Studies of nitrogen immobilization and mineralization in calcareous soils—V. Formation and distribution of isotope-labelled biomass during decomposition of 14C- and 15N-labelled plant material

Medicago littoralis leaf material, labelled with ¹⁴C and ¹⁵N, and of C:N ratio 8.7:1, decomposed rapidly in a calcareous soil. One half of the plant-C and two thirds of the plant-N remained in the soil as organic residues after 34 days. The rates of decomposition and the changes in the distribution of organic-¹⁴C and -¹⁵N residues followed similar patterns.Incorporation of ¹⁴C and ¹⁵N into microbial cells, formed during plant breakdown, reached a maximum after 62 days. At this time the microbial biomass accounted for 21.9 and 23.3%, respectively, of residual organic-¹⁴C and -¹⁵N. Thereafter, the amounts of isotope-labelled biomass decreased with the percentage decrease slightly exceeding that of the total labelled soil residues.During plant decomposition, changes occurred in the concentrations of organic-¹⁴C and -¹⁵N in some of the soil components, these having been fractionated according to density and particle size. Especially evident was the rapid and extensive decrease of labelled material from the fine clay-size components. This was partly due to the decrease in the biomass-¹⁴C of this fraction. Changes in biomass-¹⁴C of some physical fractions were approximately reflected by changes in their numbers of viable microorganisms.     

Decomposition of 14C- and 15N-labelled microbial cells in soil

To obtain detailed information on the quantities and characteristics of nitrogen derived from mineralizing dead microbial biomass in soil, ¹⁴C- and ¹⁵N-labelled microorganisms, i.e. three eukaryotic (fungal) species, two prokaryotic species or their mixture (eukaryotic to prokaryotic cells = 8:2), were grown in vitro, dried, ground and added to parabrown earth and chernozem soils, respectively. The mean percent of ¹⁴C decomposition of labelled microorganisms obtained after 10 days was 43 ± 6.3% for parabrown earth and 34 ± 4.0% for chernozem soil. About 50% of the C in the dead microorganisms was mineralized during the first 28 days of incubation. About 76% of the flush of soil organic N mineralization within 28 days, which was caused by the drying-rewetting treatment, was derived from dead microbial biomass in soil. About 33% of the added dead microbial-¹⁵N was mineralized in parabrown earth soil during 28 days of incubation and about 37% of newly immobilized ¹⁵N during the decomposition of added microorganisms was mineralized during the 28 days following a dryingrewetting treatment.     

Studies of nitrogen immobilization and mineralization in calcareous soils—II: Mineralization of immobilized nitrogen from soil fractions of different particle size and density

¹⁵NO^?₃ was immobilized in a calcareous sandy soil and a calcareous clay soil each incubated with glucose and wheat straw. Net mineralization of organic-¹⁵N was more rapid in the sandy soil, irrespective of C amendment, and in soils amended with glucose. Intermittent drying and wetting of soils during incubation stimulated mineralization of ¹⁵N-labelled and native soil organic-N in all treatments. The availability (percentage mineralization) of recently-immobilized ¹⁵N consistently exceeded that of the native soil N. Ratios of the availability of labelled and unlabelled N were similar in the sandy and clay soils but varied according to C amendment, drying and wetting cycle and incubation period.Changes in the distribution of immobilized N amongst soil extracts and soil fractions of different particle size and density were determined during periods of net N mineralization. In straw-amended soils, the organic-¹⁵N of a light fraction, sp.gr. < 1.59, decomposed relatively rapidly during the late mineralization period. Decreases of organic ¹⁵N of the fine clay fraction were also recorded. In glucose-amended soils, net N mineralization was accompanied by significant decreases in the concentrations of organic-¹⁵N of the silt and fine clay fractions.Drying and rewetting of soils hastened or magnified changes occurring in the organic-¹⁵N of soil fractions, but qualitatively, the pattern of change was similar to that observed with soils incubated under uniformly-moist conditions.The percentage distribution of labelled and unlabelled N suggested that in the long term, the silt fraction will accumulate an increasing proportion of the more stable nitrogenous residues.     

