Utilization of organic materials in soil aggregates by bacteria and fungi

Cultures of six fungi and two bacteria were added to samples of aggregates in which either ¹⁴C-labelled glucose or starch was thoroughly distributed in macro- and micropores or in control samples where the labelled substrates were added to the preformed aggregates and considered to be mainly in macropores. The release of ¹⁴CO₂ was monitored over a 24-day incubation.In the control samples with substrates mainly in macropores, the bacteria were as active as fungi in releasing ¹⁴CO₂ from both soils. When the substrates were distributed in macro- and micropores in aggregates made from a fine sandy loam, the fungi were more efficient than bacteria in releasing ¹⁴CO₂. This was not the case in a self-mulching clay.The initial flush of ¹⁴CO₂ released during incubation of the amended fine sandy loam was due mainly to fungi, which were followed by a secondary bacterial population. The change in populations occurred simultaneously with a step in the cumulative ¹⁴CO₂ release curve thought to be due to the utilization of all the labelled substrate added, followed by renewed respiration as the secondary population flourished. The results presented fit well with an efficiency of C assimilation by micro-organism in soil of about 60%.     

Mineralization of Soil Organic Matter and Added Carbon Substrates in Two Acidic Soils with High non-exchangeable Aluminum

Mineralization of soil organic matter and of added ¹⁴C labelled substrates were studied on samples from two acidic forest soils, “Cademario”-sample from the Bh-horizon of a cryptopodzolic soil rich in humus and nonexchangeable Al and “Sagno”-sample from the A-horizon of a Haplumbrept with moderate humus- and Al-content. The respiration rates for the two soils were not different when related to the content of organic matter. When treated with Na₂CO₃, the CO₂ production rate in the Sagno soil increased about three fold whereas no significant difference was observed for Cademario samples. This is attributed to the more pronounced dissolution of organic matter due to the pH increase in the Sagno soil. N-mineralization was different in the two soils. During a 28 day incubation period, 0.11% and 0.34% of the total organic N was released in the Cademario and Sagno samples, respectively. Na₂CO₃ treatment stimulated N-mineralization in both soils but the mineral N-form was primarily nitrate in the Sagno sample and ammonium in the aluminum-rich sample from Cademario. Glucose, succinate and salicylate added to the soils were mineralized in this order. However, CO₂ evolution was much slower in the case of salicylate, especially in the untreated soils, a fact which is attributed to the Al-complexing power of this substrate.     

Reponse de la biomasse microbienne a l'adjonction au sol de materiel vegetal marque au 14C: Role des racines vivantes

¹⁴C and ¹⁵N-labelled immature wheat straw was incubated in the laboratory for 450 days in either a sandy soil or a clay soil, under controlled conditions of temperature and humidity. One-half of the treatments were cropped 4 times in succession with spring wheat. After each harvest, the roots and shoots were removed from the soil. The remaining treatments were kept bare, without plants. After 277 days, 1% unlabelled wheat straw was again mixed with the soils. Microbial biomass was measured after 0, 25, 53, 80, 185, 318 and 430 days, using the fumigation technique. This paper presents the ¹⁴C-data.The half-life of the labelled compounds in soil was from 60 to 70 days. After 430 days about 10% more labelled C remained in bare soil than in cropped soil. Labelled biomass carbon reached its maximum before day 25. By then 50% of the biomass-C was labelled and the biomass represented 20% of the total labelled C remaining in the soils. This percentage decreased slowly to 15% after 430 days in bare sandy soil and to 17% in bare clay soil. A second incorporation of plant material, this time unlabelled, did not appreciably alter the shape of the curve representing the decrease of labelled C in biomass, expressed as % of the total remaining labelled C. Total biomass-C (labelled + unlabelled) in cropped soil was sometimes higher and sometimes lower than in bare soil. However, the labelled C/total C ratio in biomass was always lower; in cropped soils than in soils without plants, clearly showing the effect of rhizodeposition. From days 25 to 430 an increasing difference appeared between the ratio labelled C/total and C in CO₂ and the corresponding ratio labelled C/total C in biomass. In CO₂-C the ratio diminished rapidly, in biomass-C it remained at a high level, most probably indicating a lower turnover of C in resting but living microorganisms. Other explanations are also discussed. The amount of CO₂-C released mg^?1 of biomass-C was higher in cropped than in bare soil, presumably because the microorganisms were activated by the living (or dying) root system.     

STUDIES ON THE DECOMPOSITION OF PLANT MATERIAL IN SOIL

Soil samples taken during an experiment on the decomposition of ¹⁴C-labelled ryegrass in soil under field conditions (see Part I) were air-dried, irradiated, exposed to CHCl₃ or CH₃Br vapours, oven-dried or autoclaved. After these treatments the soils were inoculated, incubated, and the output of CO₂ measured. All these methods of partially (or, in some cases, completely) sterilizing soil rendered a small heavily labelled fraction of the soil organic matter decomposable. This fraction is postulated to be the soil biomass. Treatments involving heat or irradiation rendered small additional amounts of the soil organic matter decomposable (by processes other than the killing of organisms). Incubating unsterilized soil with partially sterilized soil did not decrease evolution of CO₂. This suggests that partial sterilization does not increase mineralization by destroying toxic substances that inhibit microbial growth, or by disturbing a host: predator balance in the unsterilized soil. The longer the labelled ryegrass was allowed to decompose in the field, the less labelled-CO₂ was evolved after partial sterilization. In contrast, the same amount of unlabelled-CO₂ was evolved from a soil that had been incubated 1 or 4 years with ryegrass. The labelled part of the biomass is considered to be largely zymogenic (with a half life of approximately 1.5 years), the unlabelled part largely autochthonous, remaining almost constant over the 3-year period. It is suggested that the size of the soil biomass can be roughly estimated from the size of the flush of CO₂ after CHCl₃ vapour treatment. Calculated on this basis, 2.3–3.5 Per cent the unlabelled-C in these soils (i.e. the C present in the soil before the labelled ryegrass was added) was in the biomass. Of the original ryegrass C added, 10–12 per cent was in the biomass after 1 year, decreasing to 4 per cent after 4 years.     

Growing crops and transformations of 14C-labelled soil organic matter

Transformations of native soil organic materials, previously labelled with ¹⁴C, were investigated in soil planted with maize or perennial ryegrass and in fallow controls. There was more ¹⁴C in cold water extracts from planted soils than from fallow controls—an effect apparently caused by suppression of processes that remove labelled organic materials from this fraction of the soil organic matter. Decomposition of the labelled organic matter to ¹⁴CO₂ was significantly less in the planted soils than in fallow controls.     

Turnover of carbon and nitrogen through the microbial biomass in a sandy loam and a clay soil incubated with [14C(U)]glucose and [15N](NH4)2So4 under different moisture regimes

Two soils, one a sandy loam and the other of relatively high clay content, were incubated with [¹⁴C(U)]gtucose and [¹⁵N](NH₄)₂SO₄ for 101 days, either under continuously moist conditions, or with intermittent drying of soils. Rates of evolution of ¹⁴CO₂, decline in residual organic ¹⁴C, and net immobilization and mineralization of N and ¹⁵N in the sandy loam soil were more rapid than in the clay soil. First order decay rates for the decomposition of residual ¹⁴C, after 10 days, were consistently twice as fast in the sandy loam soil. By contrast, the efficiency with which glucose was utilized within the first few days, and the amounts of C, ¹⁴C, N and ¹⁵N present as soil biomass throughout the incubation, were greater in the clay soil than in the sandy loam. Biomass ¹⁴C as a percentage of residual organic ¹⁴C, was consistently 1.5 times greater in the clay soil. Compared with soils held continuously moist, soils which were intermittently dried and remoistened contained smaller amounts of isotope-labelled biomass C and N, but overall similar amounts of total residual organic ¹⁴C and ¹⁵N. Remoistening of dried soils caused a temporary (4 days) flush in C and N mineralization rates.A simulation model describes C and N behaviour in the two soils. Three features of the model are proposed to expain short-term differences between soils in the rates of C and N turnover, viz. the clay soil (a) has a greater capacity to preserve biomass C and N (b) holds a higher proportion of microbial decay products in the near vicinity of surviving cells, and, to a lesser extent, (c) utilizes glucose and metabolic products more efficiently for biosynthetic reactions.     

Decomposition of straw in soil after stepwise repeated additions

Samples of a sandy soil, which had been incubated for 8 years in the field with [¹⁴C]labelled barley straw, were amended with 1, 2, 3 or 4 successive additions of [¹⁴C]labelled straw, respectively, applied at intervals of 3 months. The decomposition of the straw was studied over a 4-yr period of laboratory incubation, following the first repeated application, by determination of the total amount of labelled C in the soils and labelled C in the soil amino acids. The overall pattern of decomposition was similar whether the soil was amended with one or with several successive applications.Four years after the first repeated addition of labelled straw the soils were subjected to a number of “stress” treatments: addition of unlabelled glucose, air-drying, oven-drying, grinding and fumigation with vapour of chloroform, respectively. The CO₂ that developed during the first 10 days after the treatments, less the evolution from untreated samples, was taken as a measure of the effect of the treatments. The amount of biomass in the soils was calculated from the increase caused by the fumigation with chloroform. In soil incubated undisturbed in the field for 12 yr, biomass accounted for 2.6% of the labelled C in the soil, whereas it was only half this amount in the soil incubated for 8 yr in the field followed by 4 yr in the laboratory. In the soils amended with successive additions of labelled straw, the size of the biomass showed declining values with an increasing number of additions. Biomass thus accounted for 2.6% of the labelled C in the soil amended with one repeated addition, and 1.0% in the soil amended with 4 repeated additions.The increase in the evolution of labelled CO₂-C caused by the stress treatments ranged from 0.3 to 1.7% of the labelled C in the soil: air-drying had the least effect, grinding the most. The effect of each treatment declined with an increasing number of successive additions of straw. The ratio between CO₂ evolved after grinding and fumigation, respectively, revealed that grinding also exposed non-biomass material to accelerated decomposition.The effects of the stress treatments on the evolution of native CO₂-C was on the whole parallel to the effects on the evolution of labelled CO₂-C.     

Evolution of CO2 from soil incubated with dieldrin-C and the action of soil bacteria on labelled dieldrin

Arable soil containing 10 ppm of dieldrin-¹⁴C uniformly labelled in its chlorinated ring released 0·30 and 1.86 per cent of the activity as ¹⁴CO₂ from sterile and non-sterile samples respectively during 7 weeks incubation. In a second experiment with percolated or aerated soil samples containing 50 ppm dieldrin-¹⁴C with or without glucose about 0·30 per cent was lost as ¹⁴CO₂. The different experimental conditions only influenced the time pattern of ¹⁴CO₂ evolution. About half of 177 bacterial strains isolated from the same soil produced water-soluble dieldrin metabolites in culture. From 14 selected strains the three most active strains (Nocardia, Corynebacterium and aMicrococcus sp.) were incubated for 5 weeks with 0·4 ppm dieldrin-¹⁴C and released 0·06–0·11 per cent recoverable as BaCO₃ in aerated culture and 0·14–0·2 per cent in stationary culture.     

