Introduction - The Biogeochemistry of Submerged Soils - page 9

Phosphorus
Though orthophosphate itself is generally not reduced in submerged soils, reduc-
tion of ferric iron compounds and changes in the electrochemical properties of
the surfaces with which orthophosphate reacts strongly affect its solubility and
dynamics. In agricultural wetlands with a history of P fertilization, submergence
often results in enhanced availability of P to plants, and responses to additions
of further fertilizer in ricefields are often weak. However in natural wetlands, P
concentrations are much smaller and P washed into the soil becomes strongly
sorbed on the surfaces of reduced soil constituents. Phosphorus brought in with
sediment may also be effectively filtered out of water passing through wetlands.
Phosphorus retention is therefore a highly valuable attribute of wetlands receiving
diffuse pollution. However, the net primary productivity of most natural wetlands,
particularly freshwater wetlands, is limited by deficiency of P.
Metals and Other Pollutants
Heavy metals, toxic organics and other pollutants have often frequently been
added to wetlands both accidentally and on purpose, exploiting their buffering
and storage capacities. The chemistry of submerged soils and sediments is such
that pollutants may be effectively removed from the percolating water in redox,
sorption and precipitation reactions. But the effects of long-term accumulation of
pollutants on nutrient cycles and other wetland functions are not well understood.
1.3 TYPES OF SUBMERGED SOIL
The principal distinguishing feature of wetland soils is that they develop under
predominantly anoxic conditions. Although anoxia is also sometimes found in
other ecosystems, it prevails in wetlands and dominates soil properties. Because
of the very large organic matter content of some wetland soils, a rough separa-
tion into organic and mineral types based on organic matter content is a useful
delineation.
1.3.1 ORGANIC SOILS
The USDA (1999) defines organic wetland soils as having an organic carbon
content of at least 12% if the mineral fraction has no clay, 18% if ≥ 60% clay,
or 12-18% if < 60% clay. Further differentiation is based on the botanical origin
of the organic matter-whether mosses, herbaceous plants, or woody plants-and
its state of decomposition: fibrists contain predominantly recognizable, little-
decomposed plant debris, saprists predominantly well-decomposed plant debris,