Biogenesis of ubiquinone in rats--an alternate origin of the benzoquinone moiety

The addition of cyclohexane carboxylic acid (CCA) to the incubation medium results in a dilution of the radioactivity incorporated into ubiquinone-9 (UQ-9) from 1-¹⁴C-benzoate by rat liver slices. This effect is more pronounced when the slices are preincubated prior to addition of the labeled precursor. A similar dilution by CCA of label incorporation, is observed using U-¹⁴C-tyrosine, but not either CH₃-¹⁴C-methionine or 2-¹⁴C-mevalonate, as precursors. UQ-9, but not cholesterol, isolated from liver slices incubated with ring-U-¹⁴C-CCA is found to be labeled. The extent of labeling of UQ-9 by this precursor is enhanced by the presence of an excess of mevalonate in the incubation medium and decreased by the addition of p-hydroxybenzoate. These results suggest that aromatisation of cyclohexane derivatives may serve as a possible source of the benzoquinone nucleus of UQ-9 in the rat.     

The effect of soil pH on rhizosphere carbon flow of Lolium perenne

Perennial rye-grass plants were grown at 15°C in microcosms containing soil sampled from field plots that had been maintained at constant pH for the last 30 years. Six soil pH values were tested in the experiment, with pH ranging from 4.3–6.5. After 3 weeks growth in the microcosms, plant shoots were exposed to a pulse of ¹⁴C-CO₂. The fate of this label was determined by monitoring ¹⁴C-CO₂ respired by the plant roots/soil and by the shoots. The ¹⁴C remaining in plant roots and shoots was determined when the plants were harvested 7 days after receiving the pulse label. The amount of ¹⁴C (expressed as a percentage of the total ¹⁴C fixed by the plant) lost from the plant roots increased from 12.3 to 30.6% with increasing soil pH from 4.3 to 6. Although a greater percentage of the fixed ¹⁴C was respired by the root/soil as soil pH increased, plant biomass was greater with increasing soil pH. Possible reasons for observed changes in the pattern of ¹⁴C distribution are discussed and, it is suggested that changes in the soil microbial biomass and in plant nitrogen nutrition may, in particular be key factors which led to increased loss of carbon from plant roots with increasing soil pH.     

Decomposition of diethylstilboestrol in soil

Summary The rate of decomposition of DES-monoethyl-1-C¹⁴ in soil was followed by measurement of C¹⁴O₂ released. From 1.6 to 16% of the added C¹⁴ was recovered as C¹⁴O₂ during 3 months. After six months as much as 12 to 28 per cent was released as C¹⁴O₂.Determination of C¹⁴ in the soil samples after the experiments showed that the amount extractable with benzene decreased to a greater extent than would be expected from the production of C¹⁴O₂ and that the amount extractable with water was increased when compared with that found shortly after the addition of DES.Addition of large amounts of DES (8%) did not inhibit the CO₂ production from the soil.Experiments with -sterilized soil indicated that enzymes present in the soil are able to attack DES.     

Fate of [carbonyl-14C]methabenzthiazuron in an arid region soil — effect of organic amendment,soil disturbance and fumigation

[Carbonyl-¹⁴C] methabenzthiazuron (MBT) was applied to an arid region soil at a rate of 5mg kg⁻¹ soil to give a¹⁴C content of 2400 KB kg⁻¹ soil. After 15 weeks of incubation at 22°C and 50% of the maximum water holding capacity of the soil, 7.2% of the applied¹⁴C was mineralized to¹⁴CO₂. Where the soil was amended with wheat straw, total mineralization increased to 17.3%. Soil disturbance caused a significant increase while chloroform fumigation caused a significant decrease in the rate of¹⁴CO₂ production, both from amended and unamended soils. These results suggest that MBT is degraded mainly through microbial co-metabolism. Wheat straw amendment resulted in increased transformation of MBT into soil humus. In unamended soil, a major portion of¹⁴C was recovered in fulvic acid and in fractions extracted with organic solvents. Recovery of¹⁴C in non-extractable bound residues (humins) increased as incubation progressed and seemed to be derived from the fulvic acid fraction, which showed a concomitant decrease. More than 99% of the residual¹⁴C in unamended soil consisted of unaltered MBT; the remainder occurred as 1-methyl-1 (benzthiazolyl) urea. In amended soil, a relatively higher percentage of the extractable¹⁴C was found in the metabolite. Small amounts of three unidentified¹⁴C-labelled compounds were also observed. In amended soil, disturbance caused a decrease in extractable-¹⁴C whereas fumigation caused a significant increase, as compared to the untreated control. The effects were more pronounced when the soils were reated at an early stage of incubation. In general, soil disturbance increased the availability of MBT for further transformations while chloroform fumigation decreased the process.     

