Mineralization of carbon atoms of 14C-2,4-D side chain and degradation ability of bacteria in soil

The time-course of ¹⁴CO₂ formation in chernozem soil samples enriched with 1- or 2-¹⁴C-2, 4-dichlorophenoxyacetic acid (50 μg g g^?1 air-dry soil) was determined during incubation at 28°C. Except for the initial phase of decomposition, when the conversion of carboxyl carbon to ¹⁴CO₂ predominated over that of carbon in position 2, the rates of mineralization of the two carbon atoms of the side chain of the herbicide molecule exhibited no significant difference. The exponential phase of ¹⁴CO₂ evolution lasted from the 3rd to the 21st day of incubation; a semilogarithmic plot of its time dependence was strictly linear. The mineralization activity doubling time in this phase was 89.1 ± 3.6 h with 1-¹⁴CO-2, 4-D and 85.4 ± 5. l h with 2-¹⁴CO-2,4-D. An exponential decrease in mineralization activity was observed after 21 days, probably due to substrate exhaustion. The total proportion of radioactive carbon introduced into the soil in the form of 1- or 2-¹⁴CO-2,4-D and converted into ¹⁴CO₂ during 31 days of incubation was about 33%. Plate counts of bacteria increased during 35 days of incubation from 2.14 × 10⁸ to 2.8 × 10⁸ g^?1. The proportion of bacteria capable of producing ¹⁴CO₂ from the labelled herbicide increased in this period from 4.1 to 86.1%. This increase is probably directly responsible for the immediate onset of mineralization of the herbicide in soil treated previously with it or in soil inoculated with a suspension prepared from a sample previously incubated with the herbicide.     

Effects of 2,4-d on nitrogen fixation and carbon dioxide evolution in a soil microcosm

Two concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D) 1.7 kg ha^?1 and 3.4 kg ha^?1 were applied to oats (Avena sativa L. ‘Orbit’) grown in terrestrial microcosms in a sandy soil. Carbon dioxide evolution and non-symbiotic N₂ fixation (C₂H₂ reduction) were measured weekly. On day 70 of the study, 2,4-D was applied a second time at the same application rates and soil CO₂ evolution and N₂ fixation were measured more frequently. Soil CO₂ evolution over 24 h period was significantly decreased by 40 to 50% at both application rates of 2,4-D. This response lasted less than 7 days. Nitrogen fixation was unaffected by 2,4-D except for an unexplained decrease observed in the 1.7 kg ha^?1 treatment 35 days after 2,4-D application. This effect was not observed on the following sampling date. The second application of 2,4-D failed to produce any significant change in soil CO₂ evolution or N₂ fixation. From these studies we conclude soil microbial populations either degraded or became acclimated to 2,4-D as a result of the initial application and that 2,4-D has no significant adverse effect on N₂ fixation or soil CO₂ evolution.     

Bioavailability of the herbicide 2,4-D formulated with organoclays

Research on organoclays as sorbents of pesticides has shown the usefulness of these materials as pesticide supports to prolong the efficacy of soil-applied pesticides and to reduce the large transport losses that usually affect pesticides applied in an immediately available form. Nevertheless, little information exists on the availability of organoclay-formulated pesticides for bacterial degradation. In this work, laboratory experiments were conducted to determine the adsorption-desorption behavior of two hexadecyltrimethylammonium-treated Arizona montmorillonites (SA-HDTMA₅₀ and SA-HDTMA₁₀₀) for the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D), and to evaluate the ability of these organoclays to slow the release of the herbicide and to reduce herbicide leaching losses as compared to the free (technical) compound. The kinetics of mineralization of free and formulated 2,4-D by adapted bacteria was also determined. Organoclay-based formulations of 2,4-D displayed slow release properties in water and reduced herbicide leaching through soil columns, while maintained a herbicidal efficacy similar to that of the free (technical) 2,4-D. The total amount of ¹⁴C-2,4-D mineralized at the end of the biodegradation experiment (t=130 h) ranged between 30% and 46% of the formulated herbicide, which represented 53-81% of the amount of free 2,4-D mineralized in the same conditions. The release, leaching, and mineralization patterns of the formulated herbicide were found to depend both on the affinity of the organoclay for the herbicide and on the degree of interaction promoted during the preparation of the herbicide-organoclay complex. This suggests the possibility to select diverse preparations to achieve the desired release, leaching and biodegradation behavior.     

Identification of in situ 2,4-dichlorophenoxyacetic acid-degrading soil microorganisms using DNA-stable isotope probing

Stable isotope probing (SIP) was used to investigate the microorganisms responsible for degradation of the herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D) in soil samples. Soils were unamended or amended with either unlabeled 2,4-D or UL(ring) ¹³C-2,4-D. Degradation of 2,4-D was complete after 17 days, whereas little removal (11±3%) was observed in the sterile controls. Terminal restriction fragment length polymorphism (TRFLP) on soil DNA after 17 days indicated a consistent increase in the relative abundance of one fragment (217 bp in Hae III digests) in soils spiked with 2,4-D (both unlabeled and labeled samples) compared to the unamended soils. DNA extracts from labeled and unlabeled 2,4-D amended soils were subject to ultracentrifugation, fractionation of centrifuged samples, followed by TRFLP on each fraction. TRFLP profiles from ultracentrifugation fractions illustrated that the same fragment experienced an increase in buoyant density (BD) in samples spiked with ¹³C-labeled 2,4-D. This increase in DNA BD indicates the organisms represented by this fragment were responsible for uptake and degradation of the herbicide. 16S rRNA sequencing of the heavy, ¹³C-enriched fraction suggests the organisms belong to the β subdivision of Proteobacteria. Herein, SIP facilitated the identification of unique organisms degrading 2,4-D in soil without the need for isolation and provided more direct evidence for a functional role of these organisms than would have been possible with the molecular-based methods alone.     

Spatial variability of 2,4-dichlorophenoxyacetic acid (2,4-D) mineralisation potential at a millimetre scale in soil

We analysed the ability of soil units of millimetre size to mineralise a herbicide, 2,4-D, using incubations of individual aggregates (2-7 mm diameter) and 6×6×6 mm³ cubes dissected from soil cores, under standard conditions. Mineralisation of ¹⁴C-ring labelled 2,4-D was measured using a barite paper trap and a Phosphorimager to record the evolved ¹⁴C-CO₂ from these very small soil samples. We found a large variability of 2,4-D mineralisation potential between aggregate size classes, between individual aggregates of the same size and between the different dissected cubes from a given core. We explained this variability by an uneven distribution of the degrading microorganisms at this scale, and to a lesser extent, an uneven distribution of C, necessary for co-metabolism. Furthermore, we found that in a soil core, the dissected cubes with a large mineralisation potential were not randomly distributed, but rather organised into centimetre sized hot spots.     

Soil microbial communities response to herbicide 2,4-dichlorophenoxyacetic acid butyl ester

2,4-Dichlorophenoxyacetic acid butyl ester (2,4-D butyl ester) is extensively applied for weed control in cultivation fields in China, but its effect on soil microbial community remains obscure. This study investigated the microbial response to 2,4-D butyl ester application at different concentrations (CK, 10, 100 and 1000 μg g^?1) in the soils with two fertility levels, using soil dilution plate method and phospholipid fatty acid (PLFA) analysis. Culturable microorganisms were affected by the herbicide in both soils, particularly at the higher concentration. After treating soil with 100 μg g^?1 herbicide, culturable bacteria and actinomycetes were significantly higher, compared to other treatments. Treatment of soil with 1000 μg g^?1 2,4-D butyl ester caused a decline in culturable microbial counts, with the exception of fungal numbers, which increased over the incubation time. PLFA profiles showed that fatty acids for Gram-negative (GN) bacteria, Gram-positive (GP) bacteria, total bacteria and total fungi, as well as total PLFAs, varied with herbicide concentration for both soil samples. As herbicide concentration increased, the GN/GP ratio decreased dramatically in the two soils. The higher stress level was in the treatments with high concentrations of herbicide (1000 μg g^?1) for both soils. Principal component analysis of PLFAs showed that the addition of 2,4-D butyl ester significantly shifted the microbial community structure in the two soils. These results showed that the herbicide 2,4-D butyl ester might have substantial effects on microbial population and microbial community structure in agricultural soils. In particular, the effects of 2,4-D butyl ester were greater in soil with low organic matter and fertility level than in soil with high organic matter and fertility level.     

