Transport Processes in Submerged Soils - The Biogeochemistry of Submerged Soils - page 34

calculated values of d C L / d C from the values of D P so obtained using measured
values of θ L and f L and Equation (2.18). Flooding increased θ L f L from 0.12 to
0.21, C ∞ from 199 to 352 µ mol dm − 3 (whole soil), and d C L / d C from 1/2220
to 1/1330. Drying previously flooded soil to the original θ L f L value decreased
C ∞ to 149 µ mol dm − 3 and d C L / d C to 1/1430. The changes were consistent with
changes in soil Fe upon flooding and drying.

Equations for Sorption . The following two simple equations often adequately
describe the relation between the amount of an anion or a cation sorbed on the
soil solid and its concentration in solution:
Freundlich equation

C S = aC L b ( 2 . 23 )

Tempkin equation

C S = a ln C L ( 2 . 24 )

where C S is the concentration in the solid, C L the concentration in solution,
and a and b are coefficients (b < 1 ) . Log-log transformations of the Freundlich
equation are linear and linear-log transformations of the Tempkin are linear.
These equations correspond to mechanisms by which sorption is progressively
inhibited by the accumulation of sorbed solute on the solid. However, as applied
to heterogeneous soil systems they are empirical and more precise mechanisms
than this cannot be inferred.

2.2.2 PROPAGATION OF pH CHANGES THROUGH SOIL

An important application of the theory of solute diffusion in soil is Nye's (1972)
theory of how pH changes are propagated by acid-base transfer. Sources of pH
changes in submerged soils are legion and the resulting pH gradients through the
soil have important effects on soil processes. Examples are diurnal changes in
floodwater pH caused by algae and pH changes in the rhizosphere induced by
roots. I here give a summary of the theory.
Changes in pH are propagated by the movement of protons. But because free
protons do not exist they must move by transfer between proton donors and
acceptors, i.e. Bronsted acids and bases:

acid (proton donor) = base (proton acceptor) + H + ( 2 . 25 )

The main acid-base pairs in soils are H 3 O + -HCO 3 − and H 2 CO 3 -HCO 3 − .In
particular cases other pairs, such asNH 4 + -NH 3 , H 2 PO 4 − -HPO 4 2 − andH 2 S-HS − ,
may also be important. When a pH gradient exists in a soil, acids and bases will
move in the soil solution between the solid surfaces: acids from zones of low pH
to high and bases in the opposite direction. The acid arriving in a portion of soil
will react with it, and a local acid-base equilibrium will be established. Thus the