Multiple-Reaction Approaches - Transport and Fate of Chemicals in Soils: Principles and Applications - page 141

5
Multiple-Re action Approaches
Several studies showed that sorption-desorption of dissolved chemicals on
different soils was not adequately described by use of a single reaction of
the equilibrium or kinetic type. Failure of single reactions is not surprising
since they only describe the behavior of one species, with no consideration
given to the simultaneous reactions of others in the soil system. Multisite or
multireaction models deal with the multiple interactions of any one species
in the soil environment. Such models are empirical in nature and are based
on the assumption that retention sites are not homogeneous in nature; rather,
the sites are heterogeneous and thus have different affinities to individual
solute species. Figure 5.1 clearly illustrates the heterogeneous make up of the
soil matrix.
It is well recognized that multireaction models cannot account for all pos-
sible interactions occurring in the soil-water environment. For example,
characterization of chemical, biological, and physical interactions of nitrogen
within the soil environment is a prerequisite in the formulation of a multi-
reaction nitrogen model. One major point is how strongly such factors affect
nitrogen behavior and distribution within soil systems. Among these factors
are the effect of soil texture and structure on oxygen diffusion; distribution
of plant residues (vertically and horizontally), which affects infiltration rates,
leaching, and biological transformations, including plant uptake, mineraliza-
tion, and denitrification; and cultural practices such as tillage and fertilizer
distribution, which affect nitrogen distribution vertically and horizontally.
Since nitrate is a highly mobile nitrogen form that can leach through the soil
profile and eventually into groundwater, the goal of any nitrogen manage-
ment plan must include minimizing nitrate leaching from agricultural activ-
ities into groundwater. Nevertheless, description of N dynamics in the soil is
often simplified. To illustrate this, one needs to examine the N dynamics in
soils as described in the simplified schematic for N transformations shown
Figure 5.2. The model accounts for nitrification, denitrification, immobiliza-
tion, mineralization, and ion exchange of ammonium as a reversible first-
order kinetic process. It also accounts for uptake by plant roots and transport
in the soil water.
Based on first-order kinetic reactions governing N transformations, a set of
simultaneous equations describing the N cycle were derived by Mehran and
Tanji (1974). This set of equations was solved simultaneously to provide N
simulations. Wagenet, Biggar, and Nielseen (1977) found excellent agreement
between observed urea, ammonium, and nitrate effluent concentrations and
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