pubs.acs.org/JAFC Published on Web 12/30/2010 © 2010 American Chemical Society J. Agric. Food Chem. 2011, 59, 645–653 645 DOI:10.1021/jf102960g Effect of Soil Wetting and Drying Cycles on Metolachlor Fate in Soil Applied as a Commercial or Controlled-Release Formulation OSNAT GOLDREICH, † Y AAKOV GOLDWASSER, ‡ AND Y AEL G. MISHAEL* ,† † Department of Soil and Water and ‡ RH Smith Institute of Plant Science and Genetics in Agriculture, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot 76100, Israel A controlled-release formulation (CRF) has been developed for metolachlor, which reduced its leaching in a sandy soil and improved weed control in comparison with the commercial formula- tion. The current study tested the effect of soil wetting and drying cycles (WDCs) on metolachlor fate (desorption, leaching, and weed control) applied as the CRF and as the commercial formula- tion. Metolachlor adsorption to a heavy soil (Terra-Rosa) was predominately to the clay minerals and oxides. Metolachlor release from a heavy soil subjected to WDCs was higher than its release from the soil not subjected to WDCs. Consequently, a bioassay in soil columns treated with the commercial formulation indicated enhanced metolachlor leaching in heavy soils under WDCs. In contrast, when metolachlor was applied as the CRF, leaching was suppressed and not affected by WDCs. These results emphasize the advantages of the CRF also in heavy soils subjected to WDCs. KEYWORDS: Metolachlor; controlled-release formulations; wetting and drying cycles; leaching INTRODUCTION Applying herbicides provides substantial agronomic and eco- nomic benefits; however, in some cases their use poses environ- mental issues, due to leaching and surface migration, which cause soil, surface water, and groundwater contamination ( 1-3 ). Further- more, migration and leaching reduce herbicide concentration at the topsoil, which reduces weed control efficacy. Insufficient weed control brings an increase in herbicide application dose and frequency, which further increase treatment costs and environ- mental contamination. Herbicide leaching in the soil is governed by several factors, such as soil structure and characteristics, chemophysical proper- ties of herbicides, and the effects of climatic conditions and tillage methods ( 1 , 4 ). Among climate conditions the effects of rain and irrigation on herbicide leaching have been widely explored ( 5 , 6 ). For example, heavy rain intensities and high irrigation frequen- cies have been found to enhance the migration of metolachlor (MTC) in sandy soil ( 5 ). In contrast, very little is reported on another important impact of climate, the influence of wetting and drying cycles (WDCs). The phenomenon of WDCs implies that the soil undergoes frequent changes in water content due to rain events or irrigation accompanied by dry periods. This phenomenon is most pro- nounced in semiarid areas. WDCs of the soil affect herbicide fate in the soil and in particular their persistence, leaching, migration, sorption to soil particles, and degradation ( 7 ). A number of studies have shown that microbial degradation of herbicides is inhibited during drying cycles, which enhances their persistence in soil ( 8 -11 ). The effect of WDCs on adsorption/desorption of herbicides was less studied and is less understood. Different trends are reported on this effect; for example, imazaquin desorption from the soil increased following WDCs ( 9 ), whereas diuron ( 12 , 13 ) and atrazine ( 8 ) desorption was reduced due to WDCs. The main mechanism suggested was strong adsorption of the herbicide to soil organic matter. Metolachlor is a selective preplant herbicide that controls a broad spectrum of grass weeds and small-seeded broadleaves in many crops and is widely used worldwide mainly in corn, soybean, sunflower, sugar beet, potato, and cotton. Its adsorption to the soil is considered to be moderate and is positively correlated with soil organic matter and clay content ( 14-20 ). Its water solubility is relatively high (S w = 488 mg/L, 20 °C); therefore, it is prone to extensive leaching and has been detected in groundwater ( 14 , 21-25 ). One of the approaches pursued to reduce herbicide migration in soil while maintaining suitable weed control is developing controlled-release formulations (CRFs) ( 26 -30 ). We have designed a CRF for metolachlor based on herbicide solubilization in micelles and adsorption of the mixed micelles on clay minerals ( 31 ). This formulation was tested and found to reduce metola- chlor leaching through a sandy soil column and improve weed control in comparison with the commercial formulation. In the current study the effect of WDCs on metolachlor desorption, leaching, and weed control in the soil was investigated. We hypothesized that the CRF will moderate the negative effects of WDCs on metolachlor behavior, that is, desorption, leaching, and weed control efficiency. Therefore, the CRF’s *Corresponding author (phone 972-8-948-9171; fax 972-8-948-9856; e-mail mishael@agri.huji.ac.il).