Differences in the activity and bacterial community structure of drained grassland and forest peat soils

The microbial activity and bacterial community structure were investigated in two types of peat soil in a temperate marsh. The first, a drained grassland fen soil, has a neutral pH with partially degraded peat in the upper oxic soil horizons (16% soil organic carbon). The second, a bog soil, was sampled in a swampy forest and has a very high soil organic carbon content (45%), a low pH (4.5), and has occasional anoxic conditions in the upper soil horizons due to the high water table level. The microbial activity in the two soils was measured as the basal and substrate-induced respiration (SIR). Unexpectedly, the SIR (μl CO₂ g⁻¹ dry soil) was higher in the bog than in the fen soil, but lower when CO₂ production was expressed per volume of soil. This may be explained by the notable difference in the bulk densities of the two soils. The bacterial communities were assessed by terminal restriction fragment length polymorphism (T-RFLP) profiling of 16S rRNA genes and indicated differences between the two soils. The differences were determined by the soil characteristics rather than the season in which the soil was sampled. The 16S rRNA gene libraries, constructed from the two soils, revealed high proportions of sequences assigned to the Acidobacteria phylum. Each library contained a distinct set of phylogenetic subgroups of this important group of bacteria.     

The microbial community composition changes rapidly in the early stages of decomposition of wheat residue

It is generally accepted that during the early stages of residue decomposition, easily available compounds are decomposed, leading to a relative increase in more recalcitrant compounds in the later stages of decomposition and that these changes in substrate availability are associated with changes in microbial community composition. However most studies on residue decomposition are conducted over several weeks or months; little is known about the changes in microbial community composition in the first weeks of decomposition. To address this knowledge gap, we incubated wheat residues inoculated with a microbial suspension in mesh bags buried in sand for 30 days, with sampling on days 0, 2, 4, 6, 8, 10, 15, 20, 25 and 30. Of the C added with the residues, 10, 18 and 25% had been respired on days 10, 20 and 30, respectively. The sum of PLFAs (phospholipid fatty acids), as an indicator of microbial biomass, increased strongly in the first 4 days and then decreased. The concentration of bacterial fatty acids was maximal on days 2 and 4, whereas the concentration of fungal fatty acids peaked on day 15. Microbial community composition (based on PLFA patterns) changed rapidly, with significant changes in the first 8 days and from day 8 to day 20. There were no significant changes in microbial community composition after day 20. The concentration of water-soluble C decreased strongly in the first 8 days, suggesting that the rapid changes in microbial community in this period are related to the changes in water-soluble C. Residue C chemistry, assessed by ¹³C NMR spectroscopy, changed little during the incubation period. This study showed that microbial community composition in decomposing residues changes rapidly in the first 1-2 weeks, which is, at least partly, the result of competition for the easily available compounds in the water-soluble fraction. After depletion of the water-soluble compounds, the microbial community composition changes more slowly.     

Trophic shift of stable isotopes and fatty acids in Collembola on bacterial diets

Fatty acid (FA) analysis is a promising tool to study trophic relationships in soil food webs. We determined FA biomarkers to trace bacterial food sources (Bacillus megaterium, Pseudomonas putida, Enterobacter aerogenes) of Collembola (Heteromurus nitidus, Protaphorura fimata, Folsomia candida). In addition, δ¹⁵N, δ¹³C, C/N ratio, body weight and NLFA/PLFA ratio (neutral lipid/phospholipid fatty acids) of Collembola were assessed. These measures indicated that P. putida ranked first, B. megaterium second and E. aerogenes third in food quality. FAs specific for bacteria were found in the NLFAs of the Collembola reflecting the respective bacterial diet. Biomarker FAs for gram-positive bacteria were methyl branched i14:0, i15:0, a15:0 and i17:0. Consumption of gram-negative bacteria was reflected by the cyclic form cy17:0 (E. aerogenes, P. putida) and by 16:1ω5 (P. putida).     

The chemistry and phytotoxicity of nemagon in fertilized soil

Among the pesticides, soil fumigation (3, 5), particularly with chlorinated and brominated chemicals, is widely used for soil sterilization, chiefly for killing nematodes and root-rot fungi. Besides having nematocidal action, the fumigants interact chemically (1) and biologically (4) with soil components with beneficial and sometimes adverse results. Several mechanisms (2) of fumigant damage to roots and interference with availability of nutrients have been reported, While considerable information is available on the individual effects of fertilizers and fumigants on plants, not much is known about the interactions of the fumigants with fertilizers as measured by nutrient availability in soil, growth of plants, and phytotoxicity, A study, therefore, was undertaken to evaluate the effects of 1, 2 dibromo-3-chloropropane, commonly known as nemagon, and the more commonly used fertilizers on the growth of tomato plants.     

Bacterial diversity and community structure along different peat soils in boreal forest

Bacteria in peat forest soil play important role in global carbon cycling. The distribution of bacteria population in different peat soils as a whole and how forest management practices alter the bacterial populations are still poorly known. Using pyrosequencing analysis of 16S rRNA gene, we quantified the diversity and community structure of bacteria in eight peat forest soils (pristine and drained) and two mineral forest soils from Lakkasuo, Finland with either spruce-dominant or pine-dominant tree species. In total, 