The sulfonylurea herbicides are characterized by broad-spectrum weed control at very low use rates (c. 2–75 g ha−1), good crop selectivity, and very low acute and chronic animal toxicity. This class of herbicides acts through inhibition of acetolactate synthase (EC 4.1.3.18; also known as acetohydroxyacid synthase), thereby blocking the biosynthesis of the branched-chain amino acids valine, leucine and isoleucine. This inhibition leads to the rapid cessation of plant cell division and growth. Crop-selective sulfonylurea herbicides have been commercialized for use in wheat, barley, rice, corn, soybeans and oilseed rape, with additional crop-selective compounds in cotton, potatoes, and sugarbeet having been noted. Crop selectivity results from rapid metabolic inactivation of the herbicide in the tolerant crop. Under growth-room conditions, metabolic half-lives in tolerant crop plants range from 1–5 h, while sensitive plant species metabolize these herbicides much more slowly, with half-lives > 20 h. Pathways by which sulfonylurea herbicides are inactivated among these plants include aryl and aliphatic hydroxylation followed by glucose conjugation, sulfonylurea bridge hydrolysis and sulfonamide bond cleavage, oxidative O-demethylation and direct conjugation with (homo)glutathione. Sulfonylurea herbicides degrade in soil through a combination of bridge hydrolysis and microbial degradation. Hydrolysis is significantly faster under acidic (pH 5) than alkaline (pH 8) conditions, allowing the use of soil pH as a predictor of soil residual activity. Chemical and microbial processes combine to give typical field dissipation half-lives of 1–6 weeks, depending on the soil type, location and compound. Very short residual sulfonylurea herbicides with enhanced susceptibility to hydrolysis (DPX-L5300) and microbial degradation (thifensulfuron-methyl) have been developed.

Mode of action, crop selectivity, and soil relations of the sulfonylurea herbicides†

Sulfonylureas are a unique group of herbicides used for controlling a range of weeds and some grasses in a variety of crops and vegetables. They have been extremely popular worldwide because of their low mammalian toxicity, low use rate, and unprecedented herbicidal activity. Knowledge about the fate and behavior of sulfonylurea herbicides in the soil-water environment appears to be of utmost importance for agronomic systems and environmental protection. Because these herbicides are applied at a very low rate, and their mobility is greatly affected by the chemicals' anionic nature in alkaline soils, a thorough understanding of their degradation/hydrolysis processes and mechanisms under aqueous and soil systems is important. This review brings together published information on the hydrolysis of several sulfonylureas in aqueous and soil solutions that includes the effects of pH, temperature, functional relationship between pH vs hydrolysis rate constants, and hydrolysis behavior of sulfonylureas in the presence of minerals. In addition, the transformations of sulfonylureas in soil, under laboratory and field experiments, have been discussed in connection with the compounds' varied structural features, i.e., sulfonylueas that are with or without the pyridinic, pyrimidine, and triazinic ring.

Hydrolysis of sulfonylurea herbicides in soils and aqueous solutions: a review.

Microbial Communities.- How Resilient Are Microbial Communities to Temperature Changes During Composting?.- Survey of Fungal Diversity in Mushroom Compost Using Sequences of PCR-Amplified Genes Encoding 18S Ribosomal RNA.- Bacterial Community Structure During Yard Trimmings Composting.- Characterisation of Microbial Communities During Composting of Organic Wastes.- Microbial Succession During Composting of Source-Separated Urban Organic Household Waste Under Different Initial Temperature Conditions.- Bacterial Diversity in Livestock Manure Composts as Characterized by Terminal Restriction Fragment Length Polymorphisms (T-RLFP) of PCR-amplified 16S rRNA Gene Sequences.- Amplified 16S Ribosomal DNA Restriction Analysis of Microbial Community Structure During Rapid Degradation of a Biopolymer, PHA, by Composting.- Comparative Investigation of Vermicompost Microbial Communities.- Processes and Controls.- Heat Production During Thermophilic Decomposition of Municipal Wastes in the Dano-System Composting Plant.- Composting of Different Horticultural Wastes: Effect of Fungal Inoculation.- Backyard Composting: General Considerations and a Case Study.- N-Dynamics During Composting - Overview and Experimental Results.- Unsuitability of Anaerobic Compost from Solid Substrate Anaerobic Digestion as a Soil Amendment.- Pile Composting of Two-phase Centrifuged Olive Husks: Bioindicators of the Process.- Organic Acids as a Decisive Limitation to Process Dynamics During Composting of Organic Matter.- Effects of Interrupted Air Supply on the Composting Process - Composition of Volatile Organic Acids.- Reduction of Ammonia Emission and Waste Gas Volume by Composting at Low Oxygen Pressure.- Review of Compost Process-Control for Product Function.- Using Agricultural Wastes for Tricholoma crassum (Berk.) Sacc. Production.- Microbial Transformation of Nitrogen During Composting.- Effect of Additives on the Nitrification-Denitrification Activities During Composting of Chicken Manure.- Biodegradability.- Biodegradability Study on Films for Packaging Based on Isotactic Polypropylene Modified with Natural Terpene Resins.- Isolation and Characterization of Thermophilic Microorganisms Able to Grow on Cellulose Acetate.- PCB's Biotransformation by a White-Rot Fungus Under Composting and Liquid Culture Conditions.- Tests on Composting of Degradable Polyethylene in Respect to the Quality of the End-Product Compost.- Microbial Degradation of Sulfonylurea Herbicides:Chlorsulfuron and Metsulfuron-Methyl.- Maturity Testing.- Hydrogen Peroxide Effects on Composting and Its Use in Assessing the Degree of Maturity.- Plant Performance in Relation to Oxygen Depletion, CO2-Rate and Volatile Organic Acids in Container Media Composts of Varying Maturity.- Microbiological and Chemical Characterisation of Composts at Different Levels of Maturity, with Evaluation of Phytotoxicity and Enzymatic Activities.- Monitoring of a Composting Process: Thermal Stability of Raw Materials and Products.- Compost Maturity - Problems Associated with Testing.- Use of CLPP for Evaluating the Role of Different Organic Materials in Composting.- Evaluation of Organic Matter Stability During the Composting Process of Agroindustrial Wastes.- Characterization of Organic Substances in Stabilized Composts of Rest Wastes.- Application and Environmental Impact.- Composting of Posidonia oceanica and Its Use in Agriculture.- Practical Use of Quality Compost for Plant Health and Vitality Improvement.- Environmental Impacts of Cattle Manure Composting.- Agronomic Value and Environmental Impacts of Urban Composts Used in Agriculture.- Composting in the Framework of the EU Landfill Directive.- Occurence of Aspergillus fumigatus in a Compost Polluted with Heavy Metals.- Important Aspects of Biowaste Collection and Composting in Nigeria.- Plant and Human Pathogens.- Use of Actinobacteria in Composting of Sheep Litter.- Methods for Health Risk Assessment by Clostridium botulinum in Biocompost.- The Fate of Plant Pathogens and Seeds During Backyard Composting of Vegetable, Fruit and Garden Waste.- Survival of Phytopathogen Viruses During Semipilot-Scale Composting.- Air-Borne Emissions and Their Control.- Composting Conditions Preventing the Development of Odorous Compounds.- Odour Emissions During Yard Waste Composting: Effect of Turning Frequency.- Imission of Microorganisms from Composting Facilities.- Molecular Identification of Airborne Microorganisms from Composting Facilities.- Bioaerosols and Public Health.- Passively Aerated Composting of Straw-Rich Organic Pig Manure.

Microbiology of composting

The sulfonylurea herbicides are a relatively new group of compounds which control broad-leaved weeds and some grasses in cereal crops. This literature review emphasises work reported on chlorsulfuron and met-sulfuron-methyl. The activity of the herbicides, their fate in soil and in plants, and their mode of action are discussed. In addition some of the methods of assaying these compounds are described.

A review of the activity, fate and mode of action of sulfonylurea herbicides

Use of lysimeters to estimate leaching of pesticides in agricultural soils

Degradation of chlorsulfuron {2-chloro-N- (((4- methoxy- 6- methyl-1,3 ,5 -triazin-yl)amino) carbonyl) benzene- sulfonamide} in acidic and alkaline soils was evaluated using plant bioassay and high-performance liquid chromatography (HPLC) radiotracer techniques. Soil sterilization with either ethylene oxide (EtO) or gamma irradiation significantly reduced breakdown of chlorsulfuron; the ability for degrada- tion was restored by reinoculation with indigenous soil micro- organisms. Streptomyces griseolus (a soil actinomycete), Aspergillus niger, and Penicillium sp. (soil fungi) were demon- strated to degrade 14C-chlorsulfuron in pure culture. In addition to microbial breakdown, chemical hydrolysis was an important factor in the disappearance of chlorsulfuron from soil. The contribution of chemical hydrolysis to total degradation was a function of soil pH, with hydrolysis oc- curring most rapidly in acidic soils. Both dissipation processes slowed markedly at low temperatures. Additional index words. Soil pH, soil temperature, soil mois- ture, Streptomyces griseolus, Aspergillus niger, Penicillium

James A. Romesser

Papers

Degradation of Chlorsulfuron by Soil Microorganisms