Degradation of [14C]phosphinothricin (glufosinate) in soil under laboratory conditions: Effects of concentration and soil amendments on 14CO2 production

Summary Degradation of the herbicide phosphinothricin (L-homoalanine-4-yl-(methyl)-phosphinic acid) in a phaeozem was investigated by monitoring the ¹⁴CO₂ release from [1-¹⁴C] and [3,4-¹⁴C]phosphinothricin. The degradation was largely due to microbial activity, since the rate decreased by more than 95% when the soil was sterilized by -radiation. Data obtained with both labels suggested that decarboxylation of phosphinothricin preceded oxidation of its C-atoms 3 and 4, since a metabolite, probably 3-methylphosphinico-propanoic acid, was only labelled when [3,4-¹⁴C]phosphinothricin was used as the substrate. Maximum rates of ¹⁴CO₂ production from both the 1- and 3,4-label positions occurred without a lag phase during the breakdown of phosphinothricin as monitored for a total of 30 days at 5-day intervals. This result indicated that a phosphinothricin-degrading microbial community was already present in the soil. With low concentrations of [1-¹⁴C]phosphinothricin (10.7 mg kg^-1 soil), complete decarboxylation at 25°C was observed within 30 days of incubation, compared to 15.9% ¹⁴CO₂ release from [3,4-¹⁴C]phosphinothricin. Increasing the quantity of the herbicide in the soil (10.7–1372 mg kg^-1) resulted in increased degradation rates, irrespective of whether the herbicide was labelled in the positions 1 or 3 and 4. Addition of glucose and other carbohydrates stimulated ¹⁴CO₂ release while addition of a yeast extract had a negative effect. Glucose stimulation was reversed by ammonium nitrate, suggesting that the microorganisms were using the herbicide as a source of N.     

Biodegradation of metolachlor in a soil perfusion experiment

Summary Degradation of the herbicide metachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] was studied in a soil perfusion system. After 28 days of perfusion, the ¹⁴CO₂ evolved from a Virginia soil (soil A), which had been previously treated with metolachlor (Dual) for 5 years, accounted for 18.4% of the added ¹⁴C-metolachlor, while only 3.5% of the ¹⁴C was liberated as ¹⁴CO₂ from a soil of the same field which had no history of Dual treatment (soil B). No ¹⁴CO₂ was liberated from -irradiated soil A. After incubation, metolachlor constituted almost all the extractable ¹⁴C in sterile soil A, while about 20% of the added ¹⁴C extracted from non-sterile soil A consisted of products of metolachlor; 14.8% was identified as dechlorinated metolachlor. No mineralization occurred in actinomycete-inoculated sterile soil A, but 30% of the added ¹⁴C was recovered in the form of transformation products of metolachlor. Our results demonstrate clearly that microbial activity is responsible for the mineralization of metolachlor, and that degradation is enhanced in herbicide-acclimated soils.     

Detection of microbial groups metabolizing a substrate in soil based on the [l4C] quinone profile

Analysis of [^l4C]respiratory Quinones synthesized in soil for 6 h after spiking with [U-¹⁴C]glucose, [U-^l4C]glycine, and [1,2-^l4C]acetate enabled to fingerprint the microorganisms metabolizing each substrate in soil and to determine the whole structure of the microbial communities at the same time. The [^l4C]- Quinones synthesized from [U-^l4C] glucose were the same as those from [U-^l4C] gIycinc in soil, suggesting that the same microbial groups metabolized glucose and glycine under the given conditions. No [^l4C]quinones from [1,2- ¹⁴C] acetate were detected in soil, indicating that the metabolism of acetate by microorganisms is negligible. The profiles of [^l4C]quinones from [U-^l4C]- glucose were compared between Nagoya University Farm soils subjected to 4 different fertilizing practices. The soils receiving farmyard manure contained [^l4C]menaquinones with highly hydrated isoprenoid units, which indicated the presence of Actinobacteria metabolizing glucose. The soil receiving only chemical fertilizers contained [¹⁴C]ubiquinone with 8 isoprenoid units (Q-8), indicating the presence of beta and gamma subdivisions of Proteobacteria. All the 4 soils were characterized by the high proportions of [¹⁴C] MK-6 and a mixture of [^l4C]MK-8(H₄) and [^l4C]MK-9. The Q-9 and Q-10(H₂) indicators of fungi, were not labeled under most of the conditions.     

