Evidence for phosphorus cycling parity in nodulating and non-nodulating N2 fixing pioneer plant species in glacial primary succession

Nodulating leguminous and actinorhizal N2-fixation pioneer plants are well-known drivers of primary succession as they may facilitate soil development and the growth of neighboring non-nodulating plant species as a result of their N2-fixing capacity. However, recent studies have shown that some non-nodulating species may also obtain N through endophytic diazatrophs although the N2-fixing capacity is relatively low when compared to the traditionally nodulating species. There remains limited understanding of how these two categories of N2 fixing pioneer plant species (nodulating and non-nodulating) acquire recalcitrant resident soil phosphorus (P) pools and facilitate soil P cycling. To address this knowledge gap, we investigated whether pioneering plant species belonging to different functional groups, i.e., nodulating N2-fixing species (leguminous Astragalus mahoshanicus and actinorhizal seabuckthorn Hippophae rhamnoides) and non-nodulating endophytic N2-fixing willow species (Salix rehderiana), have distinct rhizosphere soil P chemistry when grown on barren deglaciated moraine. We also examined if plant-induced changes in soil P transformations are related to the relative abundance of microbial P transformation genes. Our results showed that pioneer plant colonization enhanced soil P cycling as indicated by higher concentrations of available P (Olsen-P), alkaline phosphatase activity, and abundance of key genes governing microbial P-cycling in rhizosphere soils compared to bulk soils. Among plant species, the astragalus and the willow had the greatest available P concentrations along with greater organic acid concentrations, total organic P transformation gene and organic P mineralization gene abundances. On the other hand, seabuckthorn had the lowest available P concentration and organic P mineralization gene abundance. The willow species, S. rehderiana, was the only plant studied that had greater total abundance of inorganic P solublization genes, gcd, ppk, as well as the organic P mineralization gene phoD than that found in bulk soil. Willow also had the greatest capability for releasing recalcitrant inorganic P in infertile barren moraine. These novel studies suggest that the nodulating N2-fixing species were not categorically better than non-nodulating endophytic N2-fixing species at accessing P as measured by soil available P concentrations in rhizosphere soils or microbial P transformation genes. This study provides new insights into potential mechanisms of ecosystem primary succession with broader implications for ecosystem management and restoration efforts. 

Methods
Data was collected on the foreland of Hailuogou Glacier (29°34′N, 102°00′E, 2982-2855 m) on Gongga Mountain on the eastern slope of the Qinghai-Tibetan Plateau. Data files are raw data. Detailed methods are described in the associated paper.