Agriculture Reference
In-Depth Information
Table 1.5
Nitrogen fixation and denitrification in wetlands
N fixation
Denitrification
Mean rate
(g m
−
2
year
−
1
)
Total
(Tg year
−
1
)
Mean rate
(g m
−
2
year
−
1
)
Total
(Tg year
−
1
)
Temperate
Bogs/fens
1.0
3.0
0.4
1.2
Floodplains
2.0
6.0
1.0
3.0
Tropical
Bogs
1.0
0.5
0.4
0.2
Swamps
3.5
7.8
1.0
2.2
Floodplains
3.5
5.2
1.0
1.5
Ricefields
3.5
5.0
7.5
10.8
Total
27.5
18.9
Total terrestrial
139
43-390
Source
: after Armentano and Verhoeven (1990).
The floodwater often has a high pH as a result of CO
2
removal by photosynthe-
sizing organisms, favouring NH
3
volatilization.
As a result of gaseous losses, and in spite of biological N
2
fixation, N is often
the most limiting nutrient in wetlands together with P. It is also often one of the
most limiting nutrients in coastal waters, so the extent of denitrification of NO
3
−
in coastal wetlands has a particular significance. Pollution with excess NO
3
−
causes hypoxia in coastal waters and lakes worldwide (Mitsch and Gosselink,
2000). Currently atmospheric N
2
is fixed artificially for N fertilizers at more
than double the rate of natural biological N
2
fixation, so the return of N to the
atmosphere through denitrification in wetlands is an important brake on excess
NO
3
−
.
Sulfur
Like nitrogen, sulfur occurs in several oxidation states in submerged soils and
its transformations are microbially mediated. Sulfate washed into wetlands or
deposited from the atmosphere is largely reduced to S
2
−
in reactions mediated
by sulfate reducing bacteria. Subsequent precipitation of S
2
−
with metals, espe-
cially Fe
2
+
, results in more or less permanent removal of the S from the global
S cycle. Wetlands are therefore a potentially important sink for excess S released
in fossil fuel burning. Some of the S
2
−
may be emitted as H
2
S in organic soils,
but in submerged mineral soils the concentration of Fe
2
+
is usually sufficient to
prevent this. Hence measured emissions of H
2
S and other forms of volatile S
from wetlands are generally modest, and in general wetlands are a net sink for
S. The concentrations and availability of S in wetland soils rarely limit biologi-
cal production.
