Does P deficiency affect nodule bacterial composition and N source utilization in a legume from nutrient-poor Mediterranean-type ecosystems?

Virgilia divaricata is an indigenous forest margin legume growing in nutrient richer soils, but it is also known to invade the N and P poorer soils of the mature fynbos, a nutrient-poor ecosystem in the Cape Floristic Region of South Africa. Although this implies that the legume has a wide functional tolerance for variable soil N and P levels, it is not known how the plant utilizes inorganic N under variable P supply. The aim of this experiment was therefore to identify the nodulating bacterial species and their biological N2 fixing (BNF) efficiencies in V. divaricata during P deficiency. Furthermore, the aim was to integrate plants C and N metabolism to the N product exported via xylem to the shoots. Plants were grown at high and low P levels, both the high and low P plants were then supplied with either 500 μM NH4NO3 as soil nitrogen (N) source or exclusively relied on BNF. Although the bacterial composition of nodules remained seemingly unchanged by P and N supply, the nodule function was greatly altered. In this regard, plants reliant on only N2 at both P levels had higher and more efficient BNF, which resulted in greater plant N. This may have resulted from two physiological strategies at high and low P, when plants relied only on N2 fixation. The declines in both sugars and organic acids may imply a reduced energy supply to the bacteroid during P stress. Furthermore altered bacteroid function may be inferred from BNF, and the N compounds synthesized and exported. At high P, plants exported more amino acids relative to inorganic N and ureides in their xylem sap, whereas at low P the plants exported more ureides relative to amino acids and NH4. The bacterial tolerance for changes in P and N via nodule metabolites and xylem export might be a major factor that underpins the growth of V. divaricata under these variable soil conditions.