Abbau von Huminstoffen durch Bodenmikroorganismen — eine Übersicht

Degradation of humic substances by soil microorganisms — a review Humic substances which represent differently extractable fractions of the soil organic matter exert multifarious effects on soil as a site for plant growth and a part of terrestrial environments. Among them especially humic acids and fulvic acids are subject to degradation and/or transformation by soil microorganisms. Several authors demonstrated the participation of different species of fungi, actinomycetes and also of non-mycelial aerobic or anaerobic bacteria in those processes under laboratory conditions. Indications exist that humic substances irrespective of their structure undergo degradation on cell surfaces due to the activity of exoenzymes. In this respect microbial phenoloxidases play an extraordinarily important role. The degradation rate of humic substances can be followed by optical, gravimetric and chemoanalytical methods as well as using biochemical and microbiological procedures (CO₂ release, microbial growth, biomass formation). An objective evaluation, however, can be hindered by the adsorption of humic substances on microbial biomass and sometimes also by formation of novel humic-like microbial metabolites. Therefore it is necessary to apply a multifactorial approach in the study of the degradation of humic substances which includes both quantitative and qualitative parameters. To better elucidate how these processes may occur under natural conditions, mixed populations of soil microorganisms should be predominantly involved in future studies.     

Decomposition and distribution of residual activity of some 13C-microbial polysaccharides and cells,glucose, cellulose and wheat straw in soil

Studies were made to determine the rate of decomposition of some ¹⁴C-labeled microbial polysaccharides, microbial cells, glucose, cellulose and wheat straw in soil, the distribution of the residual ¹⁴C in various humic fractions and the influence of the microbial products on the decomposition of plant residues in soil. During 16 weeks from 32 to 86 per cent of the C of added bacterial polysaccharides had evolved as ¹⁴CO₂. Chromobacterium violaceum polysaccharide was most resistant and Leuconostoc dextranicus polysaccharide least resistant. In general the polysaccharides, microbial cells, and glucose exerted little effect on the decomposition of the plant products. Upon incubation the ¹⁴C-activity was quickly distributed in the humic. fulvic and extracted soil fractions. The pattern of distribution depended upon the amendment and the degree of decomposition. The distribution was most uniform in the highly decomposed amendments. After 16 weeks the bulk of the residual activity from Azotobacter indicus polysaccharide remained in the NaOH extracted soil. From C. violaceum polysaccharide both the extracted soil and the humic acid fraction contained high activity. About 50–80 per cent of the residual activity from the ¹⁴C-glucose, cellulose and wheat straw amended soils could be removed by hydrolysis with 6 n HCl. The greater part of this activity in the humic acid fraction was associated with the amino acids and that from the fulvic acids and residual soils after NaOH extraction with the carbohydrates. About 8 16 per cent of the activity of the humic acid fraction was present in substances (probably aromatic) extracted by ether after reductive or oxidative degradation.     

Transformation of organic matter from maize residues into labile and humic fractions of three European soils as revealed by 13C distribution and CPMAS-NMR spectra

The dynamics of incorporation of fresh organic residues into the various fractions of soil organic matter have yet to be clarified in terms of chemical structures and mechanisms involved. We studied by ¹³C‐dilution analysis and CPMAS‐¹³C‐NMR spectroscopy the distribution of organic carbon from mixed or mulched maize residues into specific defined fractions such as carbohydrates and humic fractions isolated by selective extractants in a year‐long incubation of three European soils. The contents of carbohydrates in soil particle size fractions and relative δ¹³C values showed no retention of carbohydrates from maize but rather decomposition of those from native organic matter in the soil. By contrast, CPMAS‐¹³C‐NMR spectra of humic (HA) and fulvic acids (FA) extracted by alkaline solution generally indicated the transfer of maize C (mostly carbohydrates and peptides) into humic materials, whereas spectra of organic matter extracted with an acetone solution (HE) indicated solubilization of an aliphatic‐rich, hydrophobic fraction that seemed not to contain any C from maize. The abundance of ¹³C showed that all humic fractions behaved as a sink for C from maize residues but the FA fraction was related to the turnover of fresh organic matter more than the HA. Removal of hydrophobic components from incubated soils by acetone solution allowed a subsequent extraction of HA and, especially, FA still containing much C from maize. The combination of isotopic measurements and NMR spectra indicated that while hydrophilic compounds from maize were retained in HA and FA, hydrophobic components in the HE fraction had chemical features similar to those of humin. Our results show that the organic compounds released in soils by mineralization of fresh plant residues are stored mainly in the hydrophilic fraction of humic substances which are, in turn, stabilized against microbial degradation by the most hydrophobic humic matter. Our findings suggest that native soil humic substances contribute to the accumulation of new organic matter in soils.     

