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.     

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.     

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.     

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.     

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.     

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.     

Oxalate-metabolizing microorganisms in sagebrush steppe soil

Oxalate metabolization by soil microorganisms was assessed using a calcium oxalate clearing medium and¹⁴CO₂ release from [¹⁴C]-oxalate. Three saprophytic fungi, two bacteria, and one actinomycete tested produced¹⁴CO₂ when grown in culture with [¹⁴C]-oxalate, yet failed to test positive for oxalate degradation using a calcium-clearing medium. A field plot was then established to determine the effects of oxalate inputs on oxalate metabolism. The amount of [¹⁴C]-oxalate metabolized by soil microorganisms and the number of bacteria metabolizing oxalate increased within 24 h after the addition of oxalic acid at a concentration of 11.1 mol g^-1 soil. Oxalate metabolism and bacterial numbers returned to the baseline within 84 days. Soil phosphate concentrations increased significantly above baseline 7 days after the addition of oxalate and did not return to prespike levels. Fungi, bacteria, and actinomycetes were able to metabolize oxalate. Therefore, while oxalate can influence P cycles by increasing the amount of available phosphates, that increase is mediated by microbes that metabolize the oxalates.     

Effluxed CO2-C from sterilized and unsterilized treatments of a calcareous soil

Soil inorganic carbon (C) represents a substantial C pool in arid ecosystems, yet little data exist on the contribution of this pool to ecosystem C fluxes. A closed jar incubation study was carried out to test the hypothesis that CO₂-¹³C production and response to sterilization would differ in a calcareous (Mojave Desert) soil and a non-calcareous (Oklahoma Prairie) soil due to contributions of carbonate-derived CO₂. In addition to non-sterilized controls, soils were subjected to sterilization treatments (unbuffered HgCl₂ addition for Oklahoma soil and unbuffered HgCl₂ addition, buffered HgCl₂ addition, and autoclaving for Mojave Desert soil) to decrease biotic respiration and more readily measure abiotic CO₂ flux. Temperature and moisture treatments were also included with sterilization treatments in a factorial design.The rate of CO₂ production in both soils was significantly decreased (36-87%) by sterilization, but sterilization treatments differed in effectiveness. Sterilization had no significant effect on effluxed CO₂-¹³C values in the non-calcareous Oklahoma Prairie soil and autoclaved Mojave Desert soil as compared to their respective non-sterilized controls. However, sterilization significantly altered CO₂-¹³C values in Mojave Desert soil HgCl₂ sterilization treatments (both buffered and non-buffered). Plots of 1/CO₂ versus CO₂-δ¹³C (similar to Keeling plots) indicated that the source CO₂-δ¹³C value of the Oklahoma Prairie soil treatments was similar to the δ¹³C value of soil organic matter [(SOM); −17.76‰ VPDB] whereas the source for the (acidic) unbuffered-HgCl₂ sterilized Mojave Desert soil was similar to the δ¹³C value of carbonates (−0.93‰ VPDB). The source CO₂-δ¹³C value of non-sterilized and autoclaved (−18.4‰ VPDB) Mojave Desert soil treatments was intermediate between SOM (−21.43‰ VPDB) and carbonates and indicates up to 13% of total C efflux may be from abiotic sources in calcareous soils.     

Short-term decomposition of <Superscript>14</Superscript>C-labelled glucose in a fluvo-aquic soil as affected by lanthanum amendment

In this study, we investigated the effects of lanthanum (La), one of the rare earth elements (REEs), on microbial biomass C as well as the decomposition of ¹⁴C-labelled glucose in a fluvo-aquic soil in 28 days. The soil was collected from the field plots under maize/wheat rotation in Fengqiu Ecological Experimental Station of Chinese Academy of Sciences, Henan Province, China. Application of La decreased soil microbial biomass C during the experimental period, and there was a negative correlation (P < 0.01) between microbial biomass and application rate of La. La increased microbial biomass ¹⁴C after ¹⁴C glucose addition, and the increase was significant (P < 0.05) at the rates of more than 160 mg kg⁻¹ soil. La slightly increased ¹⁴CO₂ evolution at lower rates of application but decreased it at higher rates 1 day after ¹⁴C glucose addition, while there was no significant effect from days 2 to 28. For the cumulative ¹⁴CO₂ evolution during the incubation of 28 days, La slightly increased it at the rates of less than 120 mg kg⁻¹ soil, while significantly decreased (P < 0.05) it at the rate of 200 mg kg⁻¹ soil. The results indicated that agricultural use of REEs such as La in soil could decrease the amount of soil microbial biomass and change the pattern of microbial utilization on glucose C source in a short period.     

Taxon-specific responses of soil bacteria to the addition of low level C inputs

The addition of small or trace amounts of carbon to soils can result in the release of 2-5 times more C as CO₂ than was added in the original solution. The identity of the microorganisms responsible for these so-called trigger effects remains largely unknown. This paper reports on the response of individual bacterial taxa to the addition of a range of ¹⁴C-glucose concentrations (150, 50 and 15 and 0 μg C g⁻¹ soil) similar to the low levels of labile C found in soil. Taxon-specific responses were identified using a modification of the stable isotope probing (SIP) protocol and the recovery of [¹⁴C] labelled ribosomal RNA using equilibrium density gradient centrifugation. This provided good resolution of the ‘heavy’ fractions ([¹⁴C] labelled RNA) from the ‘light’ fractions ([¹²C] unlabelled RNA). The extent of the separation was verified using autoradiography. The addition of [¹⁴C] glucose at all concentrations was characterised by changes in the relative intensity of particular bands. Canonical correspondence analysis (CCA) showed that the rRNA response in both the ‘heavy’ and ‘light’ fractions differed according to the concentration of glucose added but was most pronounced in soils amended with 150 μg C g⁻¹ soil. In the ‘heavy RNA’ fractions there was a clear separation between soils amended with 150 μg C g⁻¹ soil and those receiving 50 and 15 μg C g⁻¹ soil indicating that at low C inputs the microbial community response is quite distinct from that seen at higher concentrations. To investigate these differences further, bands that changed in relative intensity following amendment were excised from the DGGE gels, reamplified and sequenced. Sequence analysis identified 8 taxa that responded to glucose amendment (Bacillus, Pseudomonas, Burkholderia, Bradyrhizobium, Actinobacteria, Nitrosomonas, Acidobacteria and an uncultured β-proteobacteria). These results show that radioisotope probing (RNA-RIP) can be used successfully to study the fate of labile C substrates, such as glucose, in soil.     

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.     

Repeated CO2 pulse-labelling reveals an additional net gain of soil carbon during growth of spring wheat under free air carbon dioxide enrichment (FACE)

Rising levels of atmospheric CO₂ have often been found to increase above and belowground biomass production of C3 plants. The additional translocation of organic matter into soils by increased root mass and exudates are supposed to possibly increase C pools in terrestrial ecosystems. Corresponding investigations were mostly conducted under more or less artificial indoor conditions with disturbed soils. To overcome these limitations, we conducted a ¹⁴CO₂ pulse-labelling experiment within the German FACE project to elucidate the role of an arable crop system in carbon sequestration under elevated CO₂. We cultivated spring wheat cv. “Minaret” with usual fertilisation and ample water supply in stainless steel cylinders forced into the soil of a control and a FACE plot. Between stem elongation and beginning of ripening the plants were repeatedly pulse-labelled with ¹⁴CO₂ in the field. Soil born total CO₂ and ¹⁴CO₂ was monitored daily till harvest. Thereafter, the distribution of ¹⁴C was analysed in all plant parts, soil, soil mineral fractions and soil microbial biomass. Due to the small number of grown wheat plants (40) in each ring and the inherent low statistical power, no significant above and belowground growth effect of elevated CO₂ was detected at harvest. But in comparison to ambient conditions, 28% more ¹⁴CO₂ and 12% more total CO₂ was evolved from soil under elevated CO₂ (550 μmol CO₂ mol⁻¹). In the root-free soil 27% more residual ¹⁴C was found in the FACE soil than in the soil from the ambient ring. In soil samples from both treatments about 80% of residual ¹⁴C was found in the clay fraction and 7% in the silt fraction. Very low ¹⁴C contents in the CFE extracts of microbial biomass in the soil from both CO₂ treatments did not allow assessing their influence on this parameter. Since the calculated specific radioactivity of soil born ¹⁴CO₂ gave no indication of an accelerated priming effect in the FACE soil, we conclude that wheat plants grown under elevated CO₂ can contribute to an additional net carbon gain in soils.     

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.     

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.     

Fate of Zinc Ammonia Acetate in Soils and Its Uptake by Corn

Zinc ammonium acetate (ZAA) is applied to soil with anhydrous ammonia as a carrier to improve corn (Zea mays L.) productivity. To study the fate of ZAA in the soils, a laboratory leaching experiment was conducted with ¹⁴C-ZAA and ⁶⁵Zn-ZAA using water or NH₄OH as carrier. Results showed that ZAA degraded in the soil and released CO₂. The released CO₂ was from the acetate component of ZAA and accounted for less than 0.06% of the total applied ¹⁴C. Using H₂O as carrier resulted in more CO₂ release than using NH₄OH, regardless of soil type. However, more ¹⁴C was detected in the leachate when using NH₄OH as a carrier than when using H₂O. A plant uptake experiment using ¹⁴C-ZAA and ⁶⁵Zn-ZAA indicated that four-leaf-stage corn seedlings absorbed higher amounts of ¹⁴C in the stem and root than in the leaves. The ⁶⁵Zn was detected in most tested plant parts. Our study showed that ZAA enhanced cytokinin levels in root and/or shoot tissues of corn seedlings, suggesting a secondary regulatory effect of ZAA in improving corn productivity.     

The initial rate of C substrate utilization and longer-term soil C storage

The initial reaction of microbial transformation and turnover of soil carbon inputs may influence the magnitude of longer-term net soil C storage. The objective of this study was to test the merit of the hypothesis that the more rapid substrates are initially utilized, the longer the residual products remain in the soil. We used simple model C compounds to determine their decomposition rates and persistence over time. Pure ¹⁴C compounds of glucose, acetate, arginine, oxalate, phenylalanine, and urea were incubated in soil for 125 days at 24°C. Total respired CO₂ and ¹⁴CO₂ was quantitatively measured every day for 15 days and residual soil ¹⁴C after 125 days. The percent ¹⁴C remaining in the soil after 125 days of incubation was positively and significantly correlated with the percent substrate utilized in the first day of incubation. The ¹⁴C in the microbial biomass ranged from 4–15% after 15 days and declined through day 125, contributing significantly to the ¹⁴C that evolved over the longer time period. Priming of ¹²C soil organic matter (SOM) was negative at day 3 but became positive, reaching a maximum on day 12; the total increase in soil C from added substrates was greater than the primed C. The primed C came from ¹²C SOM rather than the microbial biomass. This data supports the concept that the more rapidly a substrate is initially mineralized, the more persistent it will be in the soil over time.     

Rhizodeposition-induced decomposition increases N availability to wild and cultivated wheat genotypes under elevated CO2

Elevated CO₂ may increase nutrient availability in the rhizosphere by stimulating N release from recalcitrant soil organic matter (SOM) pools through enhanced rhizodeposition. We aimed to elucidate how CO₂-induced increases in rhizodeposition affect N release from recalcitrant SOM, and how wild versus cultivated genotypes of wheat mediated differential responses in soil N cycling under elevated CO₂. To quantify root-derived soil carbon (C) input and release of N from stable SOM pools, plants were grown for 1 month in microcosms, exposed to ¹³C labeling at ambient (392 μmol mol⁻¹) and elevated (792 μmol mol⁻¹) CO₂ concentrations, in soil containing ¹⁵N predominantly incorporated into recalcitrant SOM pools. Decomposition of stable soil C increased by 43%, root-derived soil C increased by 59%, and microbial-¹³C was enhanced by 50% under elevated compared to ambient CO₂. Concurrently, plant ¹⁵N uptake increased (+7%) under elevated CO₂ while ¹⁵N contents in the microbial biomass and mineral N pool decreased. Wild genotypes allocated more C to their roots, while cultivated genotypes allocated more C to their shoots under ambient and elevated CO₂. This led to increased stable C decomposition, but not to increased N acquisition for the wild genotypes. Data suggest that increased rhizodeposition under elevated CO₂ can stimulate mineralization of N from recalcitrant SOM pools and that contrasting C allocation patterns cannot fully explain plant mediated differential responses in soil N cycling to elevated CO₂.     

Microbial assimilation of atmospheric CO2 into soil organic matter revealed by the incubation of paddy soils under 14C-CO2 atmosphere

Similar to higher plants, microbial autotrophs possess photosynthetic systems that enable them to fix CO₂. To measure the activity of microbial autotrophs in assimilating atmospheric CO₂, five paddy soils were incubated with ¹⁴C-labeled CO₂ for 45 days to determine the amount of ¹⁴C-labeled organic C being synthesized. The results showed that a significant amount of ¹⁴C-labeled CO₂ incorporated into microbial biomass was soil specific, accounting for 0.37%–1.18% of soil organic carbon (¹⁴C-labeled organic C range: 81.6–156.9 mg C kg^?1 of the soil after 45 days). Consequently, high amounts of C-labeled organic C were synthesized (the synthesis rates ranged from 86 to 166 mg C m^?2 d^?1). The amount of atmospheric ¹⁴CO₂ incorporated into microbial biomass (¹⁴C-labeled microbial biomass) was significantly correlated with organic C components (¹⁴C-labeled organic C) in the soil (r = 0.80, p < 0.0001). Our results indicate that the microbial assimilation of atmospheric CO₂ is an important process for the sequestration and cycling of terrestrial C. Our results showed that microbial assimilation of atmospheric CO₂ has been underestimated by researchers globally, and that it should be accounted for in global terrestrial carbon cycle models.     

Sorption, microbial uptake and decomposition of acetate in soil: Transformations revealed by position-specific C labeling

Many previous studies on transformation of low molecular weight organic substances (LMWOS) in soil were based on applying ¹⁴C and/or ¹³C labeled substances. Nearly all these studies used uniformly labeled substances, i.e. all C atoms in the molecule were labeled. The underlying premise is that LMWOS transformation involves the whole molecule and it is not possible to distinguish between 1) the flux of the molecule as a whole between pools (i.e. microbial biomass, CO₂, DOM, SOM, etc.) and 2) the splitting of the substance into metabolites and tracing those metabolites within the pools.Based on position-specific¹⁴C labeling, we introduce a new approach for investigating LMWOS transformation in soil: using Na-acetate labeled with ¹⁴C either in the 1st position (carboxyl group, -COOH) or in the 2nd position (methyl group, -CH₃), we evaluated sorption by the soil matrix, decomposition to CO₂, and microbial uptake as related to both C atoms in the acetate. We showed that sorption of acetate occurred as a whole molecule. After microbial uptake, however, the acetate is split, and C from the -COOH group is converted to CO₂ more completely and faster than C from the -CH₃ group. Correspondingly, C from the -CH₃ group of acetate is mainly incorporated into microbial cells, compared to C from the -COOH group. Thus, the rates of C utilization by microorganisms of C from both positions in the acetate were independently calculated. At concentrations of 10 μmol l⁻¹, microbial uptake from soil solution was very fast (half-life time about 3 min) for both C atoms. At concentrations <100 μmol l⁻¹ the oxidation to CO₂ was similar for C atoms of both groups (about 55% of added substance). However, at acetate concentrations >100 μmol l⁻¹, the decomposition to CO₂ for C from -CH₃ decreased more strongly than for C from -COOH.We conclude that the application of position-specifically labeled substances opens new ways to investigate not only the general fluxes, but also transformations of individual C atoms from molecules. This, in turn, allows conclusions to be drawn about the steps of individual transformation processes on the submolecular level and the rates of these processes.     

Characterisation of the dynamics of C-partitioning within Lolium perenne and to the rhizosphere microbial biomass using 14C pulse chase

The dynamics of C partitioning with Lolium perenne and its associated rhizosphere was investigated in plant-soil microcosms using ¹⁴C pulse-chase labelling. The ¹⁴CO₂ pulse was introduced into the shoot chamber and the plants allowed to assimilate the label for a fixed period. The microcosm design facilitated independent monitoring of shoot and root/soil respiration during the chase period. Partitioning between above- and below-ground pools was determined between 30 min and 168 h after the pulse, and the distribution was found to vary with the length of the chase period. Initially (30 min after the pulse), the ¹⁴C was predominantly (99%) in the shoot biomass and declined thereafter. The results indicate that translocation of recent photoassimilate is rapid, with ¹⁴C detected below ground within 30 min of pulse application. The translocation rate of ¹⁴C below ground was maximal (6.2% h^-1) between 30 min and 3 h after the pulse, with greatest incorporation into the microbial biomass detected at 3 h. After 3 h, the microbial biomass ¹⁴C pool accounted for 74% of the total ¹⁴C rhizosphere pool. By 24 h, approximately 30% of ¹⁴C assimilate had been translocated below ground; thereafter ¹⁴C translocation was greatly reduced. Partitioning of recent assimilate changed with increasing CO₂ concentration. The proportion of ¹⁴C translocated below ground almost doubled from 17.76% at the ambient atmospheric CO₂ concentration (450 ppm) to 33.73% at 750 ppm CO₂ concentration. More specifically, these changes occurred in the root biomass and the total rhizosphere pools, with two- and threefold ¹⁴C increases at an elevated CO₂ concentration compared to ambient, respectively. The pulselabelling strategy developed in this study provided sufficient sensitivity to determine perturbations in C dynamics in L. perenne, in particular rhizosphere C pools, in response to an elevated atmospheric CO₂ concentration.