Soil, metabolism

All soil metabolic proce.sses are driven by enzymes. The main sources of enzymes in soil are roots, animals, and microorganisms the last are considered to be the most important (49). Once enzymes are produced and excreted from microbial cells or from root cells, they face harsh conditions most may be rapidly decomposed by organisms (50), part may be adsorbed onto soil organomineral colloids and possibly protected against microbial degradation (51), and a minor portion may stand active in soil solution (52). The fraction of extracellular enzyme activity of soil, which is not denaturated and/or inactivated through interactions with soil fabric (51), is called naturally stabilized or immobilized. Moreover, it has been hypothesized that immobilized enzymes have a peculiar behavior, for they might not require cofactors for their catalysis. 

In the soil metabolism study using [ C]pyriminobac-methyl, most of the residual pyriminobac-methyl in soil was extracted by reflux extraction with acetone. ... 

Pentachlorophenol applied to beech forest soils every 2 months for 2 years at the rate of 1.0 g/m2 markedly reduced populations of soil organisms. At 5.0 g/m2, it drastically reduced most of the soil animal species and also the microflora (Zietz et al. 1987). Reduction of the soil metabolism by PCP retards decomposition and affects the overall nutrient balance of forest ecosystems (Zietz et al. 1987). Pentachlorophenol is more toxic to earthworms in soils with comparatively low levels of organic materials. The LC50 (14-day) value for Lumbricus rubellus was 1094 mg PCP/kg DW soils with 6.1% organic matter, and 883 mg/kg DW soils with 3.7% organic matter (Van Gestel and Ma 1988). The earthworm Eisenia fetida andrei is more sensitive than Lumbricus rubellus ... 

In contrast to hydrolysis and photolysis, an enantioselective and/or regioselective degradation of pyrethroids is expected since various kinds of enzymes in microbes participate in their metabolism in soil and sediment. The diastereomers should be separated by using a chiral GC or HPLC to examine the fate of each isomer [85]. Sakata et al. [86] have conducted the first extensive aerobic soil metabolism study... 

Lee PW, Powell WR, Steams SM, McConnell OJ (1987) Comparative aerobic soil metabolism of fenvalerate isomers. J Agric Food Chem 35 384-387... 

In a static model ecosystem, several amino derivatives of fenitrothion, probably derived from the soil metabolism, were demonstrated in carp tissues, together with the nitro-containing compounds. The concentration of fenitrothion in carp, snails, daphnids and algae decreased with time, although its bioaccumulation ratio relative to the concentration in water tended to increase gradually in snails, daphnids and algae, presumably due to lower metabolic activity and/or slow excretion. 

One study (76) screened about 100 soils for bloactlvity and chose one of the more bioactlve to study the soil metabolism of EDB. Approximate half-lives were calculated from the results of this study and, even In a single agricultural soil, they varied from 1.5-18 weeks In different soil aliquots of that soil. 

These data were measured at or extrapolated to ambient temperature and pH values. The data are discussed in the text. NA = not available. b/ Kq = soil water distribution coefficient (K ) divided by the organic carbon content of the soil, cj Whenever possible, half-life for soil dissipation is derived from the field data half-lives described in the text rather than lab data. As such, it may not represent a true first-order process. Value has been estimated from the equation in ref. 20. e/ Hydrolysis of total residues (aldicarb + sulfoxide + sulfone). pK for p -phthalic acid is 3.5. The chlorine atoms of DCPA should lower the pK to about 2. Conditions optimized for soil metabolism. 

Chen Y., J.RN. Rosazza, C.P. Reese, H-Y. Chang, M.A. Nowakowski, and J.P. Kiphnger (1997). Microbial models of soil metabolism Biotransformations of danofloxacin. Journal of Industrial Microbiology and Biotechnology 19 378-384. 

A little-known source of biodegradation rates of pesticides is data developed by the California Department of Food and Agriculture (CDFA). They estimated aerobic and anaerobic soil metabolism half-lives from open scientific literature and studies submitted to CDFA from chemical companies in compliance with the data call-in requirements of the Pesticide Contamination Prevention Act. Table 12.15 tabulates these data. 