Studies on bound (14)C-chlorsulfuron residues in soil

The cause for phytotoxicity of bound residues of chlorsulfuron (2-chloro-N-[[4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl]benzenesulfonamide) to rotational crops is unknown. This study was conducted to determine the formation of nonextractable (bound) residues of chlorsulfuron in soil, and the distribution of bound residues in different organic matter fractions. The results showed that over 150 days, the extractable fraction of (14)C-residues decreased to 25.1% of applied chlorsulfuron, while bound residues concurrently increased to 47.1%. The distribution of (14)C-bound residues in soil organic matter fractions followed an order of humic acid (HA) < humin < fulvic acid (FA). Although the most bound residues were detected in the FA fraction, the amount associated with the humin fraction increased with time. After soil treatment by autoclaving, it was found that bound (14)C-chlorsulfuron residues became available again in the soil. One of the released products was 2-amino-4-hydroxyl-6-methyl-1,3,5-triazine (identified by GC-MS), which is a degradation product of chlorsulfuron.     

Degradation and humification of maize straw in soil microcosms inoculated with simple and complex microbial communities

Microbial communities are responsible for soil organic matter cycling and thus for maintaining soil fertility. A typical Orthic Luvisol was freed from organic carbon by thermal destruction at 600°C. Then the degradation and humification of ¹⁴C‐labelled maize straw by defined microbial communities was analysed. To study the role of microbial diversity on the humification of plant material, microcosms containing sterilized soil were inoculated with a natural microbial community or with microbial consortia consisting of bacterial and fungal soil isolates. Within 6 weeks, 41 ± 4% of applied ¹⁴C‐labelled maize straw was mineralized in the soil microcosms containing complex communities derived from a soil suspension, whilst the most efficient communities composed of soil isolates mineralized less than 35%. The humification products were analysed by solution state ¹³C‐NMR‐spectroscopy and gel permeation chromatography (GPC). The analyses of humic acids extracts by solution state ¹³C‐NMR‐spectroscopy revealed no difference in the development of typical chemical functional groups for humic substances during incubation. However, the increase in specific molecular size fractions of the extracted humic acids occurred only after inoculation with complex communities, but not with defined isolates. While it seems to be true that redundancy in soil microbial communities contributes to the resilience of soils, specific soil functions may no longer be performed if a microbial community is harshly affected in its diversity or growth conditions.     

Selective digestion of the peptide and polysaccharide components of synthetic humic acids by the humivorous larva of Pachnoda ephippiata (Coleoptera: Scarabaeidae)

In order to identify the potential nutrient and energy sources of humivorous beetle larvae, we carried out feeding trials with soil supplemented with specifically ¹⁴C-labeled model humic acids synthesized by peroxidase-initiated radical polymerization, using the cetoniid beetle Pachnoda ephippiata (Coleoptera: Scarabaeidae) as a model organism. Ingestion of soil by the larvae significantly increased the mineralization of humic acids labeled in their peptide (HA-*peptide) or polysaccharide components (HA-*peptidoglycan and HA-*chitin), whereas the mineralization of humic acids labeled in the aromatic components (HA-*catechol) did not increase significantly. Mineralization was accompanied by a reduction of residual radiolabel in the acid-soluble fraction and an increase in the humic acid and humin fractions of the fecal pellets. During the gut passage, the residual label in peptide or polysaccharide components was transformed into acid-soluble products, especially in the alkaline midgut. High-performance gel-permeation chromatography demonstrated that the changes in solubility were accompanied by large changes in the molecular weight of the residual material. The amount of radiolabel derived from the peptide and polysaccharide components recovered from the larval body and hemolymph was significantly higher than that derived from the aromatic component, which supports the hypothesis that humivorous beetle larvae selectively digest the peptide and polysaccharide components of humic substances, whereas the aromatic components of humic substances are not an important source of nutrients and energy. This is also the first experimental evidence that also chitin and peptidoglycan, the major structural polymers in fungal and bacterial biomass, can be protected from microbial degradation in soil by a copolymerization with phenols and might contribute substantially to the refractory nitrogen pool in soil organic matter.     

