Long-term sorption of diuron and isoproturon by a clay loam soil was investigated for nine weeks at two herbicide doses (0.6 or 3 mg kg-') and two soil moisture contents (35 or 62% w/w, i.e. 3.16 or 1 kPa) by measuring changes in herbicide concentrations in the soil solution sampled by means of glass microfibre filters in presence of sodium azide (200 mg litre-') which inhibited biodegradation for more than four weeks. After the first day equilibration period, where adsorption mainly occurred ( > 70% adsorbed), herbicide concentrations in the soil solution decreased (about 50% for diuron; up to 38% for isoproturon) for two weeks but equilibration required about one month. Small amounts of herbicides were sorbed during this process (
The adsorption of diuron and isoproturon by a clay loam soil at 35% (3‐16 kPa) and 62% (1 kPa) soil moisture content was studied by means of glass microfibre filters capable of sampling soil solution for herbicide analysis. Adsorption was rapid, with 40–80% of the final (24 h) sorption being achieved within 2 min. These equilibria were achieved more rapidly for diuron, which was also the more highly adsorbed. Adsorption of both herbicides was favoured by low soil moisture initially, but was enhanced by higher soil moistures at sorption times greater than 30 min. However, increasing the soil moisture from 31% (10 kPa) to 62% (1 kPa) had little effect on the final soil sorption capacity. Regarding the water status in the soil, it is thought that adsorption took place in small pores (<3 μm). Herbicides diffused rapidly into small pores and adsorption by wet soil was delayed for a short period of time (about 30 min).
Changes in the concentrations of [''*C]carbonylisoproturon and its degradation products in a clay-loam soil and in soil solution during incubation at 11°C and 18°C for 6 weeks, were measured following solvent extraction and soil solution sampling with glass microfibre filters. During herbicide degradation, ' "^CCh was released (up to 20%) and unextractable radioactivity increased (up to 30%). Monomethyl isoproturon was the main metabolite in soil followed by metabolite X5 (possibly hydroxy di-desmethyl isoproturon). Isoproturon and monomethyl isoproturon were mainly adsorbed by soil whereas metabolite X5 was found mainly in the soil solution. Isoproturon concentrations declined in both soil and soil solution, but the percentage of the residual herbicide dissolved in the soil solution decreased from 26 to 15%. At low temperature, herbicide degradation occurred more slowly, and the degradation products were generally less abundant. However metabolite X5 was present at unexpectedly high levels, particularly in the soil solution.
Summary:
The foliar surface of 4‐leaf maize plants was found to be poorly wettable and retained 106 μl g−1 dry matter when sprayed with a U46D (2,4‐D formulation) blank. The third leaf retained 141 μl g−1. A 7‐day cold spell (17/9°C) increased retention per unit dry matter by 53% (135% on the third leaf). Cold stress lowered epicuticular wax quantity by 29% on the third leaf. Contact angles of formulated 2,4‐D lay between 115 and 125° and were not significantly affected by cold stress. 2,4‐D rapidly entered into maize third leaf (66% in 24 h) but migration from it was less than 1.5%. 2,4‐D was readily degraded in maize (80% in 72 h). The most abundant metabolite was probably an ester conjugate; little of the hydroxy derivatives were found. Cold stress reduced 2,4‐D degradation, and 72 h after treatment the amount of undegraded 2,4‐D was 78% higher in cold‐stressed maize plants. It was concluded that 2,4‐D selectivity in maize results from low spray retention per unit dry matter and active degradation of penetrated herbicide. Cold stress affects both factors.
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