Are Northwestern Patagonian “mallín” wetland meadows reservoirs of Ochetophila trinervis infective Frankia?

Abstract

“Mallin” (plural mallines) is a particular kind of wetland occurring in Patagonian steppe and forests. In Northwest Patagonia, mallines are humid meadows with high net primary production. It was previously found that a mallin soil in the steppe devoid of actinorhizal plants had a higher Frankia nodulation capacity in Ochetophila trinervis (sin. Discaria trinervis) than other soils in the region. Under the hypothesis that mallin wetland meadows are reservoir of infective Frankia, we studied the Frankia nodulation capacity in O. trinervis of 12 mallin and their neighbouring steppe soils, by using plant bioassays. A qualitative plant bioassay showed that infective Frankia was present in most soils. The number of nodules per plant in seedlings inoculated with mallin soils was negatively correlated with soil water content while the opposite was true for plants inoculated with soils from neighbouring steppe. A quantitative bioassay was performed with eight representative soils, selected according to the number of nodules per plant produced in the qualitative assay and to the presence or not of different actinorhizal plants at the sites. Frankia nodulation units per cm3 of soil (NU) in mallin soils were higher than those in steppe. Water and organic matter content of soils were correlated with the higher nodulation capacity of mallines, which may account for the saprotrophic growth of Frankia in soils. The symbiosis was effective in plants inoculated with all soil samples. These results suggest that Northwestern Patagonian mallin wetland meadows are reservoirs of infective and effective Frankia propagules in O. trinervis.