Changes in the composition and properties of humic acids under the influence of microorganisms

By gel filtration on Sephadex G-50 and G-75, we investigated the changes in the composition and ratio of the molecular fractions of humic acids under the influence of native soil microbial complexes. The highest degree of decomposition of the initial humic acid was observed when ammonium nitrate was used as a source of additional energy in the mineral nutrient medium without a yeast extract. The appearance of a low molecular fraction of organic substances on chromatograms may be associated with the synthesis by bacteria of simple organic substances on the medium with humic acids as a single source of carbon.     

Depth-related variations in organic matter at the molecular level in a loamy soil: reference data for a long-term experiment devoted to the carbon sequestration research field

Three lipid fractions, namely the freely extractable fraction and those associated with humin and humic acid fractions, were obtained from the loamy soil of a carefully maintained long-term experiment located on Deffend ORE field, Poitiers, France. The analyses showed differences in molecular distribution, suggesting different sources and diagenetic states of the source material. Despite a major input of plant material to the soil organic matter, intensive bacterial activity was suspected. Most distributions suggested an increase in the microbial/terrestrial lipid ratio from the free to humin to humic fractions. Molecular evidence of fungal activity, especially in the top layer, was also found in the distributions of n -alkanes and n -alkanoic acids. In the surface horizon A₁ alkanes were the major compounds, followed by n -alkanoic acids and sterols. The degraded horizons, poorer in organic matter, i.e. the A₂ and B horizons, were dominated by long-chain (>C₂₀) n -alkanoic acids with a strong even-over-odd predominance and C₂₄ and C₂₆ n -alkanols. Sterols had been removed from these horizons through degradation. A comparison of humic acid and humin composition on the basis of Py(methylation)-GC-MS showed that the two fractions produce partly similar pyrolysis products. Most prominent were molecules from plant and microbial carbohydrates, lignin building blocks and linear aliphatic (carboxylic acids, esters) and nitrogen compounds. The investigation showed that while low-molecular-weight soil lipids were highly dominated by compounds derived from the overlying vegetation, pyrolysis data from the corresponding high-molecular-weight fractions reflected the incorporation of microbial biomarkers into the humic-type fractions.     

土壤中14C-甲磺隆存在形态的动态研究

利用同位素示踪技术 ,在实验室条件下研究了1 4 C -甲磺隆在 1 5种不同土壤中存在形态的动态变化。结果表明 ,土壤pH值与甲磺隆1 4 C残留物的降解半衰期、残留量及可提取态残留量呈显著的正相关 ,而与结合态残留量呈显著负相关 ;土壤微生物的活性越强 ,甲磺隆降解速率越快 ,但结合态残留量也越高 ;土壤中各腐殖质组分和粘粒的含量也影响甲磺隆在土壤中的降解速率和存在形态。土壤中甲磺隆的残留符合一级反应动力学指数方程C =C0 e-kt,拟合方程的复相关系数达到极显著水平。甲磺隆残留与土壤性质之间经逐步回归分析可得到拟合效果较好的方程 ,由各自变量的决定系数可知 ,土壤pH值、微生物生物量碳和有机碳中富啡酸碳所占的比例是影响甲磺隆在土壤中残留的主要因素     

