Agriculture Reference
In-Depth Information
•
construction of burrows: synthesis of
refractory or
inhibitory structural
products;
•
irrigation of burrows: increased transfer of soluble oxidants and nutrients,
increased re-oxidation and mineralization;
•
transport of particles: transfer between major redox zones, increased re-oxida-
tion and mineralization.
Bioturbation
Oligochaetes feed with their heads downward in the burrow and posterior ends
upward in the water. They ingest fine soil particles, extract carbon and minerals,
and deposit residues in faeces on the soil surface. The faeces may subsequently
fall into the burrow and be mixed. The net effect is a loosening and mixing of
the soil to depth. The burrows of the species found in ricefields may be several
centimetres deep and a millimetre or so in diameter. Deeper and wider burrows
are formed by species found in other wetlands (Table 5.4). Once the burrows
are constructed, the worms tend to remain in them and maintain a supply of
oxygen from the overlying water by waving their posteriors in the water and
moving their bodies in a peristaltic motion. Thereby the water in the burrows is
mixed with the overlying water and solutes diffusing into a burrow are rapidly
transferred to the surface and vice versa. The calculations in Section 2.4 show
the great sensitivity of solute transport and mixing to the geometry, density and
activity of the burrowing animals.
The effects of oligochaetes on the soil or sediment depend on the particular cir-
cumstances. Limnologists and oceanographers consider oligochaetes to be agents
of aeration, increasing the depth of the oxidized layer and stimulating mineraliza-
tion and nitrification-denitrification (Fry, 1982; Aller, 1994). For example, Davis
(1974) found that the oxidized layer
(E
H
>
200mV
)
in profundal lake sediments
was increased by 0.3-1.6 cm by tubificid populations of 800m
−
2
.
By contrast in ricefields, where primary production and the amounts of organic
matter in the floodwater may be much greater, the effect can be to enhance the
incorporation of organic matter into the soil and so to make the surface soil on aver-
age more reduced, in spite of oxygenation of the solution in the burrows. Kikuchi
and colleagues (Kikuchi
et al
., 1975; Kikuchi and Kurihara, 1977, 1982) found that
with realistic densities of tubificids and organic matter in ricefields, the oxidized
layer at the soil surface disappeared altogether. They found that weed growth was
diminished because seeds were moved to a depth at which the O
2
concentration was
too low for germination, and as a consequence oxygenation of the soil by weeds
decreased and populations of aerobes in the soil decreased and anaerobes increased.
The concentrations of NH
4
+
, ortho-P and acid soluble Fe in the floodwater increased
and the concentration of NO
2
−
+
NO
3
−
decreased (Figure 5.11).
In practice redox conditions in the burrows will oscillate as the oxygenation
of the floodwater varies over the diurnal cycle. Aller (1994) found in a wide
range of organic matter-rich sediments containing burrowing invertebrates that
