Fig. 7: Conceptual figure of the increasing contribution of microbial cycling to dissolved inorganic phosphate (PO₄) from oxic to anoxic conditions in floodplain aquifers of the Hetao Basin.

The green points show the observed δ¹⁸O isotope value of PO₄ (δ¹⁸O_PO4) in groundwater. The red rectangle stands for the range of equilibrium δ¹⁸O_PO4 values after intracellular microbial cycling. The blue rectangle represents the range of δ¹⁸O_PO4 values of igneous apatite. Along the flow path, the redox conditions gradually changed from oxic to anoxic, and the groundwater residence time increased from about 125 yr to 675 yr. Under these conditions, the green dashed line shows an increasing trend of δ¹⁸O_PO4 values towards equilibrium δ¹⁸O_PO4 values along the groundwater flow path, indicating an increasing contribution of microbial cycling to dissolved PO₄ from 53% − 75% to nearly 100%. This is probably due to the release of PO₄ with an equilibrium δ¹⁸O_PO4 value from Fe(III) oxides, as a result of Fe(III) reduction in the presence of Fe(III)- and sulfate-reducing bacteria (red dashed lines). Moreover, the increasing residence times of groundwater allow the dissolved PO₄ to be more microbially cycled and consequently the groundwater δ¹⁸O_PO4 values gradually approached isotopic equilibrium.