Temporal biological nitrogen fixation pattern in soybean inoculated with Bradyrhizobium

Biological nitrogen fixation (BNF) in soybean [Glycine max (L.) Merr.] plays an important role in sustainable agriculture, reducing the limitations associated with other sources of N such as fertilizers and soils. Our major objective was to evaluate the weekly pattern of BNF in soybean influenced by nodule formation, using three different laboratory-cultured Bradyrhizobium strains. Plants were grown in 1-m polyvinyl chloride (PVC) columns for 17 week in a greenhouse and BNF was determined using an integrated approach by assessing nodule formation, stem ureide-N, and N partitioning in plant parts. Bradyrhizobium strains showed overall similar plant performance and N-2-fixation capacity. During the beginning of flowering/full bloom (R1/R2) growth stages, nodule formation significantly increased and reached a maximum at pod-filling (R4) stage. Stem ureide-N was detected at early growth stages even with fewer small nodules, which significantly increased after the beginning of pod formation (R3). Peak N-2-fixing rate (g N kg(-1) d(-1)) started to decline after the onset of seed filling (R5.5). Relationship between BNF and nodule parameters (nodule number [R-2 = .65] and nodule weight [R-2 = .62]) suggested that they can be used as predictors of BNF.