Effects of acid rain on leaf-litter decomposition in a beech forest on calcareous soil

Summary The effects of simulated acid rain on litter decomposition in a calcareous soil (pH_{H 2 O} 5.8) were studied. Litterbags (45 m and 1 mm mesh size) containing freshly fallen beech leaf litter were exposed to different concentrations of acid in a beech forest on limestone (Göttinger Wald. Germany) for 1 year. Loss of C, the ash content, and CO₂–C production were measured at the end of the experiment. Further tests measured the ability of the litter-colonizing microflora to metabolize ¹⁴C-labelled beech leaf litter and hyphae. The simulated acid rain strongly reduced CO₂–C and ¹⁴CO₂–C production in the litter. This depression in production was very strong when the input of protons was 1.5 times greater than the normal acid deposition, but comparatively low when the input was 32 times greater. acid deposition may thus cause a very strong accumulation of primary and secondary C compounds in the litter layer of base-rich soils, even with a moderate increase in proton input. The presence of mesofauna significantly reduced the ability of the acid rain to inhibit C mineralization. The ash content to the 1-mm litterbags indicated that this was largely due to transport of base-rich mineral soil into the litter.     

Interactive effects of biochar and polyacrylamide on decomposition of maize rhizodeposits: implications from <Superscript>14</Superscript>C labeling and microbial metabolic quotient

Purpose

The applications of biochar (BC) and polyacrylamide (PAM) may have interactive effects on carbon (C) dynamics and sequestration for improving the soil quality and achieving sustainable agriculture. Relative to BC and PAM, rhizodeposits act as C and energy source for microorganisms and may change the mineralization dynamics of soil organic matter (SOM). No attempt has been made to assess the effects of BC, anionic PAM, or their combination on the decomposition of different aged ¹⁴C-labeled rhizodeposits. The objective of this study was to investigate the effects of the treatments mentioned above on the decomposition of different aged ¹⁴C-labeled maize rhizodeposits.

Materials and methods

biochar (BC) at 10 Mg ha^?1 or anionic PAM at 80 kg ha^?1 or their combination (BC + PAM) was applied to soils with/without 2-, 4-, 8-, and 16-day-aged ¹⁴C-labeled maize rhizodeposits. After that, the soil was incubated at 22 °C for 46 days.

Results and discussion

After 2 days of incubation, the total CO₂ efflux rates from the soil with rhizodeposits were 1.4–1.8 times higher than those from the soil without rhizodeposits. The cumulative ¹⁴CO₂ efflux (32 % of the ¹⁴C input) was maximal for the soil containing 2-day-aged ¹⁴C-labeled rhizodeposits. Consequently, 2-day-aged rhizodeposits were more easily and rapidly decomposed than the older rhizodeposits. However, no differences in the total respired ¹⁴CO₂ from rhizodeposits were observed at the end of the incubation. Incorporation of ¹⁴C into microbial biomass and 66–85 % of the ¹⁴C input remained in the soil after 46 days indicated that neither the age of ¹⁴C-labeled rhizodeposits nor BC, PAM, or BC + PAM changed microbial utilization of rhizodeposits.

Conclusions

Applying BC or BC + PAM to soil exerted only minor effects on the decomposition of rhizodeposits. The contribution of rhizodeposits to CO₂ efflux from soil and MBC depends on their age as young rhizodeposits contain more labile C, which is easily available for microbial uptake and utilization.