Near infrared reflectance spectroscopy for determination of organic matter fractions including microbial biomass in coniferous forest soils

The objective of this work was to investigate the usefulness of near infrared reflectance spectroscopy (NIRS) in determining some C and N fractions of soils: labile compounds, microbial biomass, compounds derived from added ¹³C- and ¹⁵N-labelled straw. Soil samples were obtained from a previous experiment where soils were labelled by addition of ¹³C- and ¹⁵N-labelled wheat straw and incubated in coniferous forests in northern Sweden (64-60°N) and south France (43°N). The incubation lasted three years with 7-9 samplings at regular time steps and four replicates at each sampling (204 samples). Samples were scanned using a near infrared reflectance spectrophotometer (NIRSystem 6500). Calibrations were obtained by using a modified partial least squares regression technique with reference data on total C and N, ¹³C, ¹⁵N, control extract-C, -N, -¹³C and -¹⁵N, fumigated extract-C, -N, -¹³C and -¹⁵N, biomass-C, -N, -¹³C and -¹⁵N contents. Mathematical treatments of the absorbance data were first or second derivative with a gap from 4 to 10 nm. The standard error of calibration (SEC)-to-standard deviation of the reference measurements ratio was ≤0.2 for 10 models, namely total C and N, ¹³C, ¹⁵N, control extract-C, fumigated extract-C and -N, biomass-C and -N and biomass-¹⁵N models and therefore considered as very good. With an R²=0.955, the fumigated extract-¹⁵N model is also good. The standard error of performance calculated on the independent set of data and SEC were within 20% of each other for all the best equations except for the biomass-¹⁵N model. The ability of NIRS to detect ¹³C and ¹⁵N in total C and N and in the extracts is noteworthy, not because of its predictive function that is not really of interest in this case, but because it indicates that the spectra kept the signature of the properties of the organic matter derived from the straw even after two- or three-year decomposition. The incorporation of the ¹³C in the biomass was less well predicted than that of the ¹⁵N. This could indicate that the biomass derived from the straw was characterised by a particular protein or amino acid composition compared to the total biomass that includes a large proportion of dormant micro-organisms. The predictive ability of NIRS for microbial biomass-C and -N is particularly interesting because the conventional analyses are time consuming. In addition, NIRS allows detecting analytical errors.     

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.     

14C-labelled material leached from the rhizosphere of plants supplied continuously with 14CO2

¹⁴C, supplied continuously to plant tops as ¹⁴CO₂, was recovered in water-soluble organic material when pots with wheat, clover or ryegrass growing in a podzolic sand were leached with distilled water at weekly intervals. Leachates and root-free soil contained, respectively, 0·15–0·3 and 2·7–5·4% of the total ¹⁴C activity recovered after 8 weeks growth. Plant derived C represented 0·8–1·3% of the total organic C in root-free soils.Water-soluble organic C decreased in successive leachings to reach a steady value, approximately 20 μgC/ml for all treatments. Labelled C represented 14·4–19·5% of this value. Total organic C recovered in the leachates accounted for ca. 0·5% of the soil C, for all treatments. Approximately 15% of the labelled material in the final leachates behaved as neutral sugar, the remainder occurring in a charged complex. A membrane filter (M.Wt. cut-off ~ 10³) retained >60% of the radioactivity.     

Nitrogen in particle size fractions of soils incubated for five years with 15N-ammonium and 14C-hemicellulose

Four soils with a range of clay and silt contents were incubated for 5 a with ¹⁵N-labelled (NH₄)SO₄ and ¹⁴C-labelled hemicellulose and then fractionated according to particle size by ultrasonic dispersion and sedimentation. The distribution of labelled and native N between clay, silt and sand fractions was determined and elated to previous results on the C distributions. Between 29% and 48% of the added N was found in organic form. The ¹⁵N atom percentage excess decreased in the order: clay > whole soil > silt > sand. For both clay and silt, the enrichment factor for labelled and native N decreased with increasing fraction weight. Clay enrichment was higher for labelled than for native N, the converse being true for silt. The distribution of whole soil labelled organic N was: clay 77–91%, silt 4–11%, and sand <0.5%. Corresponding values for native N were 69–74%, 16–22%, and 1–2%, respectively. All soils had higher proportions of labelled than of native N in the clay, the converse was true for the silt. The C/N ratio of the native silt organic matter was higher and that of clay organic matter lower than whole soil C/N ratios. Differences between the C/N ratio distributions of native and labelled organic matter were small. The relative distribution of labelled N and C was very similar confirming that the turnover of C and N in soil organic matter is closely interrelated.     

