Effects of Soil Storage on the Microbial Community and Degradation of Metsulfuron-methyl STEVEN L. TRABUE,* ,†,‡ DEBRA E. PALMQUIST, § TARA M. LYDICK, ‡,# AND SUZANNE KOCH SINGLES ‡ National Soil Tilth Laboratory, Agricultural Research Service, U.S. Department of Agriculture, 2150 Pammel Drive, Ames, Iowa 50011; Agricultural Research Service, U.S. Department of Agriculture, 1850 North University Street, Peoria, Illinois 61604; Delaware Public Health Laboratory, Smyrna, Delaware 19977; and DuPont Crop Protection, Global Technology Division, Stine-Haskell Research Center, Newark, Delaware 19714 The effect storage had on the microbial biomass in two soils (Trevino and Fargo) was compared to the effect storage had on each soil’s capacity to degrade metsulfuron-methyl. Soils were collected from the field and used fresh (<3 weeks old) or stored at 20 and 4 °C for 3 or 6 months. The phospholipid fatty acid content of the soils was used to monitor changes in the microbial biomass during storage and incubation in a flow-through apparatus. In both soils, [phenyl-U- 14 C]metsulfuron- methyl was used to monitor changes in the route and rate of degradation along with 14 CO 2 evolution (mineralization). Total microbial biomasses in both soils were significantly reduced for soils incubated in the flow-through apparatus, whereas only the Trevino soil’s microbial biomass was significantly reduced as a result of storage. The microbial communities of both soils were significantly different as a result of storage as shown by discriminant analysis. In both soils, degradation rate, pathway of degradation, and mineralization of metsulfuron-methyl were significantly affected by storage compared to fresh soil. The half-life of metsulfuron-methyl increased significantly (P < 0.05) in the Trevino soil from 45 days (fresh) to 63 days (stored soil), whereas in the Fargo soil half-lives increased significantly (P < 0.05) from 23 days (fresh) to 29 days (soils stored for 6 months). In both soils, mineralization of [ 14 C]metsulfuron-methyl was significantly (P < 0.05) higher in fresh soils compared to stored soils. The degradation pathways of metsulfuron-methyl changed with storage as evidenced by the loss of formation of one biologically derived metabolite (degradate) in stored soils compared to fresh soils. KEYWORDS: Storage; biomass; microbial community; pesticide; metsulfuron-methyl; half-life; degrada- tion pathway INTRODUCTION Soils are the primary medium through which most crop protection products (CPP) enter the environment. Regulators are interested in the metabolism and rate of degradation of these compounds in soil in order to evaluate the fate of these compounds in the environment. The registration of new and existing CPP in the United States (1) and the European Union (EU) (2) requires laboratory soil metabolism studies to be conducted following certain regulatory guidelines. These studies are used to determine half-lives, degradation pathways, and significant metabolites of CPP in the soil environment. Ter- restrial soil dissipation in the field and terrestrial ecotoxicological studies are in part dictated by the results obtained in laboratory soil metabolism studies. In addition, soil metabolism studies are one of the key studies used in determining which metabolites are to be included in the ecological risk assessment during the registration process (3). Consequently, ensuring that a laboratory soil’s intrinsic properties are maintained prior to the initiation of a study is vital because loss of a soil functional property may bias results that will eventually be used in a compound’s regulatory evaluation. The methods used in collecting, processing, and storing soil will greatly influence its integrity, and typically the soil microbial community is the most affected by storage. Improper handling of soil that negatively influences the soil microbial community can affect the degradation rates of compounds that are degraded biologically. EU guidelines (4) specify that moist soil samples be processed through a 2 mm sieve, collected during nonstressed conditions (i.e., no flooding, drought, or freezing events), and stored for no longer than 3 months at 4 °C. Storage of soil can be extended beyond 3 months by demonstrating the microbial community remained viable (5) and * Author to whom correspondence should be addressed [telephone (515) 294-0201; fax (515) 294-1209; e-mail trabue@nsric.ars.usda.gov]. † U.S. Department of Agriculture, Ames, IA. ‡ DuPont Crop Protection. § U.S. Department of Agriculture, Peoria, IL. # Delaware Public Health Laboratory. 142 J. Agric. Food Chem. 2006, 54, 142-151 10.1021/jf0512048 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/09/2005