     

Short-term partitioning of <Superscript>14</Superscript>C-[U]-glucose in the soil microbial pool under varied aeration status

The effect of soil aeration status on carbon partitioning of a labelled organic substrate (¹⁴C-[U]-glucose) into CO₂, microbial biomass, and extra-cellular metabolites is described. The soil was incubated in a continuous flow incubation apparatus under four different aeration conditions: (1) permanently aerobic, (2) permanently anaerobic, (3) shifted from anaerobic to aerobic, and (4) shifted from aerobic to anaerobic. The soil was pre-incubated for 10 days either under aerobic or under anaerobic conditions. Afterwards, glucose was added (315 g C g^–1) and the soils were incubated for 72 h according to four treatments: aerobic or anaerobic conditions maintained, aerobic conditions shifted to anaerobic conditions and anaerobic conditions shifted to aerobic conditions. Carbon partitioning was measured 0, 8, 16, 24, 48 and 72 h after the glucose addition. In permanently aerobic conditions, the largest part of the consumed glucose was built into microbial biomass (72%), much less was mineralised to CO₂ (27%), and only a negligible portion was transformed to soluble extra-cellular metabolites. Microbial metabolism was strongly inhibited when aeration conditions were changed from aerobic to anaerobic, with only about 35% of the added glucose consumed during the incubation. The consumed glucose was transformed proportionally to microbial biomass and CO₂. In permanently anaerobic conditions, 42% of the consumed glucose was transformed into microbial biomass, 30% to CO₂, and 28% to extra-cellular metabolites. After a shift of anaerobic to aerobic conditions, microbial metabolism was not suppressed and the consumed glucose was transformed mainly to microbial biomass (75%) and CO₂ (23%). Concomitant mineralisation of soil organic carbon was always lower in anaerobic than in aerobic conditions.     

Transformation of [14C] and [35S]labeled lignosulfonates during soil incubation

[¹⁴C] and [³⁵S]labeled lignosulfonates (LS) or [¹⁴C]labeled coniferyl alcohol dehydropolymer (DHP) were aerobically incubated in soil for 17 weeks. Respiratory ¹⁴CO₂ was compared with that from DHP or that from [U¹⁴C]cellulose. Less CO₂ was released from ring and side chain carbons of LS than from DHP, though similar amounts of CO₂ were released from the methoxyl groups of both compounds. After incubation, the soil samples were exhaustively extracted with water and then with a sodium pyrophosphate-NaOH solution. The water solubility of the originally completely-soluble LS carbons was greatly decreased by incubation, and a large portion of the extracted ³⁵S was detected as sulfate. The pyrophosphate extract was separated into humic and fulvie acids. The humic acid from soils incubated with LS contained low ³⁵S activity and a similar ¹⁴C activity to that from soils incubated with DHP. The fulvic acid from the soils incubated with LS contained higher amounts of ¹⁴C (and ³⁵S) than that of the soils incubated with DHP. More side chain ¹⁴C activity than other ¹⁴C activity was found in both, the water extract and the fulvic acid from soils incubated with LS. The high ³⁵S together with the high side chain ¹⁴C activity probably indicates an elimination of the side chain carbons together with sulfonic acid groups. Anaerobic incubation of soil with LS or DHP promoted breakdown and incorporation of LS and DHP into humus much less than aerobic incubation. The possible reduction in potential pollution from lignosulfonates due to the observed transformations in soil are discussed.     

Factors influencing the loss of organic carbon from wheat roots

Wheat plants were grown in an atmosphere containing ¹⁴CO₂ at temperatures of 10°C or 18°C for periods from 3–8 weeks. The plant roots were maintained under sterile or non-sterile conditions in soil contained in sealed pots which were flushed to displace respired ¹⁴CO₂. The ¹⁴C content of the shoots, roots and soil was measured at harvest. The loss of ¹⁴C from the roots, expressed either in terms of total ¹⁴C recovered from the pots or ¹⁴C translocated to the roots, ranged from 14.3–22.6%, mean 17.3% or 29.2–44.4%, mean 39.2%, respectively. The presence of soil microorganisms significantly increased ¹⁴CO₂ release from the rhizosphere but had no effect on the ¹⁴C content of the soil. Fractionation of 6 m HC1 hydrolysates from sterile and non-sterile soils showed the presence in all soils of material behaving as neutral sugars and amino acids, in quantities representing 5.9–9.2% and 13.4–17.2% of the soil ¹⁴C content for the sugar and amino acid fractions respectively. It is proposed that a major loss of root carbon resulted from autolysis of the root cortex. Root lysis was increased by soil microorganisms, apparently without penetration of the plant cell walls.     

