Abstract
Root exudates alter the rhizosphere’s physical properties, but the
impact that these changes have on solute transport is unknown. In this
study, we tested the effects of chia mucilage and wheat root exudates on
the transport of iodide and potassium in saturated or unsaturated soil.
Saturated solute breakthrough experiments, conducted in loamy sand soil
or coarser textured quartz sand, revealed that increasing the exudate
concentration in soil resulted in increasingly non-equilibrium solute
transport. This behavior was demonstrated by an initial solute
breakthrough after fewer pore volumes and the arrival of the peak solute
concentration after greater pore volumes in soil mixed with exudates
compared to soil without exudates. These patterns were more pronounced
for the quartz sand than in the loamy sand soil and in soil mixed with
mucilage than in soil mixed wheat root exudates. Parameter fits to these
breakthrough curves with a mobile-immobile transport model indicated the
fraction of immobile water increased as the concentration of exudates
increased. For example, in quartz sand the estimated immobile fraction
increased from 0 without exudates to 0.75 at a mucilage concentration of
0.2%. The solutes’ breakthrough under unsaturated conditions was also
altered by the exudates, demonstrated by a smaller volume of water
extracted from soil mixed with exudates, compared to soil without
exudates, before the arrival of the peak solute concentration. The
results indicate that exudates alter the rhizosphere’s transport
properties; we hypothesize that this is due to exudates creating
low-conducting flow paths that result in a physical non-equilibrium
solute transport.