Agriculture Reference
In-Depth Information
other factors are at work. In addition to temperature and aeration, and the quan-
tity and nature of organic matter inputs, other factors influencing decomposition
include the soil pH, which may be more favourable following submergence; the
level and balance of nutrients, which may also be more favourable; and the
communities of micro- and macro-fauna that together bring about the decom-
position. Submerged soils are never wholly anoxic and contain aerated zones
at the interfaces between the soil and floodwater and the soil and plant roots.
Burrowing oligochaete worms transport fresh and partially decomposed organic
matter between the soil and floodwater, and the activities of organisms in the soil
and floodwater are thereby linked. Hence the soil-floodwater system as a whole
behaves quite differently from its component parts in isolation. Decomposition
processes are discussed further in Chapter 5.
Studies by Olk and others of long-term
(
≤
30 years
)
changes in organic matter
in soils under intensive wetland rice cultivation have shown a gradual accumula-
tion of less humified and more phenolic material (Olk
et al
., 1996, 1998, 1999;
Mahieu
et al
., 2000a, b, 2002). These authors compared the chemical composi-
tion of organic matter from four soils with different histories of cropping and
submergence: (a) one crop of upland rice annually without soil submergence;
(b) one wetland rice crop and one soybean crop annually; (c) two wetland rice
crops annually; and (d) three wetland rice crops annually. With increasing inten-
sity of wetland rice cropping there were large increases in the proportions of
less humified material in the soil organic matter, measured as diester P, amide
N and phenolic C in nuclear magnetic resonance spectra. There were also posi-
tive correlations with visible light absorption and concentrations of free radicals,
both of which are indices of humification, and negative correlations with the
concentration of H, a negative index of humification. The authors speculate that
slower lignin decomposition under restricted O
2
supply in submerged soil leads
to incorporation of phenolic compounds into young soil organic matter as it is
turned over. Since phenolic compounds can react strongly with nitrogenous com-
pounds, they further speculate that the mineralization and immobilization of N
in intensively cropped rice soils may be adversely affected by accumulation of
phenolic material.
3.7 SOLID-SOLUTION INTERACTIONS
3.7.1 ADSORPTION
Adsorption depends on the interaction of ions and uncharged solutes with the
functional groups on soil surfaces. It is in some ways analogous to complexation
of ions with ligands in solution (Section 3.1), with the difference that the surface
functional groups are stationary and their properties depend to a greater extent
on interactions with neighbouring groups. Two types of surface complex are
distinguished:
