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Assessment of Pesticide Impacts on the
Biological Health of the Rice Ecosystem
mm
A Kumar, C Saison, S Grocke, H Doan, T Waller and R Kookana April 2007
RIRDC Publication No 07/024 RIRDC Project No CSL-16A
Increasing awareness of the potential adverse effects of pesticides has resulted in greater public pressure to assess, monitor and minimise off-site impacts. Pesticides applied to rice fields are of some concern, as during the spraying season they are sometimes detected in drainage water discharged from the rice farms. During this period the chemical residues in rice fields can adversely affect the beneficial soil micro-organisms on-site. However, it is not clear if these effects are transitory in nature and if the system recovers in the post-spraying season.
To ensure profitability and sustainability of rice farming systems, the impacts of pesticides both on the microbiological health of soils (on-site) and aquatic ecosystems (off-site) need to be properly understood. The RIRDC project (complemented by a CRC for Sustainable Rice Production funded project) focussed mainly on assessing the ecotoxicological impacts of rice pesticides.
Aims/Objectives
The main aim of the project
was to provide a better understanding of the impact of pesticides on the
soil microbiological health in rice systems and on the aquatic ecosystems
receiving drainage water from rice bays during the spray-season.
The specific objectives were as follows:
Methods used
Impacts on aquatic organisms
A monitoring study assessed
the pesticide impacts on the ecological health of the seven drainage channels
and creeks in NSW. Samplings were carried out at three different times
during 2001-2002 rice-season and four times during 2002–2003 rice-season
representing pesticide pre-spraying, spraying and post-spraying seasons.
Each sampling period involved (i) water quality assessment, (ii) analyses
of pesticides in water and sediment samples and (iii) laboratory bioassays
with the collected sediments.
Laboratory bioassays were conducted on the water, sediment and sediment pore waters collected from the Murrumbidgee and Murray irrigation areas. Laboratory cultures of waterflea, Ceriodaphnia dubia and midge, Chironomus tepperi were used as ecotoxicological test organisms. Yabby, Cherax destructor was used as an indicator species to assess the bioavailability of the sediment-bound pesticides at the sites selected for off-site monitoring. Furthermore, we also tested the toxicity of drainage water to these aquatic organisms.
Ecotoxicological tests were also carried out in experimental rice bays under field conditions to establish how persistent are the toxic effect of applied pesticides and how rapidly the system recovers.
The rice crop was sown following local practices and the pesticides (molinate, benzofenap, clomazone, fipronil, thiobencarb and chlorpyrifos) were sprayed as per the recommended rates of applications and the common combination of pesticides used. This study was conducted with a view to establish critical periods during which the accidental release of drainage water may have toxicological consequences.
In addition, through laboratory experiments, we determined the inherent toxicity of rice pesticides (single or mixtures) to a suite of organisms representing different trophic levels and also tested if the inherent toxicity of pesticides is affected by water salinity.
Utilising the ecotoxicological data generated in this project, an interim water quality guideline for a relatively newer rice pesticide (clomazone) was also derived, as discussed in latter section.
Impacts of pesticides
on soil micro-organisms
To understand the effect
of pesticides on soil micro-organisms, we carried out experiments under
field conditions in the Murrumbidgee Irrigation Area (MIA) and laboratory
experiments, to underpin the field observations. Experimental rice bays
were established under field conditions near Griffith, following a statistical
design. The rice crop was sown following local practices and the pesticides
(molinate, benzofenap, clomazone, fipronil, thiobencarb and chlorpyrifos)
were sprayed as per the recommended rates of applications and the common
combination of pesticides used. Two different combinations prevalent in
the region were applied.
In laboratory trials, two soils of the Murrumbidgee Irrigation Area (MIA) and Coleambally Irrigation Area (CIA) were chosen as representative of the two rice-growing areas. The soils were spiked with a set of four pesticides (molinate, thiobencarb, clomazone, chlorpyrifos) individually and in three different combinations (molinate + thiobencarb; clomazone + thiobencarb; and chlorpyrifos + thiobencarb).
Periodically, composite samples from surface layers (0- 10cm) of soil were collected in duplicate.
Three types of microbiological tests, as indicators of soil health, were carried out on these soils; (i) soil microbial biomass to assess general activity of soil microbes – to establish activity of total microbial community, (ii) substrate induced nitrification to assess the activity of specialist nitrifying bacteria that are often sensitive to pesticides, and (iii) a range of microbiological enzymes based assays were carried out to assess the impact of pesticides on various microbiological functions. These nitrifying bacteria and other enzymes, respectively, perform the important function of conversion of ammonium to nitrate, which is the major form of nitrogen taken up by the plants, and a range of functions such as carbon and nutrient turnover.
We chose to carry out tests for 3 enzymes: Acid/alkali phosphatase, ?-glucosidase, and chitinase.
Phosphatase enzymes are produced by various plant roots and micro-organisms. They convert insoluble forms of phosphorus from organic matter into soluble forms which plants can then absorb.
Acid phosphatase activity is predominately found in acid soils and is linked to cultivated plants and bacteria. Alkaline phosphatase activity is predominately found in neutral or alkaline soils and is linked to micro-organisms. ?-glucosidase is an enzyme which catalyses the hydrolysis of the low molecular mass glycosides and produce glucose. The enzyme is produced by micro-organisms, plant tissue and animals. Chitinase catalyses the hydrolysis of chitin into carbon and nitrogen sources and is produced by a vast array of organisms.
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