Rhizodeposits of Trifolium pratense and Lolium perenne: their comparative effects on 2,4-D mineralization in two contrasting soils

Rhizosphere enhanced biodegradation of organic pollutants has been reported frequently and a stimulatory role for specific components of rhizodeposits postulated. As rhizodeposit composition is a function of plant species and soil type, we compared the effect of Lolium perenne and Trifolium pratense grown in two different soils (a sandy silt loam: pH 4, 2.8% OC, no previous 2,4-D exposure and a silt loam: pH 6.5, 4.3% OC, previous 2,4-D exposure) on the mineralization of the herbicide 2,4-D (2,4-dichlorophenoxyacetic acid). We investigated the relationship of mineralization kinetics to dehydrogenase activity, most probable number of 2,4-D degraders (MPN_2,4-D) and 2,4-D degrader composition (using sequence analysis of the gene encoding α-ketoglutarate/2,4-D dioxygenase (tfdA)). There were significant (P<0.01) plant-soil interaction effects on MPN_2,4-D and 2,4-D mineralization kinetics (e.g. T. pratense rhizodeposits enhanced the maximum mineralization rate by 30% in the acid sandy silt loam soil, but not in the neutral silt loam soil). Differences in mineralization kinetics could not be ascribed to 2,4-D degrader composition as both soils had tfdA sequences which clustered with tfdAs representative of two distinct classes of 2,4-D degrader: canonical R. eutropha JMP134-like and oligotrophic α-proteobacterial-like. Other explanations for the differential rhizodeposit effect between soils and plants (e.g. nutrient competition effects) are discussed. Our findings stress that complexity of soil-plant-microbe interactions in the rhizosphere make the occurrence and extent of rhizosphere-enhanced xenobiotic degradation difficult to predict.     

Sorption of 2,4-D and other phenoxy herbicides to soil, organic matter, and minerals

Purpose

We review 2,4-dichlorophenoxyacetic acid (2,4-D) and other phenoxy herbicide sorption experiments.

Methods

A database with 469 soil–water distribution coefficients K _d (in liters per kilogram) was compiled: 271 coefficients are for the phenoxy herbicide 2,4-D, 9 for 4-(2,4-dichlorophenoxy)butyric acid, 18 for 2-(2,4-dichlorophenoxy)propanoic acid, 109 for 2-methyl-4-chlorophenoxyacetic acid, 5 for 4-(4-chloro-2-methylphenoxy)butanoic acid, and 57 for 2-(4-chloro-2-methylphenoxy)propanoic acid. The following parameters characterizing the soils, solutions, or experimental procedures used in the studies were also compiled if available: solution CaCl₂ concentration, pH, pre-equilibration time, temperature, soil organic carbon content (f _oc), percent sand, silt and clay, oxalate extractable aluminum, oxalate extractable iron (Oxalate Fe), dithionite–citrate–bicarbonate extractable aluminum, dithionite–citrate–bicarbonate extractable iron (DCB Fe), point of zero negative charge, anion exchange capacity, cation exchange capacity, soil type, soil horizon or depth of sampling, and geographic location. K _d data were also compiled characterizing phenoxy herbicide sorption to the following well-defined sorbent materials: quartz, calcite, α-alumina, kaolinite, ferrihydrite, goethite, lepidocrocite, soil humic acid, Fluka humic acid, and Pahokee peat.

Results

The data review suggests that sorption of 2,4-D can be rationalized based on the soil parameters pH, f _oc, Oxalate Fe, and DCB Fe in combination with sorption coefficients measured independently for humic acids and ferrihydrite, and goethite.

Conclusions

Soil organic matter and iron oxides appear to be the most relevant sorbents for phenoxy herbicides. Unfortunately, few authors report Oxalate Fe and DCB Fe data.     

Denitrification in soil: Effects of herbicides

The effects of 20 herbicides on denitrification of nitrate in three soils were studied by determining the effects of 10 and 50μgg^?1 soil of each herbicide on the amounts of nitrate lost and the amounts of nitrite, N₂O and N₂ produced when soil samples were incubated anaerobically after treatment with nitrate. The herbicides used were butylate, EPTC, chlorpropham, propham, diuron, linuron, monuron, siduron, alachlor, trifluralin, 2,4-D amine, 2,4-D ester, atrazine, cyanazine, metribuzin, simazine, dalapon, chloramben, dicamba and dinoseb.None of the herbicides studied significantly affected denitrification of nitrate when applied at the rate of 10 μg g^?1 soil, but dinoseb increased the ratio of N₂ to N₂O in the gaseous products of denitrification when applied at this rate. Butylate, EPTC, diuron, simazine and dalapon had no significant effect on denitrification when applied at the rate of 50μgg^?1 soil, whereas metribuzin and dinoseb enhanced denitrification when applied at this rate. The influence of the other herbicides on denitrification when applied at the rate of 50μgg^?1soil depended on the soil, but all enhanced or inhibited denitrification in at least one soil.     

The millimetre-scale distribution of 2,4-D and its degraders drives the fate of 2,4-D at the soil core scale

The biodegradation of organic compounds in soil is a key process that has major implications for different ecosystem services such as soil fertility, air and water quality, and climate regulation. Due to the complexity of soil, the distributions of organic compounds and microorganisms are heterogeneous on sub-cm scales, and biodegradation is therefore partly controlled by the respective localizations of organic substrates and degraders. If they are not co-localized, transfer processes become crucial for the accessibility and availability of the substrate to degraders. This spatial interaction is still poorly understood, leading to poor predictions of organic compound dynamics in soils. The objectives of this work were to better understand how the mm-scale distribution of a model pesticide, 2,4-dichlorophenoxyacetic acid (2,4-D), and its degraders drives the fate of 2,4-D at the cm soil core scale. We constructed cm-scale soil cores combining sterilized and “natural” soil aggregates in which we controlled the initial distributions of 2,4-D and soil microorganisms with the following spatial distributions: i) a homogeneous distribution of microorganisms and 2,4-D at the core-scale, ii) a co-localized distribution of microorganisms and 2,4-D in a single spot (360 mm³) and iii) a disjoint localization of microorganisms and 2,4-D in 2 soil spots (360 mm³) separated by 2 cm. Two sets of experiments were performed: one used radiolabeled ¹⁴C-2,4-D to study the fate of 2,4-D, and the other used ¹²C-2,4-D to follow the dynamics of degraders. Microcosms were incubated at 20 °C and at field capacity (−31.6 kPa). At the core scale, we followed 2,4-D mineralization over time. On three dates, soil cores with microorganisms and 2,4-D localized in soil spots, were cut out in slices and then in 360 mm³ soil cubes. The individual soil cubes were then independently analysed for extractable and non-extractable ¹⁴C and for degraders (quantitative PCR of tfdA genes). Knowing the initial position of each soil cube allowed us to establish 3D maps of 2,4-D residues and degraders in soil. The results indicated that microorganisms and pesticide localizations in soil are major driving factors of i) pesticide biodegradation, by regulating the accessibility of 2,4-D to degrading microorganisms (by diffusion); and ii) the formation of non-extractable residues (NER). These results also emphasized the dominant role of microorganisms in the formation and localization of biogenic NER at a mm-scale. To conclude, these results demonstrate the importance of considering micro-scale processes to better understand the fate of pesticides and more generally of soil organic substrates at upper scales in soil and suggest that such spatial heterogeneity should not be neglected when predicting the fate of organic compounds in soils.     

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.     

Accumulation of 2,4-D and glyphosate in fish and water hyacinth

At a concentration of approximately one hundredth and one thousandth of the 48-hr LC₅₀ values, the accumulation of radioactivity and relative concentration of 2,4-D and glyphosate in carp and tilapia were studied by using labelled and unlabelled chemicals. About 83 (at a concentration of 0.5 ppm) and 91% (of 0.05 ppm) of the radioactive matter remained in the water until 14 days after ¹⁴C-2,4-D amended, but only 17.2% of glyphosate remained in the water with 0.05 ppm concentration of glyphosate. No significant variation was shown in the accumulation of the concentration of herbicide in fish from 2 to 7 d. Although glyphosate disappeared within 3 d in water under sunlight, the radiochemicals in the water hyacinth remained constant to the 14th d.     

Dynamics of soil microbial populations involved in 2,4-D biodegradation revealed by FAME-based Stable Isotope Probing

