Persistence, Infectiveness, and Effectiveness of Bradyrhizobium Japonicum in Soybeans (Glycine max L.).

Biological nitrogen fixation (BNF) in soybeans (Glycine max L.) is the result of a symbiotic relationship with Rhizobia.  Bradyrhizobium japonicum is added as an inoculant to seeds at planting to enhance BNF. Their persistence in the soil beyond a growing season is dependent upon the competitiveness and fitness of the naturalized rhizobium from previous inoculants and management practices. However, persistent strains may lower BNF particularly as roots grow beyond the inoculation zone. As such, there is a need to understand rhizobium population dynamics and the effects of differing rotations, tillage, and fertilizer regimes on their activity and the biologically fixed N they produce. A split-split-split plot, randomized complete block design experiment was established with three replicates where main plots were crop rotation (wheat (Triticum aestivum L.)-field pea (Pisum sativum L.)-corn ( Zea mays L.)- soybean) and split plots were tillage (conventionally and no-till) and fertilizer (112 kg-N ha^-1, 56 kg-N ha^-1 as manure, and no N). Soils were sampled at 0-15 and 15-30-cm prior to planting. B. japonicum persistence in soil was estimated by nodZ gene copies and measured via qPCR. Whole soybean plants were collected and nodules were counted and weighed during the V2-4 and V6-8 growth stages. mRNA from nodules was extracted and quantified to determine the activity of the nifH (N fixation) gene. The amount of N in soybean tissue derived from BNF was measured by plant ureide-N and nitrate concentrations. Tillage system and nitrogen availability had the greatest effect on B. japonicum numbers and activity, where nutrients in no-till systems can be stratified. The lower amounts of nitrogen obtained from BNF under no-till systems reflects the reduced reliance on B. japonicum for plant nitrogen needs.  Additionally, these results suggest B. japonicum persistence in the soil is related to organic matter distribution and nitrogen dynamics from the previous crop. Manure applications add additional organic matter into the soil that can aid in harboring greater inoculant populations. Consequently, inoculated populations are important for early season BNF, but persistent populations of B. japonicum play an important role in late season BNF in the lateral root nodules.