Turnover of14C-labelled oat residues and small molecular organic compounds in two soils under different levels of mineral nutrition

Mineralization and redistribution of carbon from¹⁴C-labelled oat shoots and [¹⁴C(U)] labelled glucose, leucine, acetate and phenylacetate were studied in light loamy sand and medium clay loam under different levels of mineral nutrition. Losses of mineralized¹⁴C as CO₂ were greater in the sandy soil than in the clay soil. NPK and NPK+Ca fertilization increased the rates of decay of the introduced plant organic matter. Among the small molecular organic compounds glucose was degraded fastest and phenylacetate slowest. Incorporation of radioactive carbon into humus fractions varied and depended on the nature of the compound introduced and on the soil type. Carbon of glucose, phenylacetate and acetate was mainly incorporated into fulvic acids, whereas¹⁴C of leucine was almost evenly distributed between humic and fulvic acids and¹⁴C of oat residues in fulvic acids and humin fractions. There was significantly higher incorporation of¹⁴C into humic acids and lower incorporation into humins in the sandy soil compared to the clay soil. NPK+Ca decreased the conversion of¹⁴C from phenylacetate and acetate to bitumens and increased its content in humic acids, particularly in the clay soil. The incorporation of¹⁴C from phenylacetate to humins benefitted from mineral fertilization during the first 30 days of the experiment in both soils.     

Screening of white-rot fungi for their ability to mineralize polycyclic aromatic hydrocarbons in soil

Soil samples from an agricultural field contaminated with 10 ppm¹⁴C-benz(a)anthracene in glass tubes were brought into contact with cultures of wood-rotting fungi, precultivated on wheat straw substrate. Forty-five strains of white-rot fungi and four brown-rot fungi were tested for their ability to colonize the soil and to mineralize¹⁴C-benz(a)anthracene to¹⁴CO₂ within a 20-week incubation time. Twenty-two white-rot fungi and all brown-rot fungi were unable to colonize the soil. Twenty-three strains of white-rot fungi, all belonging to the genusPleurotus, colonized the soil. During the experiment the noncolonizing fungi and their substrate disintegrated more and more to a nonstructured pulp from which water diffused into the soil. The same phenomenon was observed in the control which contained only straw without fungus and contaminated soil. In samples with colonizing fungi the substrate as well as the mycelia in the soil remained visibly unchanged during the entire experiment. Surprisingly, most samples with fungi not colonizing the soil and the control without fungus liberated between 40 and 58 % of the applied radioactivity as¹⁴CO₂ whereas the samples with the colonizing fungi respired only 15–25 % as¹⁴CO₂. This was 3–5 times more¹⁴CO₂ than that liberated from the control (4.9 %) which contained only contaminated soil without straw and fungus. A similar result was obtained with selected colonizing and noncolonizing fungi and soil contaminated with 10 ppm¹⁴C-pyrene. However, in pure culture studies in which¹⁴C-pyrene was added to the straw substrate,Pleurotus sp. (P2), as a representative of the colonizing fungi, mineralized 40.3 % of the added radioactivity to¹⁴CO₂. The noncolonizing fungiDichomitus squalens andFlammulina velutipes liberated only 17.2 or 1.7 %, respectively, as¹⁴CO₂. These results lead to the hypothesis that the native soil microflora stimulated by the formed products of straw lysis is responsible for high degradation rates found with noncolonizing fungi.     

Model for rhizodeposition and CO2 efflux from planted soil and its validation by 14C pulse labelling of ryegrass

A model for rhizodeposition and root respiration was developed and parameterised based on ¹⁴C pulse labelling of Lolium perenne. The plants were grown in a two-compartment chamber on a loamy Haplic Luvisol under controlled laboratory conditions. The dynamics of ¹⁴CO₂ efflux from the soil and ¹⁴C content in shoots, roots, micro-organisms, dissolved organic carbon (DOC) and soil were measured during the first 11 days after labelling. Modelled parameters were estimated by fitting on measured ¹⁴C dynamics in the different pools. The model and the measured ¹⁴C dynamics in all pools corresponded well (r ²=0.977). The model describes well ¹⁴CO₂ efflux from the soil and ¹⁴C dynamics in shoots, roots and soil, but predicts unsatisfactorily the ¹⁴C content in micro-organisms and DOC. The model also allows for division of the total ¹⁴CO₂ efflux from the soil in ¹⁴CO₂ derived from root respiration and ¹⁴CO₂ derived from rhizomicrobial respiration by use of exudates and root residues. Root respiration and rhizomicrobial respiration amounted for 7.6% and 6.0% of total assimilated C, respectively, which accounts for 56% and 44% of root-derived ¹⁴CO₂ efflux from the soil planted with 43-day-old Lolium perenne, respectively. The sensitivity analysis has shown that root respiration rate affected the curve of ¹⁴CO₂ efflux from the soil mainly during the first day after labelling. The changes in the exudation rate influenced the ¹⁴CO₂ efflux later than first 24 h after labelling.     

Influence of light intensity on the distribution of carbon and consequent effects on mineralization of soil nitrogen in a barley (Hordeum vulgare L.)-soil system

A pot experiment was conducted in a ¹⁴C-labelled atmosphere to study the influence of living plants on organic-N mineralization. The soil organic matter had been labelled, by means of a 200-days incubation, with ¹⁵N. The influence of the carbon input from the roots on the formation of microbial biomass was evaluated by using two different light intensities (I). Mineralization of ¹⁵N-labelled soil N was examined by following its fate in both the soil biomass and the plants. Less dry matter accumulated in shoots and roots at the lower light intensity. Furthermore, in all the plant-soil compartments examined, with the exception of rhizosphere respiration, the proportion of net assimilated ¹⁴C was lower in the low-I treatment than in the high-I treatment. The lower rates of ¹⁴C and ¹⁵N incorporation into the soil biomass were associated with less root-derived ¹⁴C. During the chamber period (¹⁴CO₂-atmosphere), mineralized amounts of ¹⁵N (measured as plant uptake of ¹⁵N) were small and represented about 6.8 to 7.8% of the initial amount of organic ¹⁵N in the soil. Amounts of unlabelled N found in the plants, as a percentage of total soil N, were 2.5 to 3.3%. The low availability of labelled N to microorganisms was the result of its stabilization during the 210 days of soil incubation. Differences in carbon supply resulted in different rates of N mineralization which is consistent with the hypothesis that roots induce N mineralization. N mineralization was higher in the high-I treatment. On the other hand, the rate of mineralization of unlabelled stable soil N was lower than labelled soil ¹⁵N which was stabilized. The amounts of ¹⁵N mineralized in planted soil during the chamber period (43 days) which were comparable with those mineralized in unplanted soil incubated for 210 days, also suggested that living plants increased the turnover rate of soil organic matter.     

Dynamics of dissolved organic <Superscript>14</Superscript>C in throughfall and soil solution of a Norway spruce forest

Dissolved organic carbon (DOC) is an important component of the C cycle in forest ecosystems, but dynamics and origin of DOC in throughfall and soil solution are yet poorly understood. In a 2-year study, we analyzed the radiocarbon signature of DOC in throughfall and soil solution beneath the Oa horizon and at 90 cm depth in a Norway spruce forest on a Podzol soil. A two-pool mixing model revealed that throughfall DOC comprised mainly biogenic C, i.e. recently fixed C, from canopy leaching and possibly other sources. The contribution of fossil DOC from atmospheric deposition to throughfall DOC was on average 6% with maxima of 8–11% during the dormant season. In soil solution from the Oa horizon, DO¹⁴C signature was highly dynamic (range from −8‰ to +103‰), but not correlated with DOC concentration. Radiocarbon signatures suggest that DOC beneath the Oa horizon originated mainly from occluded and mineral associated organic matter fractions of the Oa horizon rather than from the Oi or Oe horizon. Relatively old C was released in the rewetting phase following a drought period in the late summer of 2006. In contrast, the DO¹⁴C signature indicated the release of younger C throughout the humid year 2007. In soil solutions from 90 cm depth, DO¹⁴C signatures were also highly dynamic (−127‰ to +3‰) despite constantly low DOC concentrations. Similar to the Oa horizon, the lowest DO¹⁴C signature at 90 cm depth was found after the rewetting phase in the late summer of 2006. Because of the variation in the DO¹⁴C signatures at this depth, we conclude that DOC was not equilibrated with the surrounding soil, but also originated from overlaying soil horizons. The dynamics of DO¹⁴C in throughfall and soil solution suggest that the sources of DOC are highly variable in time. Extended drought periods likely have a strong influence on release and translocation of DOC from relatively old and possibly stabilized soil organic matter fractions. Temporal variations as well as the input of fossil DOC needs to be considered when calibrating DOC models based on DO¹⁴C signatures.     

Partitioning sources of soil respiration in boreal black spruce forest using radiocarbon

Separating ecosystem and soil respiration into autotrophic and heterotrophic component sources is necessary for understanding how the net ecosystem exchange of carbon (C) will respond to current and future changes in climate and vegetation. Here, we use an isotope mass balance method based on radiocarbon to partition respiration sources in three mature black spruce forest stands in Alaska. Radiocarbon (Δ¹⁴C) signatures of respired C reflect the age of substrate C and can be used to differentiate source pools within ecosystems. Recently‐fixed C that fuels plant or microbial metabolism has Δ¹⁴C values close to that of current atmospheric CO₂, while C respired from litter and soil organic matter decomposition will reflect the longer residence time of C in plant and soil C pools. Contrary to our expectations, the Δ¹⁴C of C respired by recently excised black spruce roots averaged 14‰ greater than expected for recently fixed photosynthetic products, indicating that some portion of the C fueling root metabolism was derived from C storage pools with turnover times of at least several years. The Δ¹⁴C values of C respired by heterotrophs in laboratory incubations of soil organic matter averaged 60‰ higher than the contemporary atmosphere Δ¹⁴CO₂, indicating that the major contributors to decomposition are derived from a combination of sources consistent with a mean residence time of up to a decade. Comparing autotrophic and heterotrophic Δ¹⁴C end members with measurements of the Δ¹⁴C of total soil respiration, we calculated that 47–63% of soil CO₂ emissions were derived from heterotrophic respiration across all three sites. Our limited temporal sampling also observed no significant differences in the partitioning of soil respiration in the early season compared with the late season. Future work is needed to address the reasons for high Δ¹⁴C values in root respiration and issues of whether this method fully captures the contribution of rhizosphere respiration.     

