Agriculture Reference
In-Depth Information
Table 7.4
Nitrogen budget of a tidal salt marsh
in Massachusetts
Flow
(
gNm
−
2
year
−
1
)
Inputs
Surface water
?
Rainfall
0.8
Biological N fixation
6.5
Groundwater
27.0
Tidal inflow
116.0
150.3
Outputs
Denitrification
13.3
Tidal outflow
140.0
153.3
Source
: data from Mitsch and Gosselink (2000).
Acid Sulfate Soils
Acid sulfate soils are an especially difficult class of acid soil formed in former
marine sediments that have been drained. The acidity is generated from the oxi-
dation of pyrite in the soil resulting in acute aluminium toxicity, iron toxicity,
and deficiencies of most nutrients, especially phosphate which becomes immobi-
lized in ferric oxide. The development and management of acid sulfate soils are
discussed in detail in Dost and van Breemen (1983) and Dent (1986).
In brief, the steps in the formation of pyrite in marine sediments are:
(1) reduction of Fe(III) in the sediment to soluble Fe
2
+
and reduction of SO
4
2
−
from seawater to S
2
−
;
(2) partial oxidation of S
2
−
to elemental S or polysulfide, S
2
2
−
;
(3) formation of pyrite, FeS
2
, either directly from Fe
2
+
and S
2
2
−
or via FeS
formed from Fe
2
+
and S
2
−
and subsequent reaction with S.
The overall reaction is
2Fe
2
O
3
+
8SO
4
2
−
+
16CH
2
O
+
O
2
→
4FeS
2
+
16HCO
3
−
+
8H
2
O
(
7
.
2
)
The necessary conditions are sources of iron oxide, dissolved SO
4
2
−
and organic
matter, and sufficiently reducing conditions for reduction of SO
4
2
−
coupled to
intermittent or localized oxidizing conditions to produce elemental S or poly-
sulfide. Potential acidity develops by the removal of alkalinity (represented by
HCO
3
−
in Equation 7.2) from the sediment by diffusion and tidal action. What