Effect of clipping and shading on C allocation and fluxes in soil under ryegrass and alfalfa estimated by 14C labelling

Photosynthesis of higher plants drives carbon (C) allocation below-ground and controls the supply of assimilates to roots and to rhizosphere microorganisms. To investigate the effect of limited photosynthesis on C allocation, redistribution and reutilization in plant and soil microorganisms, perennial grass Lolium perenne and legume Medicago sativa were clipped or shaded. Plants were labelled with three ¹⁴C pulses to trace allocation and reutilization of C assimilated before clipping or shading. Five days after the last ¹⁴C pulse, plants were clipped or shaded and the total CO₂ and ¹⁴CO₂ efflux from the soil was measured. ¹⁴C in above- and below-ground plant biomass and bulk soil, rhizosphere soil and microorganisms was determined 10 days after clipping or shading.After clipping, 2% of the total assimilated ¹⁴C originating mainly from root reserves were detected in the newly grown shoots. This corresponded to a translocation of 5 and 8% of total ¹⁴C from reserve organs to new shoots of L. perenne and M. sativa, respectively. The total CO₂ efflux from soil decreased after shading of both plant species, whereas after clipping, this was only true for L. perenne. The ¹⁴CO₂ efflux from soil did not change after clipping of both species. An increased ¹⁴CO₂ efflux from soil under shading for both plants indicated that lower assimilation was compensated by higher utilization of the reserve C for root and rhizomicrobial respiration.We conclude that C stored in roots is an important factor for plant recovery after limiting photosynthesis. This stored C is important for shoot regrowth after clipping, whereas after shading, it is utilized mainly for maintenance of root respiration. Based on these results as well as on a review of several studies on C reutilization for regrowth after clipping, we conclude that because of the high energy demand for nitrogen fixation, legumes use a higher portion (9–10%) of stored C for regrowth compared to grasses (5–7%). The effects of limited photosynthesis were of minor importance for the exudation of the reserve C and thus, have no effect on the uptake of this C by microorganisms.     

Availability of soil nitrogen and phosphorus under elevated [CO2] and temperature in the Taihu Lake region,China

Global climate change affects the availability of soil nutrients, thereby influencing crop productivity. This study examined the effects of elevated [CO₂] and temperature on the availability of soil N and P in a paddy field in the Taihu Lake region, China. Winter wheat (Triticum aestivum L.) was planted at two levels of atmospheric [CO₂] (375 μmol L^–1 ambient; 575 μmol L^–1 elevated) and two temperature levels (ambient; ambient + 2°C). The results were as follows: Compared to ambient, the interaction effects of elevated [CO₂] and temperature significantly decreased soil NH$ _4^+ $ ‐N contents by 20.3%, 20.6%, and 18.7% in the jointing, heading, and ripening stages (p < 0.05), respectively, while soil NO₃^–‐N content had no clear variation trend under different [CO₂] and temperature conditions. Elevated [CO₂] significantly increased soil available P content by 14.3% in the jointing stage, and elevated temperature significantly decreased soil available P content by 18.8% in the jointing stage. Compared with ambient [CO₂], elevated [CO₂] significantly increased wheat biomass in jointing and heading stages (p < 0.05). The positive effect of elevated [CO₂] on wheat biomass was more significant at ambient temperature (AT) than at elevated temperature (ET) in the middle and late plant growth stages. These results explained that the availability of soil N and P varied under elevated [CO₂] and temperature conditions. The application of N and P should be adjusted to meet the need of wheat plants after the wintering stage.     

The degradation of 14C-labelled phosphatidyl choline in soil

When phosphatidyl [N-methyl-¹⁴CO]choline or phosphatidyl choline di[l-¹⁴C]palmitoyl were incubated in a low phosphorus status soil there was an early and rapid release of CO₂ and a concurrent increase in NaHCO₃-extractable inorganic phosphorus, indicating mineralization of the added organic phosphorus. Mineraiization slowed dramatically and by 20 days only 50% of the carbon from the molecule was accounted for as microbial biomass or respiration. The rates of release of ¹⁴CO₂ from the two labelled substrates indicated that ¹⁴CO₂ measured as respiration initially arose more swiftly from the carbon portion of the molecules with easiest access to enzymic degradation.     