The fate of carbon in pulse-labelled crops of barley and wheat

Wheat (cv. Gutha) and barley (cv. O'Connor) were grown as field crops on a shallow duplex soil (sand over clay) in Western Australia with their root systems contained within pvc columns. At four stages during growth, the shoots were pulse-labelled for 1.5h with¹⁴CO₂; immediately prior to labelling, the soil was isolated from the shoot atmosphere by pvc sheets. After labelling, the soil atmosphere was pumped through NaOH to trap respired CO₂ and after 2.5, 5, 7.5 and 24 h from the start of labelling, columns were destructively sampled to recover¹⁴C from the roots, soil and shoot.Both species showed similar patterns of¹⁴C distribution and changes in distribution through the growing season. During early tillering, 15–25% of the¹⁴C recovered after 24 h had been respired by the roots and rhizosphere, 17–27% was retained in the roots, 0.4–1.8% was recovered as water-soluble¹⁴C in the soil and the remainder (45–67%) was present in the shoot. These percentages changed during growth so that during grain filling only 2–3% of the¹⁴C recovered after 24 h was as respired CO₂, 2–6% was in the roots, 0.2% was in the soil and over 90% was in the shoot.The distribution of¹⁴C in components of the soil-plant system changed during the 24 h after labelling with the most rapid changes occurring generally during the first 7.5 h after labelling.Using growth measurements from adjacent plots, the amounts of C added to the soil were estimated for the whole season. Carbon input to the soil was about 48 gC m^–2 for wheat and 58 gC m^–2 for barley; the crops produced total shoot dry matter of 494 (wheat) and 735 g m^–2 (barley). Of the C input to the soil, 27.8% (wheat) and 40.3% (barley) was as respired C and only 3.3 (wheat) and 4.1% (barley) was collected as exudate (water-soluble material).     

Photosynthesis controls of CO2 efflux from maize rhizosphere

The effects of different shading periods of maize plants on rhizosphere respiration and soil organic matter decomposition were investigated by using a ¹³C natural abundance and ¹⁴C pulse labeling simultaneously. ¹³C was a tracer for total C assimilated by maize during the whole growth period, and ¹⁴C was a tracer for recently assimilated C. CO₂ efflux from bare soil was 4 times less than the total CO₂ efflux from planted soil under normal lighting. Comparing to the normal lighting control (12/12 h day/night), eight days with reduced photosynthesis (12/36 h day/night period) and strongly reduced photosynthesis (12/84 h day/night period) resulted in 39% and 68% decrease of the total CO₂ efflux from soil, respectively. The analysis of ¹³C natural abundance showed that root-derived CO₂ efflux accounted for 82%, 68% and 56% of total CO₂ efflux from the planted soil with normal, prolonged and strongly prolonged night periods, respectively. Clear diurnal dynamics of the total CO₂ efflux from soil with normal day-night period as well as its strong reduction by prolonged night period indicated tight coupling with plant photosynthetic activity. The light-on events after prolonged dark periods led to increases of root-derived and therefore of total CO₂ efflux from soil. Any factor affecting photosynthesis, or substrate supply to roots and rhizosphere microorganisms, is an important determinant of root-derived CO₂ efflux, and thereby, total CO₂ efflux from soils. ¹⁴C labeling of plants before the first light treatment did not show any significant differences in the ¹⁴CO₂ respired in the rhizosphere between different dark periods because the assimilate level in the plants was high. Second labeling, conducted after prolonged night phases, showed higher contribution of recently assimilated C (¹⁴C) to the root-derived CO₂ efflux by shaded plants. Results from ¹³C natural abundance showed that the cultivation of maize on Chromic Luvisol decreased soil organic matter (SOM) mineralization compared to unplanted soil (negative priming effect). A more important finding is the observed tight coupling of the negative rhizosphere effect on SOM decomposition with photosynthesis.     

