural abundance 15N method. Values for fixation for the best treatments in these species were A. nilotica 53%, A. tortilis ssp. heteracantha 45% and A. tortilis ssp. spirocarpa 44%. These are conservative values because of the relatively small δ15N values (−2.85‰ for A. nilotica and −2.52‰ for both species of A. tortilis) determined as the 100% fixation values. We conclude that: inoculation can result in substantial gains in nitrogen fixation in African acacias; multi-strain inoculation is preferable to single strain inoculation in some circumstances; A. nilotica and A. tortilis have at least moderate nitrogen fixation potential and the wide genetic variation found suggests that substantial improvement may be obtained by selection for this character.