Factors affecting the survival of a flavobacterium species in non-planted and rhizosphere soil

Factors affecting the survival of a Gram-negative soil Flavobacterium sp. (P25) were investigated in small-scale laboratory microcosms. Two terms for expressing P25 persistence are used: survival-describing the number of P25 reisolated from soil at a certain time point; and survival rate-expressed as the time in days taken for a 90% (t90), 99% (t99) or 99.9% (t99.9) decline in numbers of P25. Survival rates of P25 increased in rhizosphere as opposed to bulk soil and were increased further by applying the inoculum directly to the planted seedling (t99 = 21 d) rather than homogenizing it through the soil prior to planting (t99 = 10.5 d). Survival was increased by using greater inoculum densities, for example following inoculation at 1.1 x 10(9) P25 g-1 soil, numbers surviving at day 40 were 12.5-fold greater than that achieved with 1.1 x 10(4) P25 g-1. In addition, survival rates of P25 were increased by applying the inoculant to soil at moisture holding capacities (MHC) between 40 and 50% (t99.9 = 12.2 d) in comparison to either wetter (e.g. 100% MHC, t99.9 = 3.5 d) or drier (e.g. 10% MHC, t99.9 = 1 d) soils. Carbon amendments (i.e. galactose, maltose, sorbitol) significantly increased survival of P25 in non-planted soil throughout the 35 d study whereas in rhizosphere soil an increase was only recorded for the first 14 d.