Agriculture Reference
In-Depth Information
5 Biological Processes in the Soil
and Floodwater
The soil and floodwater in wetlands are busy with life, and this drives the biogeo-
chemistry. The remarkable long-term productivity of wetland rice systems depends
on the fixation of carbon and nitrogen from the atmosphere by organisms in the soil
and water, for which conditions are optimal. For example, in a long-term experi-
ment at the International Rice Research Institute in the Philippines in which three
crops of rice have been grown each year for 30 years without additions of fertil-
izers or manures and with complete removal of the rice straw, grain yields have
remained nearly constant at 3 to 3.5 t ha
−
1
per crop or a total of 9 to 10 t ha
−
1
per year (Dobermann
et al
., 2000). No other intensive agricultural system without
artificial inputs of nutrients comes close to this level of productivity. The accumula-
tion of nitrogen by crops in this experiment has remained constant at about 50 kg N
ha
−
1
per crop, largely due to additions from biological fixation in the floodwater
and floodwater-soil interface (Ladha
et al
., 2000). Comparable rates of nitrogen
fixation are attained in other fluxial wetland systems (Table 1.5).
This chapter describes the important micro- and macrobiological processes in
submerged soil and the overlying floodwater. Processes in plants and their rhizo-
spheres are discussed in Chapter 6. The microbiological processes are discussed
first and then the additional complexities caused by macrobiological processes
and the particular ecology of the floodwater-soil system.
5.1 MICROBIOLOGICAL PROCESSES
Descending through the soil from the floodwater there is a gradient of redox
potential and a sequence of zones characterized by progressively more-reduced
electron acceptors. Figure 5.1 shows hypothetical concentration profiles of redox
species with depth. At sufficient depth the only electron acceptors are CO
2
and
H
+
, and this zone is dominated by fermentation and methanogenesis. At inter-
mediate depths there are successive zones of sulfate reduction, iron reduction,
manganese reduction and denitrification. The microbes mediating these pro-
cesses are largely prokaryotic; populations of fungi and other eukaryotes that
are important in digesting organic matter under aerobic conditions are much less
significant in anaerobic soil.