Decomposition of 15N-labelled catch-crop residues in soil: evaluation of N mineralization and plant-N uptake potentials under controlled conditions

The decomposition of ¹⁵N-labelled catch-crop materials (rape, radish and rye), obtained from field experiments, was studied in a chalky Champagne soil during a 60-week incubation at 28°C. Mineralized N was assumed to come from either labile or recalcitrant fractions of plant residues. The labile fraction represented about one-third of the catch-crop N; its mineralization rate constant varied from 0.06 to 0.12 d^?1. The decomposition rate of the recalcitrant N fraction ranged from 0.03 × 10^?2 to 0.06 × 10^?2 d^?1. Catch-crop species and rate of incorporation had no effect on N residue mineralized at the end of incubation. The decomposition of labelled rye was monitored in the same soil during a 5-month pot experiment to determine the N availability to an Italian ryegrass crop and the effect of plants on the decomposition processes. The ¹⁵N-rye decomposed rapidly both in the presence or absence of Italian ryegrass, but the amounts of N mineralized were influenced by the presence of living roots: 42% of the ¹⁵N in labelled rye was present as inorganic N in the pots without plants after 5 months, compared with only 32% in the ryegrass crop. Comparison of microbial-biomass dynamics in both treatments suggested that there had been preferential utilization by soil micro-organisms of materials released from the living roots than the labelled plant residues.     

Priming effect of small glucose additions to 14C-labelled soil

Soil was freed of its organic matter by heating it to 400°C. Plants were grown in a ¹⁴CO₂ atmosphere and from them a labelled “soil organic matter” (humus) was prepared by composting the plant material for more than 3 yr in the modified soil under laboratory conditions. The influence of small additions of unlabelled glucose on the decomposition of the labelled soil organic matter was studied. Shortly after the addition of glucose there was a small extra evolution of ¹⁴CO₂, which lasted about 1 day. It is claimed that the extra evolution of ¹⁴CO₂ was caused by conversion of labelled material in the living biomass and was not due to a real priming action, i.e. an accelerated decomposition of humic substances or dead cellular material.     

Stabilization and plant uptake of N from N-labelled pea residue 16.5 years after incorporation in soil

The decline of N from ¹⁵N-labelled mature pea residues was followed in unplanted soil over 16.5 yr. Eight years after residue incorporation, 24% of the residue ¹⁵N input was still present in the soil and, after 16.5 yr, 16% of the residue ¹⁵N input remained. A double exponential model successfully described the decay of N from ¹⁵N-labelled pea residues. The total residual ¹⁵N declined with average decay constants of 1.45 yr⁻¹ for the 30 d to 1 yr period and of 0.07 yr⁻¹ for the 1-16 yr period. Sixteen years following incorporation of the residues, indicator plants growing in residues-amended soils were obtaining 1.7% of their N from residue N. This is, to our knowledge, the longest study on decay of N in soils from ¹⁵N-labelled crop residues. The current study thus provides a unique data set for our empirical understanding of N-dynamics in agricultural systems, which is a prerequisite to parameterize and validate N-simulation models.     

利用15NO3-标记法研究土壤微生物量氮的化学及生物有效性

采用加入含15N的硝态氮培养方法标记了土壤微生物量氮 ,然后利用碱解扩散法测定了标记土壤有效氮含量 ,温室培养法评价了小麦对标记的土壤微生物量氮的吸收情况。结果表明 ,碱解扩散法对土壤微生物量固持的15N的提取比率 (即提取液中15N原子百分超 /土壤15N原子百分超 )在 1 47～ 2 83之间(平均 2 0 1 ) ,碱解氮中约有 3 0 1 %～ 61 6% (平均 42 9% )来自土壤微生物固持氮。植物体15N丰度在0 749%～ 1 1 62 %之间 ,明显高于15N的自然丰度 ,表明土壤微生物固持的15N在小麦生长期间发生释放 ,为植物利用。土壤微生物固持氮对植物的有效性比率 (植物地上部分15N原子百分超 /土壤15N原子百分超 )在 3 7～ 7 1之间。可见 ,土壤微生物量固持氮有较高的化学及生物有效性     

Simulating the dynamics of glucose and NH4+ in alkaline saline soils of the former Lake Texcoco with the Detran model

The turnover of organic matter determines the availability of plant nutrients in unfertilized soils, and this applies particularly to the alkaline saline soil of the former Lake Texcoco in Mexico. We investigated the effects of alkalinity and salinity on dynamics of organic material and inorganic N added to the soil. Glucose labelled with ¹⁴C was added to soil of the former Lake Texcoco drained for different lengths of time, and dynamics of ¹⁴C, C and N were investigated with the Detran model. Soil was sampled from an undrained plot and from three drained for 1, 5 and 8 years, amended with 1000 mg ¹⁴C‐labelled glucose kg^?1 and 200 mg NH₄⁺‐N kg^?1, and incubated aerobically. Production of ¹⁴CO₂ and CO₂, dynamics of NH₄⁺, NO₂^– and NO₃^–, and microbial biomass ¹⁴C, C and N were monitored and simulated with the Detran model. A third stable microbial biomass fraction had to be introduced in the model to simulate the dynamics of glucose, because > 90 mg ¹⁴C kg^?1 soil persisted in the soil microbial biomass after 97 days. The observed priming effect was mostly due to an increased decay of soil organic matter, but an increased turnover of the microbial biomass C contributed somewhat to the phenomenon. The dynamics of NH₄⁺ and NO₃^– in the NH₄⁺‐amended soil could not be simulated unless an immobilization of NH₄⁺ into the microbial biomass occurred in the first day of the incubation without an immediate incorporation of it into microbial organic material. The dynamics of C and a priming effect could be simulated satisfactorily, but the model had to be adjusted to simulate the dynamics of N when NH₄⁺ was added to alkaline saline soils.     

Factors controlling decomposition of soil organic matter in fallow systems of the high tropical Andes: A field simulation approach using C- and N-labelled plant material

N-rich (C:N=27) and N-poor (C:N=130) wheat straw, labelled with ¹⁴C and ¹⁵N, was incubated for 2 yr in two major ecosystems of the upper elevation belt of cultivation in the high Andes: the moist Paramo (precipitation=1329 mm, altitude=3400 m asl, Andes of Merida, Venezuela) and the dry Puna (precipitation=370 mm, altitude=3800 m asl, Central Altiplano, Bolivia). The experiment was installed in young (2 yr) and old (7 yr) fallow plots. The following soil analyses were performed at nine sampling occasions: soil moisture, total-¹⁴C and -¹⁵N, and Microbial Biomass (MB)-¹⁴C and -¹⁵N. The measured data were fitted by the MOMOS-6 model (a process based model, with five compartments: labile and stable plant material, MB, and labile (HL) and stable humus (HS)) coupled with the SAHEL model (soil moisture prediction) using daily measured and/or predicted meteorological data. The aim was to understand how (1) the climatic conditions, (2) the quality of plant material, (3) the fallow age and (4) the soil properties affect the cycling of C and N within the soil organic matter system.The fallow age (2 and 7 yr) did not affect the measured data or the model predictions, indicating that in these systems the decomposition potential is not affected by fallow length. During the short initial active decomposition phase, the labile plant material was quickly exhausted, enabling a build up of MB and of HL. During the low activity phase, that covered 4/5 of the time of exposure, the MB size decreased slowly and the HL pool was progressively exhausted as it was reused by the MB as substrate. The HL compartment was directly or indirectly the major source for the inorganic ¹⁵N production. If the C:N ratio of the added plant material increased, the model predicted (1) a reduction of the decomposition rates of the plant material (essentially the stable plant material) and (2) an increased mortality of the MB which increased the production of HL (microbial cadavers and metabolites). Thus the essential effect of the slower decomposition due to the N-poor plant material was a higher accumulation of C and N in the HL and its slower recycling by the MB during the low activity phase. The labelling experiment allows to understand the higher soil native organic matter content in Paramo soils compared to Puna. The large sequestration of organic matter generally observed in the Paramo soils can be explained by two abiotic factors: the unfavourable soil microstructure and the accumulation of free aluminium linked to the climatic and acid soil conditions, inhibiting the microbial activity physically and chemically.     

Nitrogen immobilization and mineralization during initial decomposition of15N-labelled pea and barley residues

The immobilization and mineralization of N following plant residue incorporation were studied in a sandy loam soil using¹⁵N-labelled field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) straw. Both crop residues caused a net immobilization of soil-derived inorganic N during the complete incubation period of 84 days. The maximum rate of N immobilization was found to 12 and 18 mg soil-derived N g^–1 added C after incorporation of pea and barley residues, respectively. After 7 days of incubation, 21% of the pea and 17% of the barley residue N were assimilated by the soil microbial biomass. A comparison of the¹⁵N enrichments of the soil organic N and the newly formed biomass N pools indicated that either residue N may have been assimilated directly by the microbial biomass without entering the soil inorganic N pool or the biomass had a higher preference for mineralized ammonium than for soil-derived nitrate already present in the soil. In the barley residue treatment, the microbial biomass N was apparently stabilized to a higher degree than the biomass N in the pea residue treatment, which declined during the incubation period. This was probably due to N-deficiency delaying the decomposition of the barley residue. The net mineralization of residue-derived N was 2% in the barley and 22% in the pea residue treatment after 84 days of incubation. The results demonstrated that even if crop residues have a relative low C/N ratio (15), transient immobilization of soil N in the microbial biomass may contribute to improved conservation of soil N sources.     

Response of microbial biomass to alternate moist and dry conditions in a soil incubated with 14C- and 15N-labelled plant material

A red mediterranean soil was incubated for 1.5 yr, with ¹⁴C- and ¹⁵N-labelled plant material under constant temperature and moisture conditions. Then a portion of the soil was submitted to 4 rapid drying (at 40°C) and rewetting cycles. The duration of the dry periods ranged from 8 to 10 days and the wet periods from 15 to 20 days. Another portion of the soil was incubated under continuously moist conditions. At the end of each dry and moist period, biomass-C and -N were estimated, using the chloroform fumigation technique. The portion of biomass killed on drying and that restored after rewetting were calculated, by the difference between the sizes of biomass present after the dry and the moist periods.Soil drying destroyed $\text{1}\text{3}$ to $\text{1}\text{4}$ of biomass and at each cycle, after remoistening, the biomass was progressively restored to approximately the same size as before drying.The labelled-C to total-C ratio of the CO₂ released from undisturbed and continuously moist soil, ranged from 6 to 7%. In biomass, which survived the drying, the values ranged from 20 to 22%, whereas in the killed biomass they ranged between 7 and 8%, i.e. the same orders of magnitude as that of CO₂ evolved from undisturbed soil.A comparison of the labelled-C to total-C ratio of (1) CO₂C released from undisturbed and continuously moist soil, (2) the extra CO₂-C evolved as a result of alternate drying-remoistening conditions, (3) CO₂C released from the soil at the end of moist periods, (4) the C of microbial biomass which survived the drying, and (5) the C of the biomass present at the end of the moist periods, revealed that the calculated portion of biomass killed on drying essentially corresponded to a still relatively “active” fraction of biomass and that the biomass surviving rapid drying, essentially corresponded to a dormant and protected fraction.In contrast to the labelled-C to total-C ratio, the labelled-N to total-N ratio, in the fraction of biomass which was destroyed on drying, was not different from that of the surviving fraction. During incubation, labelled nitrates accumulated progressively in soil; transformations of N were probably affected by a remetabolization of the nitrates and of material made decomposable on drying, including destroyed microflora and non-biomass material.     

Influence of the rhizosphere on microbial biomass and recently formed organic matter

We have aimed to quantify the effect of roots on the size of the soil microbial biomass, and their influence on the turnover of soil organic matter and on the extent of the rhizosphere. We sampled a sandy clay loam topsoil from two subplots with different treatment histories. One had a normal arable fertilization record, the other had received only inorganic nitrogen fertilizer but no phosphorus and potassium for 30 years. Glucose labelled with ¹⁴C was added to both samples which were then incubated for 4 weeks before the soil was packed in cylinders and planted with ryegrass. In both soils, microbial biomass at the root surface doubled during the first 8 days. At day 15, the microbial biomass had further increased in the fertile soil, and the rhizosphere effect had extended 2.5 mm into the fertile soil, but to only 1 mm in the infertile soil. The microbial ¹⁴C increased threefold near the roots in the fertile soil as a result of assimilation of previously formed microbial residues, but in the infertile soil there was no increase. There was a close relation between the increase in microbial ¹⁴C and a decrease in ¹⁴C soluble in 2 m KCl, indicating that the microbial residues were more weakly adsorbed in the fertile soil. We conclude that the increased microbial population living near the root surfaces re‐assimilated part of the ¹⁴C‐labelled microbial residues in the fertile soil. In the infertile soil, microbial residues resisted decomposition because they were more strongly sorbed on to soil surfaces.     

Fate of carbon and nitrogen in water-stable aggregates during decomposition of 13C15N-labelled wheat straw in situ

When incorporated in soil, plant residues and their decomposition products are in close contact with mineral particles with which they can be bound to form aggregates. We measured the incorporation of carbon (C) and nitrogen (N) derived from crop residues in water-stable aggregate fractions of a silty soil in a field experiment in Northern France using ¹³C¹⁵N-labelled wheat straw (Triticum aestivum L.). Soil samples were taken seven times for 18 months and separated into slaking-resistant aggregate size fractions which were analysed for total C and N contents, and ¹³C and ¹⁵N enrichments. During the early stages of decomposition (approximately 200 days), the enrichment of ¹³C increased rapidly in the macro aggregates (> 250 pm) but decreased thereafter. The macro aggregates represented only < 20% of the soil mass and at any one time, they accounted for <25% of the residual ¹³C in the soil. The proportion of ¹³C recovered in the <50-μm and 50–250-μm fractions increased during decomposition of the residues; at day 574, the 50–250-μm fraction accounted for close to 50% of the residual ¹³C. A greater proportion of ¹⁵N than ¹³C was recovered in the <50-μm fraction. The results indicate that during decomposition in soil, C and N from crop residues become rapidly associated with stable aggregates. In this silty soil the 50–250-μm stable aggregates appear to be involved in the storage and stabilization of C from residues.     

Organic matter and microbial biomass in a soil incubated in the field for 20 years with 14C-labelled barley straw

¹⁴C-labelled barley straw was incubated under field conditions in a sandy soil. After 8 yr, 16% of the labelled C added remained in the soil; after 20 yr 9.3%. During the 8 to 20 yr period the labelled organic matter in the soil decayed at a rate corresponding to a half life of 15 yr. The percentage of residual labelled C in amino acids remained almost constant during the period, being on average 21%. The soil contained 1.9% native C in organic matter; during the 8 to 20 yr period this decayed at a rate corresponding to a half life of 91 yr. The percentage of native C in amino acids increased significantly, from about 14% to about 16%.During the 8 to 20 yr period, microbial biomass was determined yearly by the chloroform fumigation technique. The proportion of total labelled C in biomass remained nearly constant during this period, on average 2.7%. Labelled C in biomass decreased at a rate corresponding to a half life of 8 yr. The native biomass increased during the same period from 0.7 to 1.4% of the total native soil C.As measured by CO₂ produced during periods of 3 months, laboratory incubation increased the rate of decay of the labelled organic matter by a factor of 1.2 and that of the native organic matter by a factor of 4.3.     

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.     

Microbial community changes during humification of 14C-labelled maize straw in heat-treated and native Orthic Luvisol

The microbial communities in agricultural soils are responsible for nutrient cycling and thus for maintaining soil fertility. However, there is still a considerable lack of knowledge on anthropogenic impacts on soils, their microflora, and the associated nutrient cycles. In this microcosm study, microorganisms involved in the conversion of crop residues were investigated by means of classical microbiological and molecular methods such as denaturing gradient gel electrophoresis (DGGE) of PCR (polymerase chain reaction) amplified 16S rRNA genes. ¹⁴C‐labelled maize straw was humified by the naturally occurring microflora in native and in ashed soils, from which organic carbon was removed by heating at 600°C. The humic acids synthesized in the microcosms served as indicators of the humification process and were analysed by ¹³C‐NMR spectroscopy. Ashed, autoclaved and native soil exhibited similar microbial and physicochemical dynamics after inoculation with a soil suspension. Bacterial counts and DGGE analyses showed that in the first few weeks a small number of rapidly growing r‐strategists were principally responsible for the conversion of maize straw. As the incubation continued, the bacterial diversity increased as well as the fungal biomass. ¹³C‐NMR spectroscopy of 26‐week old soil extracts revealed that structures typical of humic substances also evolved from the plant material.