HERBICIDES in Asian rice - IRRI books - International Rice ...

Table 1. Number of papers dealing with the impact of herbicides a on microorganisms
in California ricefields, 1990.
Herbicide
Algae and
cyanobacteria
Bacteria
Lab b Pot Field Lab Pot Field
Bensulfuron methyl
Copper sulfate
2,4-D
Glyphosate
MCPA
Mollnate
Pendimethalin
Propanil
Thiobencarb
Total
2 6
21 2 1
2
5 1 1
3 1 1 1 1
7 1 3 2
5 1 1 1
45 2 14 5 20
a Listed by Crosby (1996).
b Lab = laboratory.
— Most studies are laboratory experiments with cultures of microorganisms
or with only a few grams of soil. Less than 8% of the quantitative studies
were conducted in ricefields. Experiments with microbial cultures give an
index of a strain's sensitivity to pesticides, but the results cannot be extrapolated
to field conditions where the initial concentrations of pesticides
are likely to decrease rapidly, as their degradation is hastened by soil microflora,
nonbiological decomposition, leaching, volatilization, and soil
adsorption. Concentrations of pesticides that affect microorganism growth
depend on the initial microbial population, its nutrient status, the method
of pesticide application, and the degradation products. These factors markedly
differ in vitro and in situ. For example, little agreement was found
between the responses of microbial species to glyphosate in incubated soil
samples and in pure culture (Wardle and Parkinson 1990).
— Many studies used pesticide concentrations higher than those resulting
from the recommended level for field application. Such experiments may
overestimate pesticide persistence because degradation is slowed at high
concentrations, as has been shown with trifluralin (Parr and Smith 1973)
and molinate (Deul et al 1978). The recommended levels of traditional
pesticides range from a few hundred grams to a few kilograms of active
ingredient per hectare. The median recommendation for herbicides (2.5
kg ai ha -1 ) is higher than it is for fungicides (1.7 kg ai ha -1 ) and insecticides
(1.1 kg ai ha -1 ) (Roger 1990). Experimental results should be interpreted
in the context of the range of pesticide concentrations that can be expected
in farmers' fields after application. If a herbicide applied on a nonflooded
field stays within the surface 2 cm of soil, 2.5 kg ai ha -1 would correspond
to 15 mg kg -1 dry weight of soil, for a bulk density of 1 mg m -3 . If a water-
Effect of herbicide use on soil microbiology 71