The role of phosphorus deficiency in nodule microbial composition, and carbon and nitrogen nutrition of a native legume tree in the Cape fynbos ecosystem

Abstract

In phosphorus (P)-poor ecosystems, microbial communities can play a major role in the nitrogen (N) mineral nutrition during N2 fixation in legumes. This study investigated the role of P nutrition on the composition of N2-fixing bacterial community in Virgilia divaricata root nodules, grown under glasshouse conditions. V. divaricata seeds were germinated in Fynbos soil as a natural inoculum, and, thereafter, transferred into sterile quartz-sand cultures and supplied with 500 µM P and 5 µM P, respectively. The N2-fixing bacterial communities in the rhizosphere and root nodules were examined on the basis of the polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE) banding patterns of 16S rDNA and sequencing methods. The GenBank blast results showed that V. divaricata was nodulated by a wide range of root-nodule bacterial strains also found in the rhizosphere. These included Burkholderia phytofirmans, Burkholderia sp. and Bradyrhizobium sp., during both high and low P supply. The 15N natural-abundance data also confirmed that 40–50% of the N nutrition was from symbiotic N2 fixation. This is not only evidence of nodulation, but an indication of the adaptation of a range of N2-fixing bacterial strain species to the nutrient-poor, sandy, acidic soil of the Mediterranean-type ecosystems of the fynbos vegetation in the Cape Floristic Region (CFR). Legume species V. divaricata is highly adapted to the low-nutrient soils of its native range by its association with the symbiotic N2-fixing bacteria.