Chemical studies on soil humic acids

Seventeen samples of soil humic acids, two fractions of soil fulvic acid sample, and several related compounds such as lignin, tannin, flavonoid and artificial humic substances were decomposed in conc. KOH solution at 180°C. Succinic acid, glutaric acid, phloroglucin, p-hydroxybenzoic acid, vanillic acid, protocatechuic acid, 3,4-dihydroxy-5-methoxybenzoic acid, and gallic acid were detected in the degradation products of humic acids. The amounts of these degradation products were discussed in relation to the degree of humification or the sources of the humic acid samples. Succinic acid also resulted from glucose, polymaleic acid, and the humic acid and humin prepared from glucose, but glutaric acid resulted only from glucose humic acid and glucose humin but not from glucose and polymaleic acid. Succinic acid and glutaric acid were supposed to result from the same structural portions in humic acids because of the very significant positive linear correlation between their amounts. p-Hydroxybenzoic, vanillic, protocatechuic, and 3,4-dihydroxy-S-methoxybenzoic acids were presumed to result mainly from lignin structure in humic acids. Soil humic acids yielded small amounts of gallic acid although the yields by hydrolysable tannins were in large amounts. The yields of above-mentioned degradation products from humic acids decreased with increasing degree of humification. Phloroglucin resulting from ftavonoids including condensed tannins were also found in the degradation products of humic substances. Its yield showed no linear correlation with RF value of humic acid, and is presumed to be rather related to the vegetation at the sites of soil sampling.     

Changes in the Content,Composition, and Properties of Humic Substances in Particle-Size Fractions of Soddy-Podzolic Soils under the Impact of Long-Term Drainage

Specific features of the transformation of humic substances in particle-size fractions of drained soddy-podzolic soils were studied on a field (12 ha) of the Experimental and Educational Center of Lomonosov Moscow State University in Moscow oblast. The field had a clearly pronounced microtopography. Surface-gleyed soddy-podzolic soils (Albic Stagnic Glossic Retisols (Loamic, Aric, Ochric)) of microdepressions with excessive surface moistening and nongleyed soddy-podzolic soils (Albic Glossic Retisols (Loamic, Aric, Ochric)) of elevated positions were examined. These soils were studied before the field drainage and during 25 years after drainage works in the periods differing in conditions of humification and with due account for not only drainage works but also other factors, such as topography and agrotechnology and their joint action. The specificity of transformation of humic substances in the soils and their particle-size fractions was analyzed in the basis of data on the organic carbon content, group and fractional composition of humus, the intensity of individual stages of humification (the neoformation of humic acids and the formation of humates), and the optical density of the fractions of humic acids. The results of the study of these properties in the fine soil fractions (<50 μm) made it possible to assess the response of the clay (<1 μm) and silt (1–5, 5–10, 10–50 μm) fractions to changes in the ecological situation and the role of separate particle-size fractions in the degradation of humus under adverse impacts. Overall, a clear tendency toward worsening of the humus quality was observed in both soils during the 25-year-long period, which is related to the long-term (20 years) agricultural use of the reclaimed field without application of agrochemicals. The features of humus degradation were mainly manifested in the finest (<10 μm) fractions with a general decrease in the humus content, slowing down of the formation of humic acids and humates, and considerable loss of humic acids, including their agronomically valuable fractions HA1 and HA2. The degradation of humus quality in the clay fraction was largely due to the impact of the reclamation (drainage) factor; the degradation of humus quality in the fine and medium silt fractions was mainly due to the negative changes in the agricultural background. Among negative consequences of the worsening humus quality, the lowering of soil fertility, ecological sustainability, and productivity of agrocenoses should be noted.     

Microbial Utilization and Transformation of Humic Acids Extracted from Different Soils

Humic acids (HA) extracted from Chernozem (Haplic Phaeozem), Brown Earth (Cambic Umbrisol) and Podzol (Humic Podzol) were added as a supplemental source of nutrients, or as the sole sources of carbon and nitrogen to aerobic cultures of complex microbial communities indigenous to the same individual soils. Depending on nutrient conditions in the individual cultures and origin of HA, between 14 and 86 % of the added HA was utilized. The formation of microbial biomass was enhanced up to six fold in the full-strength nutrient media supplemented with humic acids but was strongly inhibited if HA served as the sole C source. HA preparations re-isolated from the microbial cultures exhibited elemental and structural changes characteristic for early diagenetic transformations of humic substances. These included an increase in carbon content, C:N ratio, infra-red absorption typical for aromatics, and a decrease in infra-red absorption associated with aliphatic acids, nitrogenous and carbohydrate-like substances.     

