Agriculture Reference
In-Depth Information
There has been much research aimed at developing N
2
fixing systems for greater
yield levels. For practical reasons, such systems cannot entirely substitute for
mineral N fertilizers, and there is the difficult dilemma that N
2
fixation within
the ricefield is inhibited by additions of mineral N.
Table 5.2 gives estimates of N
2
fixed by various agents in wetland ricefields
and Table 5.3 gives estimates of total fixation in ricefields from a review of
211 N balances in field and pot experiments by Roger and Ladha (1992). The
values range from 0 to 100 kg N ha
−
1
per crop with averages of 30 kg N ha
−
1
in
plots without N fertilizer, 8 kg N ha
−
1
in plots with N fertilizer broadcast in the
floodwater, and 12 kg N ha
−
1
in plots where the fertilizer was placed at depth
in the soil. The beneficial effect of the presence of plants on BNF is evident,
and also the effect of illumination, especially without inorganic N, indicating
that the N
2
fixing agents are phototrophic. Rates of comparable magnitude and
variability are found in natural freshwater and coastal wetlands (Buresh
et al
.,
1980; Bowden, 1987).
Not all of the nitrogen fixed finds its way into the vegetation. The recovery will
depend on rates of decomposition of the material containing the fixed N, rates of
Table 5.2
Estimates of biological nitrogen fixation (BNF) by various agents in wetland
ricefields
BNF (kg N ha
−
1
crop
−
1
)
Heterotrophic bacteria in the rhizosphere
1-7
Heterotrophic bacteria in the rhizosphere and bulk soil
1-31
Heterotrophic and phototrophic bacteria on added straw
20-40 for 10 t straw
Free-living cyanobacteria
0-80
Azolla/Anaebena azollae
in experimental plots
20-150
Azolla/Anaebena azollae
in ricefields
10-50
Legumes as green manures
50-100 in 50 days
Source
: adapted from Roger and Ladha (1992). Reproduced with kind permission of Kluwer Academic Publishers.
Table 5.3
Estimates of nitrogen fixed in rice systems from N balance
studies
Factor
No.
observations
Mean
±
SD
(
kg N ha
−
1
crop
−
1
)
No inorganic N
166
29
.
7
±
25
.
4
With inorganic N
45
4
.
0
±
47
.
6
Planted
193
26
.
5
±
30
.
7
Unplanted
18
−
0
.
5
±
46
.
2
Soil and water light
197
25
.
0
±
33
.
9
Soil and water dark
14
13
.
2
±
13
.
8
No N, soil and water light
152
31
.
2
±
25
.
7
No N, soil and water dark
14
13
.
2
±
13
.
8
Source
: adapted from Roger and Ladha (1992). Reproduced with kind permission of
Kluwer Academic Publishers.
