Agriculture Reference
In-Depth Information
and P are particularly important. Herbicides have direct effects and insecticides
have indirect effects through grazers.
Biotic Factors
. Cyanobacteria appear to be ubiquitous in ricefields and attempts
to increase biological N
2
fixation by inoculating with improved strains developed
in laboratory cultures have not been successful (Roger, 1996). Certain bacteria,
fungi and viruses have been shown to be pathogenic to cyanobacteria and algae
under laboratory conditions, but this has yet to be confirmed under field condi-
tions. Likewise antagonistic effects have been observed between different species
of cyanobacteria and algae, and between algae and macrophytes, and vice versa,
but there is not much information on the importance of these effects under field
conditions. Invertebrates such as cladocerans, copepods, ostracods, insect lar-
vae and snails are common grazers of algae in ricefields and their population
dynamics often mirror those of algae with a lag of a week or two (below).
Dynamics Over the Crop Cycle
The plethora of variables affecting algal growth and succession mean that com-
plicated dynamics are expected. Figure 5.10 shows a generalized succession for
the dry season flora and fauna in unfertilized ricefields (Grant
et al
., 1986). The
main points are (Roger, 1996):
•
Eukaryotic algae develop first but are quickly succeeded by non-colonial, het-
erocystous cyanobacteria, shown by the increase in chlorophyll a in the figure.
The bloom of cyanobacteria produces a bloom of ostracods and molluscs,
which graze on the cyanobacteria. The resulting collapse of the cyanobacteria
population is followed by collapses of the ostracods and molluscs.
•
About 4 weeks after transplanting a population of slow-growing, mucilaginous,
colonial cyanobacteria develops, which is resistant to grazing. This continues
to grow until the soil is drained for the harvest of the rice crop.
•
Primary production typically exceeds net respiration (P:R
>
1) over the first
month, leading to accumulation of organic matter and hence a decrease in the
ratio to low values.
•
There is a peak of N
2
fixation coincident with the early cyanobacterial bloom,
shown by the increase in acetylene reducing activity in the figure. Nitrogen
fixation by the colonial cyanobacteria is slower but lasts longer.
Biological Nitrogen Fixation in the Floodwater-Soil System
The water column and soil surface are often the main sites of biological nitrogen
fixation in wetland systems (Buresh
et al
., 1980; Roger 1996). Biological nitro-
gen fixation is the process by which atmospheric N
2
is reduced to NH
4
+
and the