Stable Isotope Probing (SIP) is a powerful tool for analysing the fate of pesticides in soil. Together with FAME (Fatty Acid Methyl Esters), it can help identify biodegradation pathways and recycling into the microbial biomass. The fate of ring-labelled ¹³C-2,4-dichlorophenoxyacetic acid or 2,4-D (C_2,4-D) was determined in soil during a 6-month incubation. The distribution of ¹³C among the microbial biomass, the CO₂ respired, the water, methanol and dichloromethane soluble fractions, and the residual non-extracted bulk soil was measured. Molecular analyses were carried out on the lipid and the non-extractable fractions. After 8 days, about half of the initial amount of C_2,4-D was mineralised; the other half remained in soil as non-extractable residues (NER). C_2,4-D continued to be mineralised, suggesting that NER were still bioavailable. Analysis of C_2,4-D-enriched FAME contained in the lipid fraction suggested that a succession of microbial populations was involved in 2,4-D biodegradation. This is possibly due to the change of 2,4-D availability. The C_2,4-D yield coefficient and degrader diversity evolved during the incubation, providing corroboratory evidence that different physiological groups were active during the incubation. The ¹³C-labelled microbial community was always less diverse than the total community, even at the end of the incubation, suggesting that the cross-feeding community is also a specific part of the total community. This work shows that molecular analysis of ¹³C-labelled pesticides is a useful tool for understanding both chemical and biological aspects of their fate in soil.     

Effect of some herbicides on microbial activity in soil

The effect of 25 herbicides and herbicide combinations, in amounts comparable to those used in agriculture, on microbial activity in two soil types was determined in the laboratory. Herbicides did not affect respiration, assayed by CO₂ evolution and dehydrogenase activity, in either silty clay loam or loamy sand. Organic matter decomposition, determined by the amount of CO₂ evolved and inorganic N formed from decomposing alfalfa tissue, was also not affected. Alteration in soil pH or moisture content did not affect herbicide action. Addition of herbicides 3 weeks before amendment, or fertilizer application, also did not influence herbicide activity. Selected herbicides (trifluralin, linuron, dinoseb) at concentrations 100-fold higher than the recommended rates did not affect alfalfa decomposition. Solubilization of Ca₃(PO₄)₂ in soil was not affected by herbicides. S oxidation to SO^2?₄ in soil, however, was increased by most herbicides. In silty clay loam, 18 of the 25 herbicides and herbicide combinations increased S oxidation almost up to 3-fold. Results in loamy sand were similar. Dinoseb effectively reduced the algal population in loamy sand by more than 90%. Trifluralin, linuron, and metribuzin did not inhibit algal populations.     

Behaviour of 2,4-D Herbicide in Coastal Area of Oka River,Russia

Laboratory and field experiments werecarried out with 2,4-D herbicide(2,4-Dichlorophenoxyacetic acid) to evaluate itstransformation and migration in the coastal waterprotection zones of the Oka river, Russia. In thefirst laboratory experiment, the transformation of2,4-D was studied in various soil samples from coastalslopes (1–0°) of 480 m length soil-geochemicalcatena on the right side of the Oka river incomparison with watershed and floodplain soils. Thetransformation of 2,4-D was the lowest in soil sampleswith minimal pH values and was independent of eitherslope values or vicinity to the Oka river channel.Using indirect estimates, the surface runoff potentialwas calculated for this herbicide. In the second fieldexperiment, the vertical migration and transformationof 2,4-D was carried out in soddy sand soil (EutricArenosol) placed in the left side of the Oka river(0-100 cm) under `soft' (40 mm 2 hr<sup>-1</sup>) and `hard'(40 mm 15 min^-1) irrigation regimes. Furthermore, thetransformation of this herbicide was studied in 0–20and 40–50 cm soil layers under various temperature andmoisture regimes. After 1 day of irrigation, the mainherbicide quantity was found in the 0–30 cm layerunder both irrigation regimes. The transformation ofthe herbicide was faster in the surface, 0–20 cmlayer, than in the deeper, 40–50 cm layer.     

Abbau von Monolinuron in verschiedenen Böden

Degradation of monolinuron in different soil types The degradation of ¹⁴C-methyl, -ureido- and -phenyl-labelled monolinuron was studied in laboratory experiments with different soil types. Mineralization of the ureido-group was followed by two methods. It was found, that the mineralization rate depends on the soil. For the herbicide monolinuron there was a definite degradation characteristic. The release of ¹⁴CO₂ from the methyl- and ureido-group was faster than from the phenylring. In soil extracts monolinuron was found together with one or two radioactive substances not yet identified. Soil respiration was unchanged after the application of monolinuron. In a sterile soil no degradation occurred.     

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.     

