Metolachlor and Tillage Impacts on Microbial Dynamics during a Cotton Growing Season.

The benefits of conservation tillage including reduced erosion and runoff and increased soil organic carbon (OC) are well documented. However, relatively little is known about the interactive effects on soil pesticide dissipation and microbial ecology. A field study was conducted in 2007 which examined dissipation of the herbicide metolachlor and soil microbial biomass and composition dynamics in a cotton (Gossypium hirsutum) field as a function of tillage (strip- or conventional) and herbicide formulation (novel organoclay-based or a commercial generic product). The tillage practices have been in place since 1999. Soil samples were collected at 0-2 and 2-8 cm depths at regular intervals during the growing season (May to September) and tested for metolachlor and selected degradates, soil organic carbon and nitrogen, and microbial biomass and composition – by phospholipid fatty acid methyl ester (PLFA) analysis. Strip-till (ST) soil OC, 9.0 g kg^-1 was significantly greater than CT soil OC, 8.1 g kg^-1.  During the study active microbial biomass decreased at the surface (0-2 cm) from initially > 50 to < 10 nmol PLFA g^-1 soil in both tillage types. Below the surface (2-8 cm) CT had higher biomass across time, as compared to ST, with values of 25 and 15 nmol PLFA g^-1 soil, respectively. A metolachlor formulation impact was indicated in the in 0-2 surface zone samples. The PLFA mole percent for gram positive bacteria and fungi, with clay-based and generic product was 36 and 31, and 26 and 30%, respectively. But these differences did not impact metolachlor dissipation kinetics. In all cases soil half-lives were about 7 d. Data indicate that pesticide formulation and tillage are important regulators of microbial biomass and community composition but overall impacts on dissipation kinetics may be small.