Bradyrhizobium japonicum inoculant persistance and effectiveness in soybeans (Glycine max L.)

Background/Question/Methods

Biological nitrogen fixation (BNF) in soybeans (Glycine max L.) occurs though a symbiotic relationship with Bradyrhizobium japonicum.  B. japonicum is added as an inoculant to soybean seeds at planting to improve BNF that would otherwise be dependent upon naturalized residual Rhizobia inoculum from previous applications.  However, questions remain regarding inoculant persistence and ability to effectively fix nitrogen. We hypothesize that tillage, crop rotation, and nitrogen (N) fertilization will influence B. japonicum inoculant persistence and nodule infectiveness and effectiveness of BNF.  We utilized a long-term multi-crop experiment with a Wheat/Field Pea/Corn/Soybean rotation as blocks with tillage (no till and conventional) and fertilizers (112 kg-N ha^-1, 56 kg-N ha^-1 as manure, and 0 kg-N ha^-1) as factors. Soils were sampled from 0-15 and 15-30 cm in order to determine B. japonicum persistence using qPCR primers for the nodZ gene active in nodulation signaling. Additionally, soybeans and nodules were collected during the V2-4 and V6-8 growth stages. The amount of N in soybean tissue derived from nitrogen fixation was determined by plant ureide-N concentration and plant nitrate concentration. Soybean nodule counts and weights were assessed and mRNA was extracted and quantified to determine the activity of the nifH nitrogen fixation gene.

Results/Conclusions

Results indicate B. japonicum persists in the soil following inoculation at an average of 1.01x10⁴ nodZ copies with the highest copy numbers following soybean harvest (2.04x10⁴). Interestingly, B. japonicum populations during the V2-4 and V6-8 growth stages following inoculation are not higher than other points in the rotation (p=0.8188). B. japonicum persisted at higher numbers (p=0.0008) under manure additions (1.21x10⁴) than 112 kg-N (7.05x10³) or no fertilizer (6.83x10³). Plant nitrogen obtained through BNF increased under no-till at later growth stages (from 40 to 47%), while simultaneously decreasing in conventionally tilled soils (from 58 to 55%) (p=0.0469).  Nodule mRNA nifH gene copy numbers were greater where manure  was applied (1.01x10⁷) relative to the 112 kg-N plots (2.37x10⁶) (p=0.0325). Taproot nifH activity was greater during early growth stages (1.66x10⁷) but decreased at later growth stages (1.31x10⁶) where lateral root nifH activity significantly increased from early growth (2.15x10⁶) to later growth (1.01x10⁷) (p<0.0001). These results suggest B. japonicum persistence in the soil is related to organic matter distribution and the ability to persist in the saprophytic stage which is important for lateral nodule development and late season BNF. However, lack of correlation with ureide-N suggests the plant has ultimate control over nodule activity.