Contribution of Lolium perenne rhizodeposition to carbon turnover of pasture soil

Carbon rhizodeposition and root respiration during eight development stages of Lolium perenne were studied on a loamy Gleyic Cambisol by ¹⁴CO₂ pulse labelling of shoots in a two compartment chamber under controlled laboratory conditions. Total ¹⁴CO₂ efflux from the soil (root respiration, microbial respiration of exudates and dead roots) in the first 8 days after ¹⁴C pulse labelling decreased during plant development from 14 to 6.5% of the total ¹⁴C input. Root respiration accounted for was between 1.5 and 6.5% while microbial respiration of easily available rhizodeposits and dead root remains were between 2 and 8% of the ¹⁴C input. Both respiration processes were found to decline during plant development, but only the decrease in root respiration was significant. The average contribution of root respiration to total ¹⁴CO₂ efflux from the soil was approximately 41%. Close correlation was found between cumulative ¹⁴CO₂ efflux from the soil and the time when maximum ¹⁴CO₂ efflux occurred (r=0.97). The average total of CO₂ Defflux from the soil with Lolium perenne was approximately 21 μg C-CO₂ d⁻¹ g⁻¹. It increased slightly during plant development. The contribution of plant roots to total CO₂ efflux from the soil, calculated as the remainder from respiration of bare soil, was about 51%. The total ¹⁴C content after 8 days in the soil with roots ranged from 8.2 to 27.7% of assimilated carbon. This corresponds to an underground carbon transfer by Lolium perenne of 6–10 g C m⁻² at the beginning of the growth period and 50–65 g C m⁻² towards the end of the growth period. The conventional root washing procedure was found to be inadequate for the determination of total carbon input in the soil because 90% of the young fine roots can be lost. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date.     

Decomposition of 14C-labeled cellulose substrates in litter and soil from a beechwood on limestone

The decomposition of three different ¹⁴C-labeled cellulose substrates (plant holocellulose, plant cellulose prepared from ¹⁴C-labeled beech wood (Fagus sylvatica) and bacterial cellulose produced by Acetobacter xylinum) in samples from the litter and mineral soil layer of a beechwood on limestone was studied. In a long-term (154 day) experiment, mineralization of cellulose materials, production of ¹⁴C-labeled water-soluble compounds, and incorporation of ¹⁴C in microbial biomass was in the order Acetobacter cellulose > holocellulose > plant cellulose in both litter and soil. In general, mineralization of cellulose, production of ¹⁴C-labeled water-soluble compounds, and incorporation of ¹⁴C in microbial biomass were more pronounced, but microbial biomass ¹⁴C declined more rapidly in litter than in soil. In short-term (14 day) incubations, mineralization of cellulose substrates generally corresponded with cellulase and xylanase activities in litter and soil. Pre-incubation with trace amounts of unlabeled holocellulose significantly increased the decomposition of ¹⁴C-labeled cellulose substrates and increased cellulase activity later in the experiment but did not affect xylanase activity. The sum of ¹⁴CO₂ production, ¹⁴C in microbial biomass, and ¹⁴C in water-soluble compounds is considered to be a sensitive parameter by which to measure cellulolytic activity in soil and litter samples in short-term incubations. Shorter periods than 14 days are preferable in assays using Acetobacter cellulose, because the decomposition of this substrate is more variable than that of holocellulose and plant cellulose.Offprint requests to: S. Scheu.     

Mineralization of Parathion in the Rice Rhizosphere

We studied ¹⁴CO₂ evolution from ring-labeled [2,6-¹⁴C]parathion (O,O-diethyl-O-p-nitrophenyl phosphorothioate) in the rhizosphere of rice seedlings. The soil samples (nonflooded [60% water-holding capacity] and flooded) were treated first with technical parathion (20 μg/g) and then after 10 days with ring-labeled [¹⁴C]parathion. In unplanted soil, less than 5.5% of the ¹⁴C in the parathion was evolved as ¹⁴CO₂ in 15 days under both flooded and nonflooded conditions. In soil planted with rice, 9.2% of the radiocarbon was evolved as ¹⁴CO₂ under nonflooded conditions, and 22.6% was evolved under flooded conditions. These results suggest that soil planted with rice permits significant ring cleavage, especially under flooded conditions.     

