Agriculture Reference
In-Depth Information
7 Nutrients, Toxins and Pollutants
This chapter is concerned with the different types of wetland soil as sources,
sinks and transformers of nutrients, particular nutrient deficiencies and mineral
toxicities that commonly arise following submergence, and the fate of pollutants
that are commonly added to submerged soils, both accidentally and intentionally.
7.1 NUTRIENT AND ACIDITY BALANCES
As discussed in Chapter 1, the nutrient supplies of submerged wetland soils have
a number of special features compared with upland soils. Nutrient removal in
leaching and erosion tend to dominate the nutrient balance of upland soils. In
wetlands, inputs from inflowing water tend to exceed leaching losses, and erosion
does not occur. Rates of fixation of nitrogen from the atmosphere under wet
conditions with a good supply of other nutrients are also often greater, though so
too are losses through denitrification. Wetlands are therefore often net sinks for
nutrients, and this is reflected in their generally high productivity. However there
are large differences between the different wetland types and between locations.
7.1.1 NUTRIENT BALANCES IN RICEFIELDS
The fact that rice production has sustained huge human populations on the river
deltas of Asia for millennia is an indication of the favourable nutrient balance in
wetland ricefields. Prior to 1960 and the green revolution, yields were sustained
without artificial inputs of nutrients, other than by recycling through manures and
night soil. The requirements of the crop were met by the inflow of nutrients and
fertile sediments with floodwaters and by nitrogen fixation. The greatly increased
yields since 1960 are sustained by inputs of mineral fertilizers. Figure 7.1 shows
how increases in the yields of rice (and wheat) have been paralleled by increases
in the use of nitrogen fertilizers since 1960 to keep pace with world population.
Greenland (1997) has compiled realistic average annual nutrient balances for
wetland ricefields pre- and post-1960 from probable inputs and outputs. Inputs
come from rainfall,
R
, irrigation and floodwater,
F
, sediments,
S
, nitrogen fix-
ation,
N
, and manures and fertilizers,
M
. Outputs are due to crop removals in