Several processes may play a role in the environmental dissipation of -triazine herbicides. Dissipation processes can include microbial or chemical degradation in soil metabolism or conjugation in plants photodegradation in air, water, and on soil and plant surfaces and volatilization and transport mechanisms. This chapter will address photolytic degradation and abiotic hydrolysis of the currently used triazine herbicides, the triazinone herbicides (metribuzin and metamitron), and the triazinedione herbicide hexazinone. 

Soil. Metabolized oxidatively under paddy soil conditions C02 was the major metabolite. The calculated half-lives ranged from several weeks to several months, resp. Low mobility... 

Animals. Rapidly absorbed, metabolized via phase II conjugation and eliminated Soil. Aerobic soil metabolism DT50 336-1100 days field DT50 23-268 days... 

Plants. In apples, grapes, rice, and sugar beet, the major component is unchanged tebufenozide. Small amounts of metabolites result from oxidation of the alkyl substituents of the aromatic ring, primarily at the benzylic positions Soil. Metabolic DTJ0 in soil 7-66 days DT50 for field dissipation 4—53 days. No mobility below 30 cm... 

Grass weed Soil. Metabolizes into Chloroaniline... 

Soil. The most significant mechanisms are indirect photolysis and soil metabolism, together with chemical hydrolysis... 

Soil. Metabolism occurs by loss of the vinyl group, cleavage of the 5-membered ring and eventual formation of 3, 5,-dichloroaniline. Soil degradation takes place with half-lives of several weeks, and mainly leads to the formation of bound residues... 

Thin-layer radiochromatography (radio-TLC) is widely applied for a variety of environmental studies involving radiolabeled pesticides, such as plant uptake from soil, bioaccumulation in fish, dissipation from soil, metabolism in soil, plants, and fish, and environmental fate. The determination of the lipophiUdty of pesticides is important because their bioaccumulation and tendency for degradation and biotransformation are related to lipid solubility. TLC has advantages for lipohilicity studies compared to traditional partition coefficient measurement in an octanol-water system. 

As said above, plant root chemistry may also influence deeply alpine soil microorganism s biomass. It turns out that the particular chemical composition of exudates is a strong selective force in favour of bacteria that can catabolize particular compounds. Plants support heterotrophic microorganisms by way of rhizodeposition of root exudates and litter from dead tissue that include phenolic acids, flavonoids, terpenoids, carbohydrates, hydroxamic acids, aminoacids, denatured protein from dying root cells, CO2, and ethylene (Wardle, 1992). In certain plants, as much as 20-30% of fixed carbon may be lost as rhizodeposition (Lynch and Whipps, 1990). Most of these compounds enter the soil nutrient cycle by way of the soil microbiota, giving rise to competition between the myriad species living there, from microarthropods and nematodes to mycorrhiza and bacteria, for these resources (e.g. Hoover and Crossley, 1995). There is evidence that root phenolic exudates are metabolized preferentially by some soil microbes, while the same compounds are toxic to others. Phenolic acids usually occur in small concentration in soil chiefly because of soil metabolism while adsorption in clay and other soil particles plays a minor role (Bliun et al., 1999). However, their phytotoxicity is compounded by synergism between particular mixtures (Blum, 1996). 

Bromethalin is stable to hydrolysis over the pH range of 5-9 when incubated in the dark for up to 30 days. Data from an anaerobic soil metabolism study indicate that bromethalin is relatively stable to... 

The persistence of fenthion in the environment is dependent on several factors, including photolysis, metabolism in plants and insects, and microbial degradation. Estimates of the half-life of fenthion in soil vary from < 1 day in studies cited by the US EPA for aerobic soil metabolism to 3-6 weeks, cited by Extoxnet. Half-lives for aquatic degradation range from 2.9 to 21.1 days for various ocean, river, swamp, or lake aquatic conditions. Sunlight accelerates degradation of fenthion 20-fold in river water and fivefold in seawater. 

Both compounds were field tested at a number of locations. The level of activity observed in the field trials, however, was not sufficient to warrant continued evaluation. Subsequent greenhouse testing suggested that the failure of AC 247,909 to perform in the field may be due to photodecomposition in postemergence tests and to soil metabolism and volatility in preemergence tests. 

Mayo, B.C. (1995b). Piperonyl Butoxide Aerobic Soil Metabolism. Unpublished report no. PBT7/951484. Huntingdon Life Sciences, PO Box 2, Huntingdon, Cambridge PEIS 6ES. Undertaken for the PBO Task Force, Washington DC. USA. 

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