Biological Processes in the Soil and Floodwater - The Biogeochemistry of Submerged Soils - page 149

N limited. Nutrient availability also affects the composition of plant communities:
species richness and the presence of rare species often decline as nutrient avail-
ability increases beyond some threshold (Bedford et al ., 1999). In addition to the
inflow of nutrients with the water and sediment, there are complicated interactions
between hydrology and nutrient availability that affect productivity and decom-
position. Transformations of N and P under anaerobic conditions are discussed
in Section 4.3.
5.2.2 THE FLOODWATER-SOIL SYSTEM
Five zones can be distinguished: the floodwater standing on the soil per se ,the
floodwater-soil interface, the anaerobic bulk soil, the rhizosphere, and the sub-
soil. These are to some extent continuous with each other, and they are certainly
linked so that the function of the system as a whole is greater than the sum of
its parts. But they provide convenient boundaries for discussion.
The floodwater is photic and aerobic. It contains photosynthetic and chemosyn-
thetic producers of fixed carbon-bacteria, algae and aquatic weeds—and inver-
tebrate and vertebrate consumers that graze on the producers. The community
of producers and consumers provides organic matter to the underlying soil and
recycles inorganic nutrients.
The floodwater-soil interface is also photic and aerobic. The boundary with
the overlying water is diffuse and the bulk density increases from near zero to
1g cm − 3 or more in the underlying anaerobic soil. The depth to the underly-
ing soil varies from a few mm to a few cm, depending on the aeration of the
floodwater, reducing conditions in the soil, rates of percolation, and mixing by
invertebrates. Nitrate, Mn(III,IV), Fe(III), SO 4 2 − and CO 2 are stable and algae
and aerobic bacteria predominate. In the early stages of land preparation for rice,
algae develop on the wet soil surface and support populations of grazers. As
organic matter accumulates during the crop, populations of benthic filters and
deposit feeders develop. The activities of the invertebrates affect nutrient cycling
both directly through their excretions and indirectly by moving soil particles and
organic matter.
The anaerobic soil is non-photic and reduction processes predominate. The
value of pe + pH is generally below the range at which Fe(III) is reduced, unless
organic substrates are limiting or there are large concentrations of more oxidized
reductants such as Mn(III,IV). Microbial activity is concentrated within water-
stable aggregates containing organic matter, and produces NH 4 + , S 2 − ,organic
acids and CH 4 . Decomposing organic matter in the anaerobic soil sustains pop-
ulations of aquatic oligochaete worms and chironomid larvae.
The subsoil at greater depths may be aerobic in well-drained soils with a
perched water table owing to an impermeable layer—such as the traffic pan in
ricefields; or anaerobic in soils that are poorly drained throughout. It may provide
significant quantities of nutrients to plants growing in the soil if their roots can
reach them.