Transport Processes in Submerged Soils - The Biogeochemistry of Submerged Soils - page 27

0.9

0.8

0.7

0.6

0.5

Iloilo
Maahas
N. Ecija
Tarlac

0.4

0.3

0.2

0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

Bulk density, r (g cm − 3 )

Figure 2.4 Relation between diffusion impedance factor, f L , and bulk density, ρ ,in
four water-saturated rice soils. Dotted line is the theoretical relation between f L and ρ for
a mixture of different-sized spherical particles (Kirk et al ., 2003). Iloilo: Epiaquult; clay
21%; org C 1.04%; pH 3.93. Maahas: Haplaquoll; clay 54%; org C 1.83%; pH 5.89.
Nueva Ecija: Epiaquert; clay 35%; org C 1.57%; pH 5.25. Tarlac: Tropaquept; clay 33%;
org C 1.06%; pH 6.02. Reproduced by permission of Blackwell Publishing

given ρ . These interactions increase as ρ increases and an increasing proportion
of the pores are fine, and as clay content increases. Hence the regression lines are
steeper than the theoretical line and farther from it with increasing clay content.
Also the approximation of spherical shape is increasingly invalid with increasing
clay content because small surface irregularities become an increasing proportion
of the whole.
Figure 2.5 shows the effects of changes in redox conditions on f L in the four
soils with time following flooding. In three of the soils, f L decreases during the
first few weeks following flooding but then gradually returns to its initial value.
Since ρ and θ L were constant over this period, the changes in f L were evidently
due to changes in chemical and biological conditions in the soils following flood-
ing. This is explained as follows.
Following flooding O 2 entrapped in the soil is rapidly consumed in aerobic
microbial respiration, and then other inorganic electron acceptors are used in
microbial respiration in the sequence NO 3 − , Mn(IV), Fe(III), SO 4 2 − (Chapter 4).
Concomitantly, organic matter is oxidized, dissolved CO 2 accumulates, and the
pH of acid soils tends to increase and that of alkaline soils to decrease, stabilizing
in the range 6.5 to 7. Figure 2.6 shows the changes in E H , pH, the concentration
of HCO 3 − in solution and cation exchange capacity (CEC) in the four soils
following flooding, and Figure 2.7 shows the corresponding changes in soil Fe.
In all four soils, Fe(III) is the main inorganic reductant and large concentrations
of Fe 2 + develop in the soil solution in the weeks following flooding as Fe(III) is