Abstract:
Granite, which is a systemic herbicide formulation, is widely applied by wet zone paddy
farmers in Sri Lanka to control weeds during the initial vegetative growth phase of
paddy. The active compound, Penoxsulam; 2-(2,2-difluoroethoxy)-N-(5,8-
demethoxy[1,2,4]triazolo[1,5c]pyrimidine-2-yl)-6-trifluoromethyl) benenesulfonamide,
destroys both grass and broad leaf weeds through inhibiting Aceto Lactate Synthase
(ALS) enzyme. Paddy soil inhabiting microorganisms play a vital role in soil fertility
management and suppression of diseases. But, a considerable attention has not been
paid to see the effect of Granite on non-target soil microbiota. Thus, the present study
evaluated the effect of granite on total microbial activity of rice fields inhabiting
microorganisms. After basic soil characterization, 25 g uncontaminated soil was
amended with the herbicide Granite to prepare a laboratory contaminated soil series
(2.5, 5.0, 10, 15, and 20 ppm of Granite kg-1 of soil). The control consisted with no
Granite. All treatments were replicated thrice. Three parallel experiments with 24 (T24h),
48 (T48h) and 168 (T168h) h experimental time were carried out. At the end of each
experiment, soil samples were taken from the treatments and soil total microbial activity
(TMA) was measured. The highest TMA was measured from the treatment 2.5 ppm of
Granite kg-1 of soil and T24h, T48h and T168h exposure times for that treatment were
12.5±0.06, 12.1±0.04 and 10.8±0.34, respectively. The lowest values of TMA recorded
from the treatment with the highest contamination level (20.0 ppm of Granite kg-1 of
soil ) with T24h, and T168h exposure times of 9.6±0.33, and 8.1±0.20, respectively.
Although, the results imply an increasing trend of TMA at acute exposure to lower
concentration, the overall results showed a time and concentration dependent
irreversible significant (p<0.05), decrease in TMA of treatments with chronic exposure
to higher contamination levels of Granite. The overall results infer the potential of
negative impact of Granite on rice field inhabiting microbiota due to repeated exposure
at high concentrations.
Keywords: Granite, Total microbial activity, Herbicide concentration, Exposure,
Irreversible effect
