Microbiological Characteristics of a Vegetative Buffer Strip Soil and Degradation and Sorption of Metolachlor

Abstract

Numerous studies have documented efficacy of vegetated buffer strips (VBS) in removing herbicides from surface runoff. Little is known about the fate of herbicides after deposition in buffer strip soil. Soil samples (0‐ to 2‐cm depth) were collected from a buffer strip and an adjacent bare field (BF). Soil organic C was two‐fold higher in VBS than in BF, and VBS soil maintained about 0.7 log(10) greater propagule density of total fungi and bacteria and 2 log(10) greater gram‐negative bacteria and fluorescent pseudomonads. Corresponding with enhanced microbial populations, VBS exhibited higher endogenous levels of alkaline phosphatase, tetrazolium chloride dehydrogenase, aryl acylamidase, and fluorescein diacetate hydrolytic activity (1.6‐ to 3.8‐fold greater than BF). Batch studies of metolachlor[2‐chloro‐ N ‐(2‐ethyl‐6‐methyphenyl)‐ N ‐(2‐methoxy‐1‐methylethyl)acetamide] sorption showed greater capacity to sorb metolachlor in VBS soil than in BF ( K d 2.25 vs. 1.60 mL g −1 ). Soil treated with 14 C metolachlor (1.25 mg kg −1 ) was incubated for 46 d in a laboratory study. Limited mineralization (<4%) was observed for both VBS and BF. Less 14 C (43%) was extracted from VBS samples 46 d after treatment. Extractable fractions consisted primarily of metolachlor, although increasing amounts of polar (e.g., sulfonic acid) and nonpolar metabolites were recovered over time, especially in VBS. Metolachlor half‐life was 10 and 23 d for soils from VBS and BF, respectively, attributed to higher levels of organic matter and microbial activity in VBS soils. Data suggest retention and enhanced degradation of metolachlor as it passes through VBS strips may limit further transport.