Degradation and adsorption of 14 C‐MCPA in soil—influence of concentration, temperature and moisture content on degradation

Summary MCPA was weakly absorbed in soils with 2.4, 3.0 and 2.9% humus. K d ‐values were 0.7, 0.9 and 1.0, respectively. In soil, not previously treated with MCPA, the degradation of 0.05 mg kg −1 14 C‐MCPA followed first‐order reaction kinetics whereas degradation of 5 mg kg −1 was only first‐order for 2 weeks; exponentially increasing degradation rates followed indicating enrichment of the soil with MCPA decomposers. Degradation was monitored by evolution of 14 CO 2 . The influence of temperature on degradation of MCPA (4 mg kg −1 ) could initially be described by Q 10 values or by the Arrhenius equation. After 1 day of incubation in two field soils Q 10 values were 3.3 and 2.9, respectively, between 0°C and 29°C; the activation energies were 87 and 76 kj mol −1 . Exponentially increasing degradation rates followed with doubling times of about 4.0, 1.8, 1.2 and 0.6 days at 6,10, 15 and 21°C, respectively. After 51 days of incubation, at temperatures between 6°C and 29°C, about 60% 14 C was evolved in CO 2 and only traces of MCPA were left in the soil. At 0°C and at 40°C only 1% and 10% 14 C, respectively, were evolved as CO 2 after 51 days. 14 C‐MCPA (4 mg kg −1 ) was incubated at moisture contents from that in air‐dried soil to 2.3 times field capacity. Optimum for degradation was from 0.6 to 1.2. field capacity. Degradation was very slow where water contents were below the level of wilting point and was nil in air‐dried soil. In wet soil degradation was delayed, but even in water‐logged soil (2.3 times field capacity) MCPA was decomposed after 4 to 5 weeks at 10°C.