Naturalised populations of mesorhizobia in chickpea (Cicer arietinum L.) cropping soils: effects on nodule occupancy and productivity of commercial chickpea

Chickpea rhizobia did not occur naturally in Australian cropping soils, necessitating inoculation at sowing. Now, after more than 30 years of chickpea cultivation using a single inoculant strain, CC1192, it is likely that chickpea rhizobia are established in 1.0-1.5 Mha cropping land. The aims of this study were to examine effects of the naturalised chickpea rhizobia on nodulation and productivity (total crop N, crop N fixed and grain yield) of commercial chickpea. Soil was sampled from 26 fields to estimate chickpea rhizobial numbers, relate numbers to edaphic factors and years since previous chickpea crop, determine the proportions of CC1192 and novel strains using RAPD-PCR and subject a subset of novel strains from one site to 16S rRNA analysis. Nodules were harvested from 15 inoculated, commercial chickpea crops to determine occupancy by CC1192. The symbiotic effectiveness of a second subset of novel strains was assessed. The mean number of rhizobia in the soils varied from log 0.08 to log 5.16 rhizobia g soil(-1) with population size positively correlated with soil moisture content and negatively correlated with salt concentration (ECe). RAPD-PCR analysis of 570 strains of chickpea rhizobia isolated from the soils indicated only 14 % with molecular fingerprints similar to CC1192. Occupancy by CC1192 of nodules harvested from commercial crops ranged 0-100 %, with an average of 53 %. Occupancy by CC1192 declined by an average 17 % with each log unit increase in numbers of novel chickpea rhizobia. We found no evidence that the novel mesorhizobia in the chickpea soils compromised N-2 fixation or productivity of commercial chickpea crops, even though individual strains had generally reduced symbiotic effectiveness relative to CC1192.