Agriculture Reference
In-Depth Information
Table 5.1
Compositions of N
2
-fixing cyanobacteria and aquatic
macrophytes in ricefields. Data are means with ranges in parentheses
Cyanobacteria
Aquatic
macrophytes
Dry matter (% fresh wt)
4.0 (0.9-7.0)
8.0 (4.5-12.0)
Ash (% dry wt)
45.0 (27-71)
20.0 (12-50)
C (% dry wt ash-free)
40.0 (37-47)
-
N (% dry wt ash-free)
5.0 (3.8-7.4)
2.1 (1.3-2.9)
C:N ratio
8 (5-12)
24 (18-47)
P (% dry wt ash-free)
0.2 (0.05-0.39)
0.3 (0.1-0.6)
Source
: Roger (1996). Reproduced by permission of IRRI.
algae and macrophytes in the field during a cropping season (range 0.1-0.6 t
ha
−
1
). For freshwater bodies and irrigation canals the average was 4.5 t ha
−
1
(range 1-13 t ha
−
1
). In experimental plots of rice under a range of fertilizer
treatments, values for cyanobacteria were 177 kg dry wt ha
−
1
,28kgCha
−
1
and
4kgNha
−
1
. Net primary production in ricefield floodwater may reach 1 to
2gCm
−
2
day
−
1
but more usually it ranges between 0.2 and 1 g C m
−
2
day
−
1
.
The total production over a season is equivalent to 10-15% of the carbon accu-
mulated by the rice crop.
Factors Affecting Primary Production
Climate
. The principal variable is light intensity, which affects both the total
biomass and its composition. Generally the algal biomass increases following
flooding until shading by the rice canopy becomes limiting. In general green
algae prefer high light intensity, cyanobacteria prefer low light, though not in all
cases, and diatoms are indifferent. This results in a characteristic succession of
species over the growing season. The effect of light is moderated by temperature,
and both low and high temperatures can be inhibitory. Dessication and re-wetting
of the soil are also inhibitory and affect the proportions of species present.
Enhanced mineralization of soil N following drying and re-wetting favours algae
over cyanobacteria.
Soil
. Soil pH and the content of available P have the most consistent
effect (Roger, 1996). Overall algal growth increases as pH increases and higher
pH favours N
2
-fixing cyanobacteria over eukaryotic algae. Liming of acidic soils
increases the proportion of cyanobacteria in the biomass, and the proportion
of cyanobacteria may increase as the soil pH increases during reduction.
Applications of mineral fertilizers and organic manures have large effects on
algal growth and succession. Effects of changes in acidity and availabilities of N