The effects of biocidal treatments on metabolism in soil—II. Gamma irradiation,autoclaving, air-drying and fumigation

Respiration and mineralisation of N were measured in a set of contrasting soils that had either been autoclaved, air-dried, fumigated (with chloroform or methyl bromide) or exposed to gamma radiation. The soils used were a manured and an unmanured arable soil, an acid and a neutral woodland soil, an arable sandy soil and an organic soil under grass. With the exception of the acid woodland soil, the flushes of decomposition (i.e. the increases in O₂ consumption, CO₂ evolution and N mineralisation that occurred when the treated soil was inoculated and incubated for 10 days) were in the order: air-drying < CH₃Br ? CHCl₃ < irradiation < autoclaving. All of the treatments, except air-drying, decreased the ratio (C mineralised after treatmcnt)/(N mineralised after treatment). All of the treatments increased the amount of 1N K₂SO₄ extractable organic C, autoclaving causing by far the greatest increase.Neither of the fumigants increased respiration in the acid soil over the whole 10 day period, although N mineralisation was slightly increased. Irradiation, air-drying and autoclaving did, however, produce a flush in the acid soil, the order being: irradiation < air-drying < autoclaving. A soluble substrate, extracted from yeast cells by ultrasonic disintegration, decomposed to about the same extent in neutral and in acid soil. When ¹⁴C labelled glucose was added to the acid soil and incubated for 52 days, the retention of labelled C was slightly greater (31·6%) than in a comparable near-neutral soil (28·8%). However, the flush that followed fumigation of the acid soil was only half that in the near-neutral soil, suggesting that less biomass is formed under acid conditions. Liming increased the size of the flush in an acid soil.For soils from the same field but under different management, the size of the flush caused by CHCl₃ is in the order: grassland > cropped arable > bare fallow. The flush is much more sensitive to differences in soil management than is the total amount of soil organic matter; a fallowed soil lost half its organic C in 10 yr whereas the increase in respiration that followed fumigation fell to one-seventh its original value. Two Nigerian soils behaved similarly; a soil that had been 2 years under cultivation contained only 16% less total organic C than an adjacent soil still under secondary forest, yet the flush in the cultivated soil was half that in the forest soil. The amount of substrate metabolised during the flush is thus very sensitive to changes in soil management that alter the amount of fresh organic matter entering the soil each year.     

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.     

Carbon-nitrogen relationships during the humification of cellulose in soils containing different amounts of clay

¹⁴C-labelled cellulose and ¹⁵N-labelled (NH₄)₂SO₄ were added to four soils with clay contents of 4, 11, 18 and 34%, respectively. Labelled cellulose was added to each soil in amounts corresponding to 1, 2 and 4 mg C g^?1 soil, respectively, and labelled NH₄⁺ at the rate of 1 mg N per 25 mg labelled C.After the first month of incubation at temperatures of 10, 20 and 30°C, respectively, from 38 to 65% of the labelled C added in cellulose had disappeared from the soils as CO₂, and from 60 to nearly 100% of the labelled N added as NH₄⁺ were incorporated into organic forms. The ratio of labelled C remaining in the soils to labelled N in organic forms was close to 25 after 10 days of incubation, decreasing to about 15 after 1 month and about 10 after 4 yr.The retention of total labelled C was largest in the soil with the highest content of clay where after 4 yr it was 25% of that added, compared to 12 in the soil with the lowest content of clay. The incorporation of labelled N in organic forms and its retention in these forms was not directly related to the content of clay in the soils, presumably because the two soils with the high content of clay had a relatively high content of available unlabelled soil-N which was used for synthesis of metabolic material.The proportionate retention of labelled C for a given soil was largely independent of the size of the amendments, whereas the proportionate amount of labelled N incorporated into organic forms increased in the clay-rich soils with increasing size of amendments. Presumably this is because the dilution with unlabelled soil-N was less with the large amendments.From 50 to 70% of the total labelled C remaining in the soils after the first month of incubation was acid hydrolyzable, as compared to 80–100% of the total remaining labelled organic N. This relationship held throughout the incubation and was independent of the size of the amendment and of the temperature of incubation.During the second, third and fourth year of incubation the half-life of labelled amino acid-N in the soils was longer than the half-life of labelled amino acid-C, presumably due to immobilization reactions. Some of the labelled organic N when mineralized was re-incorporated into organic compounds containing increasing proportions of native soil-C. whereas labelled C when mineralized as CO₂ disappeared from the soils.In general, native C and native organic N were less acid hydrolyzable and were accounted for less in amino acid form than labelled C and N.The amount of labelled amino acid-C, formed during decomposition of the labelled cellulose, and retained in the soil, was proportional to the clay content. This amount was about three times as large in the soil with the highest content of clay as in the soil with the lowest content. This difference between the soils was established during the first 10 days of incubation when biological activity was most intense, and it held throughout the 4 yr of incubation; proportionally it was independent of the amount of cellulose added and the temperature.In contrast, the labelled amino acid-N content was not directly related to the amount of clay in the soil, presumably because more unlabelled soil-N was available for synthesis of metabolic material in the two clay-rich soils than in those soils with less clay. The wider ratio between labelled amino acid-C and labelled amino acid-N in the two clay-rich soils as compared with those obtained with the soils with less clay indicates this.The effect of clay in increasing the content of organic matter in soil is possibly caused by newly synthesized matter, extracellular metabolites, as well as cellular material, forming biostable complexes and aggregates with clay. The higher the concentration of clay the more readily the interactions take place. The presence of clay may also increase the efficiency of using substrate for synthesis.     

Effects of polyacrylamide,biopolymer and biochar on the decomposition of 14C‐labelled maize residues and on their stabilization in soil aggregates

The efficacy of applying plant residues to agricultural soils as a carbon (C) source for microorganisms and C sequestration is dependent on soil physiochemical properties, which can be improved by aggregation using soil conditioners. However, no attempt has been made to assess the effects of soil conditioners such as biochar (BC), biopolymer (BP) or polyacrylamide (PAM) on plant residue decomposition. We assessed the effects of BC, synthesized BP and anionic PAM on the decomposition of ¹⁴C‐labelled maize residues and on their stabilization in aggregate fractions in sandy and sandy loam soils. Polyacrylamide and BP were applied at 400 kg ha^?1 and BC was applied at 5000 kg ha^?1, and the soils were incubated for 80 days at 22°C. The conditioners improved the physical and biological properties of both soils, as shown by a 24% increase in the 1–2 mm aggregates. Biochar and BP accelerated the decomposition of plant residues as indicated by ¹⁴CO₂ efflux, and resulted in reduced stabilization of residues in both soils relative to that observed in the control and PAM treatments. The reduction in ¹⁴C incorporation and C stabilization in the BC‐ and BP‐treated soils was observed mainly in the < 0.25‐mm aggregates. This was confirmed by reduction of activity of hydrolytic enzymes (β‐cellobiosidase and β‐glucosidase). Decomposition of plant residues in sandy soil was more sensitive to BP and PAM application than that in sandy loam soil. Improved soil structure after applying BC and BP increased aeration and decreased the contact between plant residues and mineral soil particles and consequently accelerated plant residue decomposition and reduced C sequestration.     

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

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

Diffusion‐controlled mobilization of water‐dispersible colloids from three German silt loam topsoils: effect of temperature

The effects of time and temperature on the release kinetics of water‐dispersible colloids (WDCs) from three German silt loam topsoils in deionized water were investigated in batch experiments under low‐energy rotating shaking conditions. The measured critical coagulation concentrations of Ca²⁺ and Na⁺ for extracted WDC were much larger than the experimental ionic conditions. This indicates a fast dispersion rate in the first detachment step of WDC mobilization from soil aggregates. The cumulative released WDC fraction F(t) (released WDC/clay content in bulk soil) was satisfactorily fitted to the square root of shaking time by a linear function in three soils with a similar clay content. This implies diffusion‐controlled release kinetics in the second step of the WDC mobilization process. The mobilization kinetics were modelled by considering a diffusion‐controlled transport through an immobile water layer in the macropores of soil aggregates formed by silt and sand particles. The effects of temperature on the mobilization kinetics and sedimentation volumes of saturated soils were compared at 7, 23 and 35°C. A linear correlation was found between immobile water layer thickness in soil macropores (l_t) and the water volume (V_water) in soil sediment, which indicates a strong dependence of l_t on the soil texture. Temperature‐sensitive l_t and V_water influenced the effect of temperature on WDC release, which counteracts the estimated effect of temperature on particle diffusion according to the Stokes‐Einstein relation. A larger decrease in F(t) was found in grassland and forest soils than in an arable soil and can be related to greater stagnant water contents (larger l_t and V_water) in soil macropores, where particulate organic matter and polyvalent cations in their oxide forms at acidic pH will thus contribute to water immobilization.     

The effects of biocidal treatments on metabolism in soil—I. Fumigation with chloroform

This series of five papers is a study of how biocidal treatments influence metabolism in soil, directed particularly towards the flush of decomposition caused by fumigation, and designed to see if the size of this flush can be used as a measure of the soil biomass.Chloroform fumigation caused an immediate increase in the amounts of ammonium and organic C extracted from a soil by 1 N K₂SO₄. When the CHCl₃-treated soil was then inoculated with fresh soil and incubated for 10 days. it consumed 2·8 times more O₂, evolved 2·2 times more CO₂ and mineralised 7·3 times more N than an unfumigated soil. Extractable organic C decreased by about 40% when the fumigated soil was incubated for 10 days. A second fumigation given immediately after the first produced no further increase in the flush, but some recovery occurred if the soil was incubated between fumigations. However, this recovery was slow and incomplete; a second fumigation given 53 days after the first gave a flush only one-seventh the size of the first. Glucose (or ryegrass) added to the soil and allowed to decompose before fumigation increased the size of the flush. After a 52-day incubation, 29% of the C originally added as ¹⁴C labelled glucose remained in the soil; fumigation on the 52nd day increased the evolution of labelled CO₂ during the subsequent 10-day period by a factor of 8. Fumigation of a soil that had already been sterilized by 2·5 Mrads of gamma radiation increased the flush slightly; the amount of O₂ consumed in 10 days increased from 123 to 137 mg/100 g soil. It is proposed that the flush of decomposition following CHCl₃ fumigation is caused by the decomposition of killed organisms by the survivors (or by organisms added in the inoculum) and that organisms are more rapidly and completely attacked after exposure to CHCl₃ than after irradiation. On this hypothesis. 10% of the glucose C originally added to the soil was located in the soil biomass after 52 days.     

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.     

Fixation of carbon dioxide by chemoautotrophic bacteria in grassland soil under dark conditions

Grassland is one of the most important terrestrial ecosystems for carbon (C) and nitrogen (N) cycling. However, while CO₂ fixation by phototrophic bacteria is relatively well studied, little is known about microbial CO₂ fixation without light by chemoautotrophic bacteria in grassland soils. Therefore, in this study, the isotope ¹⁴C-CO₂ was used to investigate the CO₂-fixing process in grassland soils. Soil samples were collected from both fenced and adjacent continuous grazing grassland sites in Inner Mongolia and then incubated for 120 days under dark conditions. Meanwhile, the cbbL genes (red- and green-like) were analyzed to isolate chemoautotrophic bacteria, which are responsible for CO₂ fixation. After incubation, ¹⁴C was fixed into soil organic carbon (¹⁴C-SOC) and microbial biomass carbon (¹⁴C-MBC) were found in both the fenced and grazing soils, and the fixation rate of ¹⁴C-SOC in the fenced soils (48.55‰) was significantly higher than in the grazing soils (22.11‰). The fixation rate of ¹⁴C-MBC in the fenced soils (14.05‰) was higher than in the grazing soils (7.08‰), but the difference was not significant. The red-like cbbL genes could be detected in all the soil samples, but the green-like cbbL genes could not be amplified. A greater number of identified operational taxonomic units were observed in the fenced soils compared with the grazing soils. The chemoautotrophic bacteria were mainly affiliated with Alphaproteobacteria and Actinobacteria. However, Chloroflexi was detected in only the fenced soils. The results suggested that CO₂ fixation by chemoautotrophic bacteria might be significant in carbon cycling in grassland.     

Long-term effects of temperature on carbon mineralisation processes

The long-term effects of temperature on soil C mineralisation were investigated in two experiments using ¹⁴C labelled wheat straw incubated in organic soils from five coniferous forests located in different climate zones of Western Europe. In the first experiment, samples were incubated in the laboratory at 4, 10, 16, 23 or 30°C, with constant moisture, and the loss of ¹⁴C was monitored for 550 days. Double negative exponential functions fitted to the ¹⁴C loss data at different temperatures were used to define the relative proportions of labile and recalcitrant components in the original straw. The estimated proportions of these constituents were related to incubation temperatures with the amount of C reflecting the labile fraction increasing with increasing temperature. In the second experiment samples mixed with the labelled straw were incubated at 4, 16 or 30°C until the same percentage of ¹⁴C loss was reached. The samples were then incubated again at a common temperature for 30 days and CO₂ production was measured to assess the lability of the remaining material. For all the soils, the amount of readily decomposed material was higher in samples conditioned at 4° than at 30°C. It was concluded that in addition to temperature controlling rates of C mineralisation in soil it also affects the processes of decomposition so that material produced at higher temperatures was more recalcitrant than at lower temperatures.     

Seasonal transfers of assimilated 14C in grassland: Plant production and turnover,soil and plant respiration

Six areas of native grassland were labelled with ¹⁴C during a growing season. Transfers from the foliage to the roots and root respiration were measured. Plant production and turnover rates were determined by sampling the labelled material at different periods following exposure to ¹⁴CO₂.Above to beneath ground plant production ratios ranged between 1.1 and 1.9 with maximal translocation to the roots occurring during the drier summer months. The distribution of the photosynthates in the roots at different depths changed with time and soil moisture content. The upper part of the soil (0–10 cm) contained 49–77% of the labelled C found beneath the soil surface. Measurement of transfers with time of the above ground labelled C from living to dead plant and litter categories gave an insight into foliage dynamics and made it possible to estimate the seasonal shoot production at 130g Cm^?2 (1300kg ha^?1). Root growth represented 100g Cm^?2 (1000 kg ha^?1).Calculations of root and soil respiration were based on the CO₂ profiles in the soil. The fluxes of labelled and unlabelled CO₂ at the soil surface were estimated using the diffusion equation method. Respiration by roots and closely associated soil organisms accounted for 12 per cent of the net assimilation of CO₂ by the plants. This proportion was constant throughout the season and represented 19 per cent of the total CO₂ evolved at the soil surface.     

Chloroform fumigation and the release of soil nitrogen: A rapid direct extraction method to measure microbial biomass nitrogen in soil

A new “direct extraction” method for measuring soil microbial biomass nitrogen (biomass N) is described. The new method (fumigation-extraction) is based on CHC1₃ fumigation, followed by immediate extraction with 0.5 M K₂SO₄ and measurement of total N released by CHC1₃ in the soil extracts. The amounts of NH₄-N and total N extracted by K₂SO₄ immediately after fumigation increased with fumigation time up to 5 days. Total N released by CHC1₃ after 1 day fumigation (1 day CHC1₃-N) and after 5 days fumigation (5 day CHC1₃-N) were positively correlated with the flush of mineral N (F_N) in 37 soils that had been fumigated, the fumigant removed and the soils incubated for 10 days (fumigation-incubation). The regression equations were 1 day CHC1₃-N = (0.79 ± 0.022) F_N and 5 day CHC1₃-N = (1.01 ± 0.027) F_N, both regressions accounting for 92% of the variance in the data.In field soils previously treated with ¹⁵N-labelled fertilizer, the amounts of labelled N, measured after fumigation-extraction, were very similar to the amounts of labelled N mineralized during fumigation-incubation; both were about 4 times as heavily labelled as the soil N as a whole. These results suggest that fumigation-extraction and fumigation-incubation both measure the same fraction of the soil organic N (probably the cytoplasmic component of the soil microbial biomass) and that measurement of the total N released by CHC1₃ fumigation for 24 h provides a rapid method for measuring biomass N.