Agriculture Reference
In-Depth Information
1
Introduction
Submerged soils behave and affect the environment in substantially different ways
to dryland soils. This chapter discusses the main characteristics and environmental
effects of submerged soils and the wetlands they support, and their extent across
the globe.
1.1 GLOBAL EXTENT OF SUBMERGED SOILS AND WETLANDS
For the purposes of the topic I define wetlands as lands that are intermittently
or permanently inundated with water to a depth of no more than a few metres.
Depending on the precise definition applied, estimates of the total global wetland
area range from 700 to 1000 Mha (Aselmann and Crutzen, 1989; Scharpenseel,
1997; Mitsch and Gosselink, 2000). Figure 1.1 shows their approximate distri-
bution and Table 1.1 the extents of different types distinguished by hydrology,
vegetation and soil characteristics. The largest areas are the bogs and fens in
polar and boreal regions in North America and Russia (34% of total area); trop-
ical swamps, especially in East Africa and South America (14% of total area);
tropical floodplains, especially of the Amazon and the rivers of South East Asia
(10%); and temperate and tropical ricefields (4 and 12%, respectively). Almost
half the global wetland area is in the tropics. There has been considerable loss
of wetlands in many parts of the world over the past 200 years as a result of
conversion to agricultural and aquacultural uses. In the US for example, it is
estimated that the area has declined from 89 Mha in the 1780s to 49 Mha in the
1980s (Mitsch and Gosselink, 2000).
A special class of wetland is the lowland ricefield, which accounts for almost a
fifth of the wetland area worldwide. Much of our knowledge and understanding
of submerged soils has been gained from research on rice soils. The success
of rice as a food crop stems from its origins as a wetland plant and its ability
to withstand soil submergence with the attendant improvements in water and
nutrient supplies. A corollary is that rice is more sensitive to water deficiency
than most other crops, and the critical factors in its productivity are the supply
of water to the soil, from rain, river, reservoir or groundwater, and the ability
of the soil to retain water. Hence most rice is produced and the highest yields
attained on the alluvial deposits associated with major rivers and their deltas.
More than 90% of the production is in Asia, distributed unevenly over four rice
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