Mycorrhizal inoculation effect on nodulation and N accumulation in Cajanus cajan at soil P concentrations sufficient or inadequate for mycorrhiza-free growth

Abstract

The interaction of Cajanus cajan with Rhizobium and vesicular-arbuscular mycorrhizal fungi (VAMF) was investigated in a greenhouse experiment. C. cajan was planted in soil that had been inoculated with Glomus aggregatum or treated with benlate to suppress VAMF activity. Initial soil solution P concentrations of 0.06, 0.2, 0.4, and 0.8 mg l-1 were established to test the interaction at external P levels that ranged from inadequate to nonlimiting for the host plant. At 0.06 and 0.2 mg P l-1, mycorrhizal inoculation significantly increased plant P concentrations as well as nodule numbers and shoot dry weight. Mycorrhizal inoculation also significantly increased nodule dry weight at a soil P concentration of 0.4 mg l-1 but did not significantly influence any of the other variables. The mycorrhizal inoculation effect observed at this soil solution P concentration could not be explained by any of the measures of plant P status. At 0.8 mg P l-1, none of the measured variables were affected significantly by mycorrhizal inoculation. The results indicate that the enhanced nodulation associated with mycorrhizal inoculation at soil P concentrations lower than 0.4 mg l-1 was explainable by mycorrhizal-mediated P uptake. The small but significant increase in nodule mass due to VAMF inoculation at 0.4 mg P l-1 suggests that factors not related to plant P nutrition may be involved. On the other hand, the lack of a VAMF inoculation effect at 0.8 mg P l-1 despite VAMF colonization at a level comparable to that observed at the former P concentration appear to discount this hypothesis. This observation is also supported by the lack of response of plant N status and nodule number to VAMF inoculation at this soil P concentration.