Geoscience Reference
In-Depth Information
Paddy rice cultivation, gleying and
the nitrogen cycle
APPLICATIONS
The cultivation of rice in paddies is the most vital agricultural system on Earth from the viewpoint of the number of
people who depend on it for survival. Not only is this true for Asia (
Plate 18.16
),
but increasingly for Africa and South
America, where the large yields obtainable from rice cultivation make it a popular option in agricultural development
projects. This is in addition to its importance as a commercial crop in southern parts of Europe and the United States,
as well as Australasia.
Waterlogged soils form the basis of the paddy system of rice cultivation in many tropical and subtropical regions.
The 'paddy' is deliberately flooded after the surface soil structure has been destroyed by 'puddling', either by buffalo
or tractors. The water level in the paddy is constantly raised to keep pace with the height of the growing crop. The
typical soil profile of a rice paddy is shown in
Figure 18.12
.
It is important that the quality of water is good as regards
its salinity; rice is the most sensitive to salt of all the major cereals. The sheet of water at the soil surface supports
algae which keep the water oxygenated, and some blue-green algae (Cyanophyta) are able to fix atmospheric nitrogen.
Below the surface aerobic layer it is a brown aerobic horizon, and below that a blue-grey anaerobic gleyed zone.
High-yielding varieties of rice require large quantities of nitrogen (see pp. 524-5 for a discussion of the nitrogen cycle).
Nitrates formed in the surface layer will diffuse into the anaerobic horizon below, where they are reduced to the
gaseous forms of nitrogen (N
2
) and nitrous oxide (N
2
O), and thus lost to the atmosphere by denitrification. Fertilizers
in which the nitrogen is in the reduced chemical state, as in ammonium sulphate and urea, are therefore recommended
for use in the paddy system. To ensure that losses by denitrification are kept to a minimum, the fertilizers should be
placed directly into the anaerobic layer to prevent the oxidation of the [NH] groups to nitrates.
Also present in rice paddies are methanogenic bacteria which produce intermediate by-products like methane (CH
4
)
from the anaerobic breakdown of plant sugars and polysaccharides. Methane production depends on waterlogging,
and falls off rapidly where the paddies are drained and partially aerated when the crop is mature. However, the
anaerobic bacteria can survive, even though not active, and CH
4
production returns when the paddies are reflooded.
The use of manure and soil organic matter such as straw increases CH
4
production. Reducing the length of time the
paddy is flooded without affecting yields is recommended. However, agronomists predict that to feed the rapidly
increasing populations of China, India, Indonesia, Malaysia and Sri Lanka, the area of land under rice cultivation is
expected to increase by 60 per cent in the next two decades, leading to a similar increase in CH
4
emissions.
the lack of aerobic decomposer organisms and micro-
organisms. Waterlogging may result from climate, a
groundwater table or topography. The type of peat, i.e.
acid or basic, depends on the plants which produced it,
which in turn depends on the hydrology and water
chemistry of the site.
Hill peat or blanket peat (Soil Survey: Raw Peat
Soils, FAO: Oligotrophic Histosol) has the following
characteristics:
Peat soils
Peatlands are extremely important land resources for fuel,
horticultural use, farming, forestry, wildlife conservation
and wilderness character. Stratigraphies of peat at many
sites have been studied by biogeographers to establish the
postglacial history of vegetation and the hydroseral
development of ecosystems. It is estimated that peat soils
cover 360,000 ha in England, and 160,000 ha in Wales.
Most is upland peat under rough grazing, but there are
188,000 ha of lowland peat with a high agricultural
potential in East Anglia, Lancashire, Somerset and
Lincolnshire (Bourton and Hodgson 1987). Peat soils are
the accumulated remains of plant material under water-
logged conditions where decomposition is suppressed by
1
Widespread distribution in upland Britain above
300 m.
2
Saturated with water for long periods.
3
Profile sequence of O1, O2, O3, O4, etc., horizons.
4
Black or dark reddish-brown colour.
