Influence of Water Potential on the Survival of Rhizobia in a Goldsboro Loamy Sand

Inoculation of soybeans [Glycine max (L.) Merr.] at planting in North Carolina is apparently ineffective due to unfavorable environmental conditions. Water potential has been suspected to be a primary factor in inoculation failure; however, the lack of a suitable method to artifically control or create desired water potentials in laboratory environments has prevented investigation. The objectives of this study were to (i) evaluate a technique designed to maintain soil water potentials for incubation studies, and (ii) to investigate the influence of water potential on the survival of 10 Rhizobium japonicum strains in a Goldsboro loamy sand (Aquic Paleudult). Dow Ziploc Storage Bags were employed to maintain desired water potentials while the most probable number (MPN) technique and plant infectivity tests were used to enumerate rhizobia. Water potential was found to have a profound effect on numbers of rhizobia in a Goldsboro loamy sand. Rhizobial numbers were from one to three log numbers lower under water potentials of −15 bars than at potentials near field capacity. There was usually a depression of one log number at −5 bars. In most cases the 9-week rhizobial numbers were less at water potentials of −0.10 bar than at potentials of −0.50 and −0.33 bars. Total aerobic bacteria and aerobic sporeformer numbers also declined at both high and low water potentials. Differences in ability to tolerate low water potentials were noted between the 10 serogroups tested. In general, isolates from serogroups 110, 122, and 138 survived in much greater numbers at water potentials of −15 bars than the other seven serogroup isolates. Isolates of serogroups 24, 31, 94, and 123 were particularly vulnerable to water stress as populations at 9 weeks were more than four log numbers less than the initial populations. Field soil water potential data confirmed that −15 and −5 bar values which reduce rhizobial numbers are found to be quite common in North Carolina.