Metabolism of Benzene, Toluene, and Xylene Hydrocarbons in Soil

Enrichment cultures obtained from soil exposed to benzene, toluene, and xylene (BTX) mineralized benzene and toluene but cometabolized only xylene isomers, forming polymeric residues. This observation prompted us to investigate the metabolism of ¹⁴C-labeled BTX hydrocarbons in soil, either individually or as mixtures. BTX-supplemented soil was incubated aerobically for up to 4 weeks in a sealed system that automatically replenished any O₂ consumed. The decrease in solvent vapors and the production of ¹⁴CO₂ were monitored. At the conclusion of each experiment, ¹⁴C distribution in solvent-extractable polymers, biomass, and humic material was determined, obtaining ¹⁴C mass balances of 85 to 98%. BTX compounds were extensively mineralized in soil, regardless of whether they were presented singly or in combinations. No evidence was obtained for the formation of solvent-extractable polymers from xylenes in soil, but ¹⁴C distribution in biomass (5 to 10%) and humus (12 to 32%) was unusual for all BTX compounds and especially for toluene and the xylenes. The results suggest that catechol intermediates of BTX degradation are preferentially polymerized into the soil humus and that the methyl substituents of the catechols derived from toluene and especially from xylenes enhance this incorporation. In contrast to inhibitory residues formed from xylene cometabolism in culture, the humus-incorporated xylene residues showed no significant toxicity in the Microtox assay.     

Herbivore-induced changes in plant carbon allocation: assessment of below-ground C fluxes using carbon-14

Effects of above-ground herbivory on short-term plant carbon allocation were studied using maize (Zea mays) and a generalist lubber grasshopper (Romalea guttata). We hypothesized that above-ground herbivory stimulates current net carbon assimilate allocation to below-ground components, such as roots, root exudation and root and soil respiration. Maize plants 24 days old were grazed (c. 25–50% leaf area removed) by caging grasshoppers around individual plants and 18 h later pulse-labelled with¹⁴CO₂. During the next 8 h,¹⁴C assimilates were traced to shoots, roots, root plus soil respiration, root exudates, rhizosphere soil, and bulk soil using carbon-14 techniques. Significant positive relationships were observed between herbivory and carbon allocated to roots, root exudates, and root and soil respiration, and a significant negative relationship between herbivory and carbon allocated to shoots. No relationship was observed between herbivory and¹⁴C recovered from soil. While herbivory increased root and soil respiration, the peak time for¹⁴CO₂ evolved as respiration was not altered, thereby suggesting that herbivory only increases the magnitude of respiration, not patterns of translocation through time. Although there was a trend for lower photosynthetic rates of grazed plants than photosynthetic rates of ungrazed plants, no significant differences were observed among grazed and ungrazed plants. We conclude that above-ground herbivory can increase plant carbon fluxes below ground (roots, root exudates, and rhizosphere respiration), thus increasing resources (e.g., root exudates) available to soil organisms, especially microbial populations.     

Identification of metolachlor mineralizing bacteria in aerobic and anaerobic soils using DNA-stable isotope probing

The influence of soil environmental factors such as aeration on the ecology of microorganisms involved in the mineralization and degradation of the popular soil-applied pre-emergent herbicide, metolachlor is unknown. To address this knowledge gap, we utilized DNA-based stable isotope probing (SIP) where soil microcosms were incubated aerobically or anaerobically and received herbicide treatments with unlabeled metolachlor or ¹³C-metolachlor. Mineralization of metolachlor was confirmed as noted from the evolution of ¹⁴CO₂ from ¹⁴C-metolachlor-treated microcosms and clearly demonstrated the efficient utilization of the herbicide as a carbon source. Terminal restriction fragment length polymorphisms (T-RFLP) bacterial community profiling performed on soil DNA extracts indicated that fragment 307 bp from aerobic soil and 212 bp from anaerobic soil were detected only in the herbicide-treated (both unlabeled metolachlor and ¹³C-metolachlor) soils when compared to the untreated control microcosms. T-RFLP profiles from the ultracentrifugation fractions illustrated that these individual fragments experienced an increase in relative abundance at a higher buoyant density (BD) in the labeled fractions when compared to the unlabeled herbicide amendment fractions. The shift in BD of individual T-RFLP fragments in the density-resolved fractions suggested the incorporation of ¹³C from labeled herbicide into the bacterial DNA and enabled the identification of organisms responsible for metolachlor uptake from the soil. Subsequent cloning and 16S rRNA gene sequencing of the ¹³C-enriched fractions implicated the role of organisms closely related to Bacillus spp. in aerobic mineralization and members of Acidobacteria phylum in anaerobic mineralization of metolachlor in soil.     

