Agriculture Reference
In-Depth Information
Table 7.3
Additions of P in sediments in ricefields in Asia and riparian wetlands
in North America
Rate of deposition
(
gm
−
2
year
−
1
)
Ricefields in Asia
Calculated from contents of tropical soils and assumed
high sedimentation rate
(
1kgm
−
2
year
−
1
)
0.4-0.7
Calculated from measured sediment contents and
assumed high sedimentation rate
0.4-1.1
Measured additions
Guangdong, China
0.06
Bangladesh deepwater sites
0.2
Mekong delta
0.1
Riparian wetlands in North America
Southern Illinois
3.6
Central Florida
3.25
North Carolina
0.17
North Carolina from input-output balance
0.32-0.73
Northwestern Illinois
1.36
Sources
: ricefields, Greenland (1997); others, Mitsch and Gosselink (2000).
Nutrient balances in tidal wetland systems have been studied at length and
the picture is variable. In general the net exchanges are small in relation to the
overall nutrient budgets, though nutrients may be transformed between dissolved
and particulate inorganic and organic forms and oxidation states (Nixon, 1980;
Childers
et al
., 2000; Mitsch and Gosselink, 2000). Nitrogen generally flows into
the marsh largely as nitrate but is exported in dissolved and particulate reduced
forms, and is denitrified. Table 7.4 shows a representative N budget, obtained
in a large tidal salt marsh in Massachusetts. Though surface water inflows were
not measured, there is a rough balance between inflows and outflows. The tidal
exchange is far greater than any of the other components. Tidal marshes tend to
be net sinks for total phosphorus, entering in estuarine water in dissolved organic
and inorganic forms but there may be a remobilization of inorganic phosphate
leached out of sediments by saline water.
7.2 TOXINS
7.2.1 ACIDITY
In general high acidity and resulting high concentrations of toxic aluminium in
solution do not occur in submerged soils because the electrochemical changes
accompanying submergence tend to neutralize acidity present in the unflooded
soil. Acid sulfate soils are a notable exception.
