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.