Agriculture Reference
In-Depth Information
concentration in soil demonstrating that pyroxasulfone becomes less bioavailable when soil
organic carbon content is high (Figure 4). Soil organic matter is typically estimated to contain
58% carbon. Soil organic matter has colloidal structure and contains a mixture of complex
organic compounds with many functional groups that strongly sorbs various chemicals
including herbicides [19].
80
0% biochar
0.25% biochar
0.5% biochar
1.0% biochar
2.0% biochar
60
40
20
0
0
50
100
150
200
250
300
Pyroxasulfone (
g/kg)
Figure 4. Effect of biochar content in soil on pyroxasulfone bioactivity determined by the 7-day sugar
beet shoot length bioassay.
Herbicide dissipation.
The rate of herbicide dissipation in agricultural environments
determines whether the herbicide will provide long term weed control through the season of
application, and whether there is a potential for herbicide carry-over injury to subsequent
crops. Because the dissipation rate varies with soil characteristics, bioassays are an effective
tool for evaluating herbicide persistence in soil. Typically high organic matter and clay
content and low soil pH decrease the dissipation rate by reducing the amount of herbicide
available for degradation [6, 20, 21]. The rate of dissipation also depends on environmental
conditions in the season of herbicide application, particularly on moisture and temperature
[20, 22, 23]. Determination of dissipation half-life for herbicides under controlled laboratory
conditions is illustrated in the following example.
Pyroxsulam and pyroxasulfone dissipation in soil was evaluated under conditions of soil
moisture content of 80% field capacity and 25°C temperature. Soil samples with added water
and pyroxsulam at 3.45 μg/kg or pyroxasulfone at 92 μg/kg level in capped Styrofoam cups
were incubated and soil moisture content was kept nearly constant by adding water daily to
predetermined weight. At each sampling time, residual pyroxsulam was determined by the 4-
day sugar beet root length bioassay, while residual pyroxasulfone was assessed by the 7-day
sugar beet shoot length bioassay. Herbicide concentration remaining in soil at each sampling
time was calculated using dose-response curves (Figure 2), and was plotted vs incubation
time (Figure 5). The rate of dissipation was faster for pyroxsulam than pyroxasulfone, and the
estimated half-lives were 11 and 30 days for pyroxsulam and pyroxasulfone, respectively.
Moisture content of 80% field capacity and temperature of 25°C represent warm and moist
conditions that typically facilitate herbicide degradation. When pyroxsulam dissipation was
assessed at 30% moisture content and 25°C, and at 80% moisture content and 15°C (data not
