## 4 Discussion  
### Adsorption-Desorption  
The \(K_{ads}\) values of metolachlor from this study were within the range of the published metolachlor adsorption coefficients of 1.0-26.7 L Kg\({}^{-1}\)[11; 15; 16; 17]. A lower \(K_{ads}\) indicates less adsorption affinity of the herbicide for soils. The isotherm slopes (\(1/n_{ads}\)) were less than one for all the soils, implying a non-linear relationship between the amount of herbicide adsorbed and the concentration in the applied solution. This suggests the leaching potential of metolachlor leaching when accumulated at higher application rates within the soil profiles.  
Under both aerobic and anaerobic soil conditions, metolachlor adsorption was highest in the Drummer soil, followed by Flanagan, and it was lowest in the Catlin soil. Such an adsorption increase corresponds to an increase in the soil organic matter content (Table 1) and emphasizes the involvement of organic matter in the adsorption process of metolachlor, which has already been communicated in previous studies [6; 18]. Moreover, the difference in adsorption between the anaerobic and aerobic soils was more prominent in soils with high organic matter contents.  
Another remarkable observation from the study is the noticeable reduction in the adsorption of \({}^{14}\)C-metolachlor in the anaerobic soils, as evident from their respective low \(K_{des}\) values compared with the aerobic incubations. There are some explanations proposed for reduced adsorption for herbicides in anaerobic soils. One theory suggests that modifications to functional groups on soil organic matter under anoxic soil conditions lead to reduced adsorption [19]. Another explanation proposed for relatively low herbicide adsorption is the decrease in the inorganic surface area caused by the reduction of ferric to ferrous ions [20]. However, the more significant decline in adsorption observed under anaerobic conditions in the soil with higher organic matter provides more support for the former theory.  
The desorption coefficient (\(K_{des}\)) of metolachlor from the current study was in agreement with the previously reported values of 1.3-6.5 L kg\({}^{-1}\)[15] and 5.9-51.6 L kg\({}^{-1}\)[16]. Relatively low \(K_{des}\) values under anaerobic conditions for all three tested soils indicate that the desorption process was enhanced in the anaerobic or reduced soils. Such desorption patterns of the herbicide could improve its bioavailability and subsequently expedite the  
Figure 2: Degradation kinetics (**a–c**) and mineralization patterns (**d–f**) of \({}^{14}\)C-metolachlor in aerobic (Aer) and anaerobic (An) soil conditions in Catlin, Flanagan, and Drummer soils. Error bars, whenever visible, represent standard errors (\(n=3\)).  
degradation or bioactivity of the herbicide in soil. Furthermore, metolachlor desorption decreased with an increase in the organic matter content regardless of the soil environmental conditions, suggesting a role for soil organic matter in the desorption kinetics of metolachlor in soils.  
The intriguing observation of the adsorption-desorption patterns of metolachlor in anaerobic soils suggests that soil redox is one of the governing factors for these processes in soil. The decreased adsorption of metolachlor under anaerobic conditions in different soil types may have significant implications for its bioavailability and environmental transport as an herbicide compound. For instance, weak adsorption suggests a high potential for metolachlor to lose herbicidal potency faster on sites where anaerobic conditions are predominant and a high risk for metolachlor to enter a lake or other water bodies along with soil sediments.