Effect of concentration and environmental form of tetradecenyl succinic acid on its mineralization in soil

Tetradecenyl succinic acid (TSA) is the major component of a detergent builder (C12-C14 alkenyl succinic acid), which is inherently biodegradable. ¹⁴C-TSA was dosed as a component of sewage sludge into a soil with a history of sludge amendment at final added concentrations of 1.5 and 30 mg (kg soil)^-1. In addition, it was dosed to the soil in an aqueous solution to a final added concentration of 30 mg (kg soil)^-1. Dose and form were found to have a pronouced effect on the mineralization kinetics. When dosed in a realistic form and concentration (i.e. 1.5 mg (kg soil)^-1 as a component of sludge), TSA was mineralized at its highest rate and to its greatest extent, and the mineralization half-life was 2.4 days. When dosed at 30 mg (kg soil)^-1 as a component of sludge, mineralization began immediately, and the half-life was 23 days. In contrast, when dosed at this concentration in aqueous solution, the onset of mineralization was preceded by a 13 day lag period and the mineralization half-life was 69 days. Primary biodegradation and mineralization rates of TSA were very similar. Approximately, half the radioactivity was evolved as ¹⁴CO₂, while the remaining radioactivity became non-extractable, having presumably been incorporated into biomass or natural soil organic matter (humics). This study demonstrated that TSA is effectively removed from sludge-amended soils as a result of biodegradation. Furthermore, it showed the effect that dose form and concentration have on the biodegradation kinetics and the importance of dosing a chemical not only at a relevant concentration but also in the environmental form in which it enters the soil environment.     

14C – a tool for separation of autotrophic and heterotrophic soil respiration

We assessed the potential of using ¹⁴C contents of soil respired CO₂ to calculate the contributions of heterotrophic and autotrophic respiration to total soil respiration. The partitioning of these fluxes is of utmost importance to evaluate implications of environmental change on soil carbon cycling and sequestration. At three girdled forest stands in Sweden and Germany, where the tree root (autotrophic) respiration had been eliminated, we measured both flux rates and ¹⁴C contents of soil respired CO₂ in girdled and control plots in the summers of 2001 or 2002. At all stands, CO₂ flux rates were slightly higher in the control plots, whereas the ¹⁴C contents of respired CO₂ tended to be higher in the girdled plots. This was expected and confirmed that heterotrophically respired CO₂ cycles more slowly through the forest ecosystem than autotrophically respired CO₂. On the basis of these data, the contributions of hetero‐ and autotrophic respiration to total soil respiration were calculated using two separate approaches (i.e. based on flux rates or ¹⁴C). Fractions of heterotrophic respiration ranged from 53% to 87%. Values calculated by both approaches did not differ significantly from each other. Finally, we compared the ¹⁴C contents of soil respired CO₂ in the girdled plots with the ¹⁴C contents of heterotrophically respired CO₂ calculated by three different ¹⁴C models. None of the models matched the measured data sufficiently. In addition, we suspect that inherent effects of girdling may cause the ¹⁴C content of CO₂ respired in the girdled plots to be lower than ‘true’ heterotrophically respired CO₂ in an undisturbed plot. Nevertheless, we argue that measurements and modeling of ¹⁴C can be developed into a valuable tool for separating heterotrophic and autotrophic soil respiration (e.g. when girdling cannot be performed).