Discrete functional pools of soil organic matter in a UK grassland soil are differentially affected by temperature and priming

We show that both temperature and priming act differently on distinct C pools in a temperate grassland soil. We used SOM which was ¹⁴C-labelled in four different ways: by labelling soil with ¹⁴C-glucose, by adding leaf litter from plants pre-labelled with ¹⁴CO₂, and by labelling in situ with ¹⁴CO₂ applied to the ryegrass canopy either 6 or 18 months earlier. Samples of each type of ¹⁴C labelled soil were incubated at either 4, 10, 15, or 20 °C and the exponential loss of ¹⁴CO₂ used to characterise treatment effects. ¹⁴C allocation to microbial fractions was greater, and so overall mineralization by microbes was greater, as temperature rose, but turnover of the microbial labile pool was temperature-insensitive, and the turnover of microbial structural material was reduced as temperature rose. The ability of the microbial population to degrade just one fraction of plant litter was increased greatly by temperature. A pool of SOM with a half-life of about 70 d was degraded faster at higher temperatures. Less tractable but abundant pools of SOM were not accessed more readily at higher temperatures by the microbial population. Priming with glucose or amino-acids only speeded the mineralization of recent SOM (probably from the living microbial biomass), and was not altered by temperature. These results have implications for the impacts of climate change on soil C cycling.     

The influence of nitrogen on atrazine and 2,4-dichlorophenoxyacetic acid mineralization in grassland soils

The influence of fertilizer N on the mineralization of atrazine [2-chloro-4(ethylamino)-6(isopropylamino)-s-triazine] and 2,4-D (2,4-dichlorophenoxyacetic acid) in soils was assessed in microcosms using radiometric techniques. N equivalent to 0, 250, and 500 kg N as NH₄NO₃ ha^-1 was added to three grassland soils. Compared to the control, the 250- and 500-kg treatments suppressed mineralization of atrazine by 75 and 54%, respectively, and inhibited mineralization of 2,4-D by 89 and 30%, respectively. Active fungal biomass responded to the N treatments in an opposite manner to herbicide mineralization. Compared to the control, the 250- and 500-kg treatments increased the active fungal biomass by more than 300 and 30%, respectively. These results agree with other observations that N can suppress the decomposition of resistant compounds but stimulate the primary growth of fungi. The degree of suppression was not related to the amount of N added nor to the inherent soil N levels before treatment. The interaction between the N additions and the active fungal biomass in affecting herbicide mineralization suggests that N may alter microbial processes and their use of C sources and thus influence rates of herbicide degradation in the field.     

The fate of catechol in soil as affected by earthworms and clay

The effect of endogeic earthworms (Octolasion tyrtaeum) and the availability of clay (Montmorillonite) on the mobilization and stabilization of uniformly ¹⁴C-labelled catechol mixed into arable and forest soil was investigated in a short- and a long-term microcosm experiment. By using arable and forest soil the effect of earthworms and clay in soils differing in the saturation of the mineral matrix with organic matter was investigated. In the short-term experiment microcosms were destructively sampled when the soil had been transformed into casts. In the long-term experiment earthworm casts produced during 7 days and non-processed soil were incubated for three further months. Production of CO₂ and ¹⁴CO₂ were measured at regular intervals. Accumulation of ¹⁴C in humic fractions (DOM, fulvic acids, humic acids and humin) of the casts and the non-processed soil and incorporation of ¹⁴C into earthworm tissue were determined.Incorporation of ¹⁴C into earthworm tissue was low, with 0.1 and 0.44% recovered in the short- and long-term experiment, respectively, suggesting that endogeic earthworms preferentially assimilate non-phenolic soil carbon. Cumulative production of CO₂-C was significantly increased in casts produced from the arable soil, but lower in casts produced from the forest soil; generally, the production of CO₂-C was higher in forest than in arable soil. Both soils differed in the pattern of ¹⁴CO₂-C production; initially it was higher in the forest soil than in the arable soil, whereas later the opposite was true. Octolasion tyrtaeum did not affect ¹⁴CO₂-C production in the forest soil, but increased it in the arable soil early in the experiment; clay counteracted this effect. Clay and O. tyrtaeum did not affect integration of ¹⁴C into humic fractions of the forest soil. In contrast, in the arable soil O. tyrtaeum increased the amount of ¹⁴C in the labile fractions, whereas clay increased it in the humin fraction.The results indicate that endogeic earthworms increase microbial activity and thus mineralization of phenolic compounds, whereas clay decreases it presumably by binding phenolic compounds to clay particles when passing through the earthworm gut. Endogeic earthworms and clay are only of minor importance for the fate of catechol in soils with high organic matter, clay and microbial biomass concentrations, but in contrast affect the fate of phenolic compounds in low clay soils.