Nodular diagnosis of contrasting recombinant inbred lines of Phaseolus vulgaris in multi-local field tests under Mediterranean climate

• The nodular diagnosis is an innovative methodology to assess the spatial variability of legume nodulation in agro-ecosystem.
• Nodulation in agro-ecosystems varies considerably between the genotypes in space and time.
• Soil P availability is a limiting factor of the rhizobial symbiosis.
• Significant correlation between nodulation and Olsen-P content in soil.

Common bean (Phaseolus vulgaris L.) has the capacity to fix atmospheric N2 into the biosphere through its aptitude to establish a symbiosis with soil rhizobia. In order to search for environmental constraints that might limit this symbiosis a nodular diagnosis was performed in eighteen field sites chosen with farmers of the Setif agro-ecosystem. Common bean was used as a model grain-legume with six recombinant inbred lines (RILs) and one local genotype Djadida. At flowering stage, the biomass of plants and nodules was determined by excavating 20 cm in depth and around the root-system of ten plants per genotype and per site. The results indicate a large spatial variation in nodulation and growth between genotypes, and the distribution of soils in four soil clusters, based on physico-chemical properties. The inhibition of nodulation of all genotypes in soil of clusters A and B was associated with high residual soil mineral nitrogen (2.23 ± 0.49 g kg−1 soil). The low nodulation of all genotypes in the phosphorus (P) deficient soil of cluster C (6.73 ± 3.63 mg kg−1 soil) was partly compensated by increasing their efficiency in use of the rhizobial symbiosis (13%), estimated by the slope of the regression model of shoot biomass as a function of nodule biomass. Interestingly, significant correlations were found between nodulation of all genotypes and Olsen-P content in soils of clusters C (R2 = 0.97, P < 0.001) and D (R2 = 0.94, P < 0.05). It is concluded that the RILs selected for their efficient use of P for symbiotic nitrogen fixation show the highest nodulation and growth and that the nodular diagnosis can be used to assess the growth response of N2-dependent grain-legume to soils with low availabilities of N and P.