Low temperature tolerant Bradyrhizobium japonicum strains allowing improved nodulation and nitrogen fixation of soybean in a short season (cool spring) area

In short-season soybean production areas, low soil temperature is a major factor limiting soybean growth and yield. Although Bradyrhizobium japonicum strain 532 C is widely used in Canadian conditions its ability to nodulate and fix nitrogen in symbiosis with soybean is inhibited by low temperatures. We have attempted to identify strains that perform better than 532 C under cool soil conditions. We selected 39 B. japonicum strains from the USDA collection based on their isolation from northern locations in North America. These 39 strains were observed for their ability to grow at a low temperature (15 (degree)C) and the best two (USDA 30 and 31) were selected for evaluation in comparison with 532 C under field conditions. Two years (1998 and 1999) of field experiments were conducted at the Lods Agronomy Research Centre in southwestern Quebec. The treatments consisted of factorial combinations of inoculant type (no inoculant (control) and inoculants containing 532 C, USDA 30 or 31) and soybean cultivar (OAC Bayfield and Maple Glen). The experiment was structured following a randomized complete block design with four blocks. Data were collected on nodule number, nodule weight, and shoot nitrogen yield at four stages: (1) Three nodes on main stem beginning with the unifoliolate node (V3), (2) One flower at any node (R1), (3) Pod 2 cm long at one of the four uppermost nodes with a completely unrolled leaf (R4), and (4) 95% of pods brown (R8). Soybean plants inoculated with USDA 30 and 31 performed similarly and, averaged over the two seasons, resulted in increased nodule number, nodule weight, shoot nitrogen yield at harvest 3 (R4) and final nitrogen fixed by 12.7, 12.0, 12.8, and 11.9%, respectively, as compared to strain 532 C.