Studies on the humus forms of forest soils

Samples described in the previous paper were analyzed for humus composition by the method of Kumada el al,, elementary composition of humic acids, nitrogen distribution among humic acid, fulvic acid, and humin, and organic matter composition by the modified Waksman method. The samples obtained by physical fractionation from each horizon of Higashiyama soil were as follows: f₁ and f₂ from the L layer, f₁, f₂ and f₃ from the F layer, f₁ f₂, sand, silt, and clay fractions from the H-A and A horizons.

With the progress of decomposition, the following tendencies were rather clearly observed.

The extraction ratio of soluble humus, amounts of humic acid and fulvic acid, and PQ, value tended to increase with some exceptions. The degree of humification of humic acid proceeded. Most humic acids belonged to the Rp type, but those of the clay fractions belonged to the B type.

As for the elementary composition of humic acid, transitional changes from the Lf₁ to the clay fraction of the A horizon were observed. But differences in elementary composition among humic acids were far less, compared with those among whole fractions.

Nitrogen contents in humic, fulvic, and humin fractions increased with the progress of decomposition and humiliation, and the largest relative increase was found in fulvic acid nitrogen.

According to the modified Waksman's method, the amounts of residues and protein increased, while the total amounts of each extract, except for the HCl extract, and the amounts of sugars and starch, phenolic substances, hemicelluloses and pectin, and cellulose decreased. Sugars and starch comprised only a small portion of the hot water extract, and polyphenols substances comparable to sugars and starch were also found in the extract. Hemicelluloses and pectin accounted for only about one-half of the HCl extract. Several characteristic differences in the elementary composition of extracts and residues were found.

Pheopigments existed in benzene-ethanol extracts and their amounts seemed to decrease from Lf₁ to Ff₂.     

Enhanced non-extractable residue formation of 14C-metalaxyl catalyzed by an immobilized laccase

We studied the influence of an immobilized laccase from Trametes versicolor on non-extractable residue (NER) formation of the systemic fungicide ¹⁴C-metalaxyl in soil. We added the enzyme (130 mU/g DW) to soil sterilized by gamma irradiation and observed that the amount of NER (6.3 % of applied radioactivity) after 10 days of incubation was enhanced about twofold compared to the sterile soil without laccase addition. Residues formed within samples without enhanced enzyme activity were mainly bound via ester linkages to all fractions of humic matter, i.e., fulvic acids, humic acids, non-humines, and humines, respectively. In contrast, residues formed in presence of immobilized laccase were more strongly bound by covalent linkages such as ether and C-C bonds, especially to humic acids. After chemical degradation of the humic matter, it could be observed that all NER contained the first major transformation product, i.e., metalaxyl acid. The findings underline that the residue formation of metalaxyl in soil may be partly catalyzed by immobilized extracellular oxidative enzymes through oxidative coupling reactions within the humic matter.     

Effects of Humic Acids Extracted from Mined Lignite or Composted Vegetable Residues on Plant Growth and Soil Microbial Populations

Growing interest in the market for humic substances with agricultural applications has led to the development of new potential sources of these compounds other than fossil matrices (e.g. different kinds of lignite), which, until now, have represented the main raw material for the extraction of humus-like products. High quality compost (green compost) obtained through the aerobic biostabilization of selected organic residues, such as vegetable waste from source-collection at garden-produce markets, may be considered for this purpose. Beyond the primary need to develop technically and economically reliable procedures for the extraction of humic substances from compost at the industrial scale, importance must be placed on controlling the influence of such compounds on soil-plant systems. Humates from leonardite, representative of the active agents among humus-based commercial preparations, have been compared in pot trials with humic acids, potassium salts, from green compost in order to evaluate their respective effects on soil microbial activity and plant productivity. Differences between pot blocks amended with humic acids have suggested that humus-like substances extracted from compost seem to exert higher stimulative effects on microbial growth and vegetative biomass production than fossil humates.     

The process of manganese depositton in paddy soils

Abstract

The changes in quality and quantity of phenolic substances in the decaying process of rice straw in a soil were compared under moist and flooded conditions for 200 days. The amounts of phenolic substances divided into fractions of humic acid and fulvic acid, ether- and butanol-extractable and organic solvent-unextractable fractions, then the amounts of individual phenolic acids were determined. The following results were obtained.

1) Alkali-extractable total phenolics as well as individual phenolic acids decreased more rapidly under moist, than under flooded, conditions as rice straw decayed in the soil. The phenolics present were mainly attributable to the straw, not to the soil.

2) The decrease in the level of total phenolics in the early stage of the decaying process was mainly due to the decrease in ether-extractable phenolic compounds in the fulvic acid fraction, and in the later stage, was mainly due to the decrease in butanol-extractable phenolics in the humic acid fraction.

3) The amounts of butanol-extractable phenolics and organic solvent-unextractable phenolics were larger in humic acid than in fulvic acid. On the other hand, a larger amount of organic solvent-extractable phenolics, especially ether-extractable phenolics, was present in fulvic acid.

4) The degradation patterns and pathways of individual phenolic acids in the decaying process of rice straw in soil were found to be the lame as those of decaying straw without soil which were reported previously.

5) The level of phenolic substances in the humic acid was not greatly changed during the decaying process, but the phenolic substances in fulvic acid rapidly increased for 30 days and then rapidly decreased to a constant level.     

Acid properties of fulvic and humic acids isolated from two acid forest soils under different vegetation cover and soil depth

We studied the acid‐base properties of 16 fulvic acids and 16 humic acids isolated from the surface (3–15 cm) and subsurface (> 45 cm) horizons of two types of acid forest soils, derived respectively from amphibolite and granite rocks, under five different types of vegetation. The observed differences between the contents of humic substances in the two types of soils were related to the degree of Al‐saturation of the soil organic matter, as indicated by the molar ratio between pyrophosphate extractable Al and C. Humic fractions were characterized in terms of elemental composition, and CPMAS ¹³C NMR spectrometry. The contents of carboxylic and phenolic groups were estimated by potentiometric titrations conducted in 0.1 m KNO₃ in a nitrogen atmosphere. The fulvic acids contained more carboxylic groups but less phenolic groups than the humic acids: the ratio of phenolic to carboxylic groups in the humic acids was 0.48 ± 0.10 and in the fulvic acids 0.23 ± 0.05. The mean values of the protonation constants of each of the humic substance fractions can be used as generic parameters for describing the proton binding properties. The fulvic acids isolated from the subsurface horizon of the soil contained between 2.6 and 23% more carboxylic groups, and the humic acids between 8 and 43% more carboxylic groups than those isolated from the surface horizon of the same soil.     

High energy ultraviolet photo-oxidation: a novel technique for studying physically protected organic matter in clay- and silt-sized aggregates

A novel approach to the study of organic-matter distribution in soil microaggregates (<20 μm) using high-energy ultraviolet (UV) radiation in the presence of oxygen (photo-oxidation) is reported. The method quantitatively destroyed complex organic materials through oxidation, even in the presence of clay, provided the organic materials were directly exposed to the UV radiation. Photo-oxidation of clay and silt fractions for periods up to 8 h demonstrated that a considerable proportion of the organic matter was physically protected within clay- and silt-sized aggregates. In some clay fractions, up to 23% of the organic carbon could be considered as physically protected whereas in silt fractions this was as high as 36%. Infrared spectroscopy demonstrated that the materials external to both clay- and silt-sized aggregates were largely proteinaecous in nature, while the materials in the interior of the aggregates resembled humic acids. These humic materials appeared to be physically shielded against photo-oxidation, rather than being chemically recalcitrant. Using the clay- and silt-sized fractions from one soil, ¹⁴C accelerator mass spectrometry demonstrated that, although both clay and silt fractions contained essentially modern carbon, after 4 h of photo-oxidation much older organic carbon with a mean resonance time (MRT) of between 200±80 and 320±80 years before the present (BP) remained. This protection from photo-oxidation, therefore, appears to mirror the process which physically protects organic substances in soils against microbial degradation. Photo-oxidation of the clay-plus-silt fractions also resulted in a considerable reduction in particle size as the organic-cementing agents, consisting of proteinaecous and humic materials, were oxidized. Using data from the photo-oxidation method along with infrared spectroscopy, radiocarbon dating and scanning electron microscopy, a simple model is proposed that spatially relates the various organic structures present to their positions in the mineral aggregates.     

Über Veränderungen im Humuskomplex bei der Degradation von Schwarzerden (Tschernosemen)

Dynamics of organic matter during the degradation of Chernozems In the course of pedogenesis especially the amount and the composition of soil organic matter changes. This is investigated in the presented paper at 6 different developmentstages of a Chernozem-degradation-sequence from Hildesheim region. The mechanisms of this process can be deduced from the ‘degradation products’, found in the different soils. Using several methods for extraction of organic substances by successively stronger reagents or conditions, different amounts of fulvic and humic acids were got. The findings give indications to the different stability of bondings between mineral and organic substances in the soils and their change during degradation. The ascending exhaustive fractionating method was found to be the best for obtaining and-simultaneously - separating completely the organic matter in distinct franctions and also to earn a humus-clay and a silica humic acid component. Using this method, 5 fractions of the humus complex at any of profile-horizons were got in greater amounts and quantitative as far as qualitative determined. It was found that the loss of organic matter is differently distributed to the single humus fractions in the profiles and in the horizon-zones. Therefore it is not sufficient to determine only the total C- and N-content. Comparing the increase or decrease of humic substances in the upper resp. lower zones of the profiles of the sequence, the degradation process can be pursued retrospectively.     

Abbau und Umwandlung von Pflanzenrückständen und ihren Inhaltsstoffen durch die Mikroflora des Bodens

Degradation and transformation of plant residues and their components by the microflora of the soil This review describes recent results of publications in this area. It condsiders the dynamics of degradation under field and laboratory conditions and indicates some of the problems of simulation models. The preponderant part of transformation processes apparently takes place in a relatively small fraction of soil organic matter which also includes the biomass. Methods for the quantitative measurements of the biomass have been considerably improved. The turnover rates of plant residues are effectively influenced by their chemical composition, for instance by their C/N ratio and their content of lignin or polysaccharides. C/N ratios also seem to influence the socalled priming effect and the transformation of increasing amounts of plant residues added to the soil. Some progress has been also made in the transformation of plant, residues under different climatic conditions. Experiments with polysaccharides and glucose have indicated that a major portion of the residual carbon residues in soil are contained in the biomass or N-containing microbial metabolites. A small part is also present in the phenolic constituents of humic compounds. Root excretions also contribute carbohydrates. Recent experiments indicated the transportation of a considerable amount of photosynthetic products from sprouts through roots into the soil. They cause intensive microbial turnover processes in the root zone. N-containing compounds are stabilized in the soil biomass. Proteins and aminopolysaccharides are furthermore stabilized by sorption on humic compounds or clay particles. They are even more effectively stabilized by linkage into humic compounds. Plant residues contain appreciable amounts of free and polymerized phenols which are degraded by microorganisms or incorporated into humic compounds. Easily oxidisable phenols are more slowly degraded in soil than the more stable ones. This is especially true for low concentrations of phenols added to the soil. The pH-values of the soil and its content of humic compounds influence essentially the degradation rate. Lignins belong to the most important natural biopolymers. Methods for their specific labelling with 14°C have essentially promoted studies about their degradation and transformation in soil and have enlarged the knowledge about lignin degrading organisms. Beside lignins and other plant polyphenols, melanins may play an important role in the formation of humic compounds. These melanins are synthesized form carbohydrates by some microscopic fungi through secondary metabolic reactions. They are formed from phenols, quinones, polyenes, aminoacid and aminosugar derivatives. Similar to humic acids they are relatively stabile against rapid microbial degradation. Analytical methods developed in the last years indicate that both groups of polymers contain similar building blocks.