Ochetophila-infective Frankia colonization patterns of volcanic ash in Patagonia

Frankia actinobacteria occur in northwest Patagonian steppe soils and form nitrogen-fixing actinorhizal symbiotic root nodules with Ochetophila trinervis (Rhamnaceae). Soil disturbances affect soilborne Frankia, reducing the likelihood of establishing symbiosis, as found for Pinus ponderosa plantations in the region. The effect on local actinorhizal symbionts of other kinds of disturbance such as the ash that was deposited by the eruption of the Puyehue-Cordon Caulle (PCC) volcanic complex was unknown. We studied the initial succession steps for the Frankia colonization process and the soil change that might affect the establishment of symbiosis as a result of the combined action of volcanic ash deposition and P. ponderosa afforestation by comparing pre- and post-eruption time points. We collected soil and ash samples at three depths to use in plant bioassays with O. trinervis. These samples were also used to determine chemical properties (soil and ashes) and elemental composition (soil). Infective Frankia gradually colonized the ash deposited in steppe soils via soil legacy and immigration. The higher nodulation frequency found in plants inoculated with buried soil and deep ash than in plants inoculated with surface ash suggests that the colonization process was mainly due to soil legacy, but also, though at a slower rate, to immigration of Frankia from elsewhere. Nevertheless, the colonization process seemed to be hindered by the pine plantation establishment processes. C, N and P increased over time, favoring the establishment of Frankia and therefore the nodulation capacity of ash. The chemical and elemental characteristics of soils under natural vegetation seemed to be closer to pre-eruptive conditions than those of soils under the afforestation, and to favor nodulation capacity. The combination of ash deposition and the pine plantation appeared to have the strongest effect in impairing the restoration of previous conditions for the establishment of symbiosis with Frankia.