Carbon turnover in the rhizosphere under continuous plant labeling with <Superscript>13</Superscript>CO<Subscript>2</Subscript>: Partitioning of root,microbial, and rhizomicrobial respiration

The input of labeled C into the pool of soil organic matter, the CO₂ fluxes from the soil, and the contribution of root and microbial respiration to the CO₂ emission were studied in a greenhouse experiment with continuous labeling of oat plants with ¹³CO₂ using the method of the natural ¹³C abundance in the air. The carbon of the microbial biomass composed 56 and 39% of the total amounts of ¹³C photoassimilates in the rhizosphere and in the bulk soil, respectively. The contribution of root respiration to the CO₂ emission from the soil reached 61–92%, including 4–23% of the rhizomicrobial respiration. The contribution of the microbial respiration to the total CO₂ emission from the soil varied from 8 to 39%. The soil organic matter served as the major carbon-containing substrate for microorganisms in the bulk soil and in the rhizosphere: 81–91% of the total amount of carbon involved in the microbial metabolism was derived from the soil organic matter.     

Formation and degradation of soil-bound [14C] fenitrothion residues in two agricultural soils

The formation of non-extractable residues of [¹⁴C]fenitrothion and their degradation in a black earth and a red yellow podzolic soil was studied. After a 65-day incubation, non-extractable radioactivity represented 73.7 and 59.35% of the applied radioactivity in the black earth and podzolic soil respectively, while evolved ¹⁴CO₂ accounted for only 9.4 and 12.7%. The effects of various amendments and treatments on the degradation of the non-extractable residues to ¹⁴CO₂ was studied. All of the amendments produced a priming effect and significantly increased the formation of ¹⁴CO₂ in both soils. The addition of unlabelled fenitrothion and 3-methyl-4-nitrophenol to the black earth and 3-methyl-4-nitrophenol to the podzolic soil produced the greatest increases in ¹⁴CO₂ evolution. The evidence presented suggests that a part of the unextractable residue is either fenitrothion or 3-methyl-4-nitrophenol.     

Biomineralization of atrazine and analysis of 16S rRNA and catabolic genes of atrazine degraders in a former pesticide mixing site and a machinery washing area

Purpose

The purpose of this study was to determine the first-order rate constants and half-lives of aerobic and anaerobic biomineralization of atrazine in soil samples from an agricultural farm site that had been previously used for mixing pesticide formulations and washing application equipment. Atrazine catabolic genes and atrazine-degrading bacteria in the soil samples were analyzed by molecular methods.

Materials and methods

Biomineralization of atrazine was measured in soil samples with a [U-ring-¹⁴C]-atrazine biometer technique in soil samples. Enrichment cultures growing with atrazine were derived from soil samples and they were analyzed for bacterial diversity by constructing 16S rDNA clone libraries and sequencing. Bacterial isolates were also obtained and they were screened for atrazine catabolic genes.

Results and discussion

The soils contained active atrazine-metabolizing microbial communities and both aerobic and anaerobic biomineralization of [U-ring-¹⁴C]-atrazine to ¹⁴CO₂ was demonstrated. In contrast to aerobic incubations, anaerobic biometers displayed considerable differences in the kinetics of atrazine mineralization between duplicates. Sequence analysis of 16S rDNA clone libraries constructed from the enrichment cultures revealed a preponderance of Variovorax spp. (51 %) and Schlesneria (16 %). Analysis of 16S rRNA gene sequences from pure cultures (n?=?12) isolated from enrichment cultures yielded almost exclusively Arthrobacter spp. (83 %; 10/12 isolates). PCR screening of pure culture isolates for atrazine catabolic genes detected atzB, atzC, trzD, trzN, and possibly atzA. The presence of a complete metabolic pathway was not demonstrated by the amplification of catabolic genes among these isolates.

