Aged and bound herbicide residues in soil and their bioavailability: Part 1: Uptake of aged and non-extractable (bound) [3-14c]metamitron residues by sugar beets

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Abstract
In spring 1983 the herbicide active substance [3‐14C] metamitron was applied to sugar beet grown in a degraded loess soil ("Parabraunerde") in a 0.5 m2 lysimeter, corresponding to 10 kg Goltix/ha under field conditions. 190 days after pre‐emergence spraying, and harvesting the sugar beet the 0–2.5 cm and the 2.5–5 cm soil layers were removed from the lysimeter for plant experiments on the bioavailability of aged and bound metamitron residues in microeco‐systems. Sugar‐beets were planted at the 2‐leaf stage and cultivated for 22 days until the 8‐leaf stage. The uptake of radioactivity by the sugar‐beet from the soils containing aged metamitron residues was about 0.03% of the 14C‐activity still present in the soil. The uptake increased to 0.14 and 0.17% when plants grow in soil pretreated with organic solvents. If [3‐14C] metamitron was freshly added to the soil, the bioavail‐ability to the sugar‐beet roots in this microecosystem decreased by a factor of 90. Although the bioavailability to the roots is thus extremely low, 3.3–7.6% of the radioactivity present in the 0–5 cm layer after one vegetation period was mineralized to 14CO2 during the 22‐day plant experiment. After the plant experiment between 2.5 and 4.7% were ex‐tractable from the soil with organic solvents and water and 4–9% with a simulated soil solution (0.01 M CaCl2). Approximately 90% of the radioactivity remained bound in the soil; ca. 60% in the humin and inorganic fraction, 7–11% in the humic acid and about 17% in the fulvic acid fraction. No significant differences between the two soil layers (0–2.5 and 2.5–5 cm) with respect to the distribution of the radioactivity in the various fractions became apparent, irrespective of pretreatment of the soil.