Long-term tillage effects on soil metolachlor sorption and desorption behavior

Area of Science:

• Environmental Chemistry
• Soil Science
• Agronomy

Background:

• Pesticide retention in soils is governed by sorption and desorption processes.
• The influence of soil tillage management on pesticide sorption mechanisms remains unclear.
• Metolachlor is a widely used herbicide whose soil behavior is critical for environmental risk assessment.

Purpose of the Study:

• To investigate the sorption and desorption characteristics of metolachlor.
• To determine the impact of long-term conservation tillage (CnT) versus conventional tillage (CT) on metolachlor sorption.
• To elucidate the role of soil organic matter (SOM) fractions in mediating these processes.

Main Methods:

• Soil samples from long-term CnT and CT plots were analyzed.
• Sorption and desorption experiments were conducted using a batch-equilibration method on whole soils and extracted humic acid (HA) and humin.
• Three sequential desorption rinses were performed to assess desorption behavior.

Main Results:

• Hysteresis in metolachlor sorption and desorption was observed across all soil samples and SOM fractions.
• Sorption nonlinearity (N) and hysteresis were dependent on SOM structure and composition.
• Freundlich isotherm exponents (N) for HA and humin were higher in CnT soils compared to CT soils, potentially due to differences in SOM aromaticity. Sorption capacity (K'f) correlated positively with soil organic carbon (SOC).

Conclusions:

• Long-term tillage management significantly affects metolachlor sorption and desorption.
• Qualitative differences in the structural characteristics of soil humic substances under different tillage regimes are likely responsible for altered pesticide behavior.
• Understanding these SOM-mediated mechanisms is crucial for predicting pesticide fate and transport in agricultural soils.