Conclusions

The soils contained active atrazine-metabolizing microbial communities. The anaerobic biometer data showed variable response of atrazine biomineralization to external electron acceptor conditions. Partial pathways are inevitable in soil microbial communities, with metabolites linking into other catabolic and assimilative pathways of carbon and nitrogen. There was no evidence for the complete set of functional genes of the known pathways of atrazine biomineralization among the isolates.

     

Interactions between 14C-labelled atrazine and the soil microbial biomass in relation to herbicide degradation

A laboratory incubation experiment was set up to determine the effects of atrazine herbicide on the size and activity of the soil microbial biomass. This experiment was of a factorial design (0, 5, and 50 g g^–1 soil of non-labelled atrazine and 6.6×10³ Bq g^–1 soil of ¹⁴C-labelled atrazine) x (0, 20, and 100 g g^–1 soil of urea-N) x (pasture or arable soil with a previous history of atrazine application). Microbial biomass, measured by substrate-induced respiration and the fumigation-incubation method, basal respiration, incorporation of ¹⁴C into the microbial biomass, degradation of atrazine, and ¹⁴C remaining in soil were monitored over 81 days. The amount of microbial biomass was unaffected by atrazine although atrazine caused a significant enhancement of CO₂ release in the non-fumigated controls. Generally, the amounts of atrazine incorporated into the microbial biomass were negligible, indicating that microbial incorporation of C from atrazine is not an important mechanism of herbicide breakdown. Depending on the type of soil and the rate of atrazine application, 18–65% of atrazine was degraded by the end of the experiment. Although the pasture soil had twice the amount of microbial biomass as the arable soil, and the addition of urea approximately doubled the microbial biomass, this did not significantly enhance the degradation of atrazine. This suggests that degradation of atrazine is largely independent of the size of the microbial biomass and suggests that other factors (e.g., solubility, chemical hydrolysis) regulate atrazine breakdown. A separate experiment conducted to determine total amounts of ¹⁴C-labelled atrazine converted into CO₂ by pasture and arable soils showed that less than 25% of the added ¹⁴C-labelled atrazine was oxidised to ¹⁴CO₂ during a 15-week period. The rate of degradation was significantly greater in the arable soil at 24%, compared to 18% in the pasture soil. This indicates that soil microbes with previous exposure to atrazine can degrade the applied atrazine at a faster rate.     

CO2 efflux by rapid decomposition of low molecular organic substances in soils

Decomposition rates of the [2-¹⁴C]-glucose and [2-¹⁴C]-glycine in four different soils of the long-term field trial of Moscow were investigated in a 3-months laboratory experiment in which ¹⁴CO₂ respiration was measured. A model with three decomposition components and two distribution parameters was developed and validated with the data of the experiment. The decay rate constants of free [2-¹⁴C]-glucose (4–32 day^-1) were slower than those of [2-¹⁴C]-glycine (16–44 day^-1). The calculated use efficiency for microbial biosynthesis of the second carbon atom was 47% for glucose and 31% for glycine. The potential half-life of labelled carbon in the microbial soil biomass ranged from 0.6 to 4.4 days, depending on the soil type and the initial amount of added substrate. The calculated total utilisation of carbon by the soil biomass from glycine was about 2–5 times lower than that of glucose.The modelled ¹⁴C incorporation into the microbial soil biomass reached its maximum on the first day of the incubation experiment and did not exceed 22% of the ¹⁴C input. Both of the investigated substances decomposed most rapidly in the soil samples from sites that have not being fertilised with organic or mineral fertilisers during an 81-years period.     

A simple chamber technique for the in situ labelling of pasture sward with carbon (14C)

Abstract

Information on carbon (C) flows and transformations in the rhizosphere provides a basis for understanding the functioning of the system. However, the sophisticated growth cabinet facilities required for collecting quantitative data, with ¹⁴C labelling, generally limit their application under field conditions. We determined the feasibility of ‘pulse‐labelling’ pasture swards with ¹⁴C [exposing the plants to a single large ¹⁴C‐carbon dioxide (CO₂) pulse] to monitor C transformations under field conditions using a simple chamber modified to form a sealed hemisphere over an area of pasture. The ¹⁴C‐CO₂ was introduced into the hemisphere to ¹⁴C label the plant material. Assimilation of ¹⁴C‐CO₂ was checked by taking samples of the chamber atmosphere. Any leakage of ¹⁴C‐CO₂ from the chamber was also checked by taking air samples from around and outside the chamber during the assimilation period. The chamber was subsequently removed, and the pasture was opened to natural conditions. Cores were taken periodically from the treated area. Herbage, roots and soils were separated and analyzed for ¹⁴C. Incorporation of ¹⁴C‐CO₂ into the pasture sward was rapid and the variability was non‐limiting. Up to 78% of the calculated ¹⁴C‐CO₂ produced in the syringe and injected into the chamber could be accounted for in the plant/soil system four hours after labelling. The fate of the ¹⁴C label was monitored after an allocation period of 4 hour to 35 days in the plant/soil system using well established methods of analysis. This simple chamber technique appears to be useful for studying C transfers through the pasture plant/soil system and for understanding C dynamics in the field.     

Decomposition de corps microbiens dans des sols fumiges au chloroforme: Effets du type de sol et de microorganisme

Five microbial species (Aspergillus flavus, Trichoderma viride, Streptomyces sp., Arthrobacter sp., Achromobacter liquefaciens) were cultivated in liquid media containing ¹⁴C-labelled glucose. The decomposition of these microorganisms was recorded in four different soils after chloroform fumigation by a technique related to that proposed by Jenkinson and Powlson, to determine the mineralization rate of microbial organic matter (K_c coefficient). Three treatments were used: untreated soil, fumigated soil alone and fumigated soil supplied with ¹⁴C-labelled cells. Total evolved CO₂ and ¹⁴CO₂ were measured after 7 and 14 days at 28°C.The labelled microorganisms enabled the calculation of mineralization rate K_c (K_c = mineralized microbial carbon/supplied microbial carbon). The extent of mineralization of labelled microbial carbon depended on the type of soil and on the microbial species. Statistical analysis of results at 7 days showed that 58% of the variance is taken in account by the soil effect and 32% by the microorganism effect. Between 35 and 49% of the supplied microbial C was mineralized in 7 days according to the soil type and the species of microorganism. Our results confirmed that the average value for K_c = 0.41 is acceptable, but K_c variability according to soil type must be considered.The priming effect on organic C and native microbial biomass mineralization, due to microbial carbon addition was obtained by comparison between the amount of non-labelled CO₂-C produced by fumigated soils with or without added labelled microorganisms: this priming effect was generally negligible.These results indicate that the major portion of the error of microbial biomass measurement comes from the K_c estimation.     

Fate of bound methyl parathion residues in soils as affected by agronomic practices

The fate of [ring-¹⁴C]methyl parathion in a silt loam soil was monitored during a 49-day incubation. After this period, 54% of the initial ¹⁴C remained in the soil; of this, 13% was soxhletextractable with methanol and 87% was bound residue. Soils were then treated with inorganic and organic amendments and incubated for an additional 70 days. Release of methyl parathion bound residues could not be demonstrated, but both bound and extractable ¹⁴C were mineralized to ¹⁴CO₂, CO₂ was evolved slowly and continuously by the controls and where soil was amended with H₂SO₄, (NH₄)₂SO₄, NH₄OH, chitin, oat seedlings or oat straw. Glucose and asparagine caused higher rates of ¹⁴CO₂ production. HgCl₂ gave very high initial rates of ¹⁴CO₂ loss; the rate declined to that of the control only after 9–10 weeks. The lime treatment exceeded the controls after 1 week, declining only slightly with time. The effects of sewage sludge and dairy manure were similar to the controls except that: sludge caused a very high initial release of ¹⁴CO₂, and both treatments gave an unaccountable loss of ¹⁴C, perhaps as ¹⁴CH₄ resulting from the formation of anaerobic conditions. By 70 days, amounts of extractable ¹⁴C and bound ¹⁴C had both declined twice as rapidly in certain soils as in unamended controls.Studies carried out with soxhlet-extracted soils, containing only bound residues, indicated that the soil microflora able to mineralize bound residues without any appreciable buildup of ¹⁴C activity in the extractable phase.