Understanding pesticide metabolism in plants and microorganisms is necessary for pesticide development, for safe and efficient use, as well as for developing pesticide bioremediation strategies for contaminated soil and water. Pesticide biotransformation may occur via multistep processes known as metabolism or cometabolism. Cometabolism is the biotransformation of an organic compound that is not used as an energy source or as a constitutive element of the organism. Individual reactions of degradation–detoxification pathways include oxidation, reduction, hydrolysis, and conjugation. Metabolic pathway diversity depends on the chemical structure of the xenobiotic compound, the organism, environmental conditions, metabolic factors, and the regulating expression of these biochemical pathways. Knowledge of these enzymatic processes, especially concepts related to pesticide mechanism of action, resistance, selectivity, tolerance, and environmental fate, has advanced our understanding of pesticide science, and of plant and microbial biochemistry and physiology. There are some fundamental similarities and differences between plant and microbial pesticide metabolism. In this review, directed to researchers in weed science, we present concepts that were discussed at a symposium of the American Chemical Society (ACS) in 1999 and in the subsequent book Pesticide Biotransformation in Plants and Microorganism: Similarities and Divergences, edited by J. C. Hall, R. E. Hoagland, and R. M. Zablotowicz, and published by Oxford University Press, 2001.

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Core Ideas Light frosts did not increase phosphorus release from cover crops.Heavy frosts released more water‐extractable phosphorus than light frosts.Herbicide induced termination increased phosphorus and ammonium losses.Frost tolerant species released less phosphorus than frost‐intolerant species.Cover crops remain a suitable management practice in temperate regions. Cover crops (CC) are planted into fields during the non‐growing season as a best management practice (BMP) for agronomic and environmental benefits. However, freeze–thaw cycles (FTC) may increase the availability of water extractable P (WEP) from damaged plant tissues, leading some to question their efficacy as a nutrient BMP due to their potential to release P during snowmelt. The objectives of this study were to experimentally determine the influence of: (1) FTC magnitude (4°C, −4 to 4°C, –18 to 4°C, and –18 to 10°C), (2) CC species [cereal rye (Secale cereale L.), oilseed radish (Raphanus sativus L. var. oleoferus Metzg Stokes), red clover (Trifolium pratense L.), oat (Avena sativa L.), and hairy vetch (Vicia villosa Roth)], and (3) termination using herbicide on the magnitude of WEP, NH4+, and NO3− release. Shoot tissue clippings underwent five FTC followed by extraction. Large magnitude FTC from –18 to 4 and –18 to 10°C (heavy frost) elevated WEP release, whereas the −4 to 4°C (light frost) treatment did not. Responses varied with plant type, where frost‐intolerant species released more WEP than frost‐tolerant species. In contrast, NH4+, and NO3− release did not increase following FTC. Termination elevated WEP and NH4+ release across all temperature treatments. The use of CC as a nutrient BMP should be used with caution in some regions, but in areas with mild winter climates, growing frost tolerant species without termination may reduce the risk of P leaching from vegetation in winter and early spring.

In temperate climates, corn (Zea mays L.) is often harvested too late for sufficient cover crop growth to meet grower objectives. This study was conducted to evaluate intersowing into standing corn in terms of cover crop establishment and growth and the impact on corn yield. Three experiments were conducted from 2009 to 2011 in southwestern Ontario sweet and hybrid seed corn production systems to assess timing of cover crop intersowing, utility of alfalfa (Medicago sativa L.) and 17 other cover crops species or multispecies mixes. In all 22 locations over 3 yr, corn yield was not affected in any of the three cover crop experiments. Sweet corn cover crop treatments exhibited poor stands of limited growth (<1% ground coverage) at corn harvest, attributed to sweet corn canopy closure. At seed corn harvest, early‐sown (corn V4–V6) cover crops accumulated 1116 kg ha−1 dry biomass and 42.4 kg N ha−1, which were 33% greater than the late‐sown (V10–V12) treatments. A lack of a cover crop effect compared with the no‐cover‐crop treatment in soil mineral N and corn yield indicates little N competition. Hairy vetch (Vicia villosa Roth), oilseed radish (OSR, Raphanus sativus L. var. oleoferus Stokes) and three of six cover crop blends were the only treatments to accumulate dry biomass over 1000 kg ha−1 by corn harvest. Of the cover crops evaluated, interseeding into hybrid seed corn production systems appears to be of little risk to yield and can provide ground cover during postharvest fallow periods.

Vegetables are important horticultural commodities with high farm gate values and nutritional quality. For many vegetables, growers apply large amounts of N fertilizer ([200 kg N ha -1 ) to increase yield and profits, but such high N fertilizer applications can pose a significant threat for N loss and environmental contamination via denitrification, volatilization, leaching, runoff, and erosion. Nitrogen losses can reduce air and water quality by contributing to greenhouse gas emissions, ground-level ozone and particulate matter production, ground and surface water contamination, and eutrophication. The processes governing N loss include a complex of biological, physical, and chemical factors, which are impacted by management practices, climatic conditions and soil properties. Therefore, we reviewed and evaluated various management practices for minimizing N loss in N-intensive vegetable production within a temperate climate. Most soil nutrient management practices have focused on reducing N loss throughout the growing season, but the risk for N loss is very high after harvesting vegetables with low N harvest indices, low C:N ratios, and high quantities of N in crop residues, such as most Brassica oleracea L. crops. Amending soil with organic C material may present a novel strategy for reducing N losses after harvest by 37 %, compared to the typical practice of incorporating N-rich vegetable crop residues. Research must focus on testing new and innovative methods of minimizing post-harvest N loss in intensive horticulture.

Cover crops (CC) have both agronomic and environmental benefits but also have the potential to increase losses of dissolved reactive P after freeze-thaw cycles (FTC). This field study, conducted over one nongrowing season (NGS) in Ontario, Canada, characterized water-extractable P (WEP) content in different CC species and compared observed changes in plant WEP content with changes in P content in soil, surface runoff, and shallow groundwater (5-25 cm). Five plots (0.4 ha) of cereal rye (Secale cereal L.), oilseed radish (Raphanus sativus L. var. oleoferus Metzg Stokes), oat (Avena sativa L.), and hairy vetch (Vicia villosa Roth) were established after winter wheat (Triticum aestivum L.) harvest. Throughout the NGS (October-April), CC shoot tissues and surface soil were routinely sampled for WEP analyses, and groundwater and runoff water samples were collected after rain and snowmelt. Responses to FTC varied among CC species, with P released from frost-intolerant species but not frost-tolerant species. Although CC released P, the top 5 cm of soil contained greater WEP than plants at all times, and the changing WEP content in CC over the NGS was not reflected in soil or water P concentrations. These results suggest that the degree of frost exposure should be considered in the selection of CC species in cold regions; however, in temperate regions with snow cover that insulates the soil surface from heavy frost, P release from vegetation may not lead to increased P loss in runoff.