Agriculture Reference
In-Depth Information
Chlorophyll a (mg m
−
2
) and
Ostracods (no. m
−
2
×
10
2
)
Colonial algae (g m
−
2
)
3
2
14
1
0
12
Chlo
r
ophyll a
10
20
30
40
50
60
DAT
10
1000
Colonial
cyanobacteria
ARA
Ostracods
8
200
500
6
150
2000
1500
100
4
100
1000
Harvest
Molluscs
50
2
50
500
0
0
0
0
0 0 0 0 0 0 0 0 0 0 0
Days after transplanting
Figure 5.10
Generalized fluctuations of algae, acetylene-reducing activity (ARA), and
grazers in floodwater of unfertilized ricefields. P:R is the ratio of primary production to net
respiration (Grant
et al
., 1986). Reproduced by permission of AB Academic Publishers
NH
4
+
incorporated into organic compounds. The reduction requires a pe less than
about
−
4
.
5 at pH 7 (Section 4.3). This is less negative than the pe required for
reduction of CO
2
to CH
2
O, hence cyanobacteria (blue-green algae) and certain
other photosynthetic bacteria are able to mediate the reduction at the negative pe
levels generated in photosynthesis. Non-photosynthetic N
2
fixers require anoxic
conditions or must exclude O
2
from site of fixation. Because of the large acti-
vation energy required to break the N
≡
N triple bond, there is a kinetic barrier
to be overcome, and this is achieved in N
2
fixing organisms through the enzyme
nitrogenase.
In wetlands N
2
fixation can occur in the water column, in the aerobic water-soil
interface, in the anaerobic soil bulk, in the rhizosphere, and on the leaves and
stems of plants. Phototrophic bacteria in the water and at the water-soil interface
are generally more important than non-photosynthetic, heterotrophic bacteria in
the soil and on plant roots (Buresh
et al
., 1980; Roger 1996). The phototrophs
comprise bacteria that are epiphytic on plants and cyanobacteria that are both
free-living and epiphytic. A particularly favourable site for cyanobacteria is below
the leaf surface of the water fern
Azolla
, which forms a very efficient symbiosis
with the cyanobacterium
Anabaena azollae
. This symbiosis and those in various
leguminous plants have been exploited in traditional rice production systems to
sustain yields of 2 to 4 t ha
−
1
of grain without fertilizer for hundreds of years.
