Kinetics and effect of moisture content and preincubation on the decomposition of 14C‐labeled herbicides in soil

Abstract

The kinetics and the influence of soil moisture content (MC) on the mineralization of 14C-labeled linuron were investigated at 15 different MCs in the range from air-dried soil to 100% of the water-holding capacity (WHC). For all MCs used, the data on the liberated amount of 14C (x) were initially described by Eq. (1) x = k1t + a—after which the data were described by Eq. (2) x = k2t1/2 + b—where t is time, k1 and k2 are rate constants, and a and b are constants. The rate constants k1 and k2 for MCs < 100% of WHC were mathematically described by the equation k = l + m. MC12, where k is either of the rate constants, l is a constant that can account for a threshold value of MC below which no decomposition occurs, and m is a constant. The validity of this equation for first-order rate constants was tested by using data from the literature. The kinetics of mineralization of 14C-labeled glyphosate were investigated by adding glyphosate after different times of preincubation (tp) of a previously frozen soil. An initial phase of 11 days linear with t [Eq. (1)] was obtained when the herbicide was added immediately after the thawing, after which a phase linear with t1/2 [Eq. (2)] ensued. The length of the initial phase decreased and k1 increased with increasing tp, and for tp ≥ 8 days the initial phase could not be confirmed. The following phase was almost unaffected by the preincubation. Thus, the phases represented by Eqs. (1) and (2) are separate, since they can be affected independently of each other. It is suggested that the initial phase is an induction phase, or lag phase, which reflects the disturbances introduced by handling the soil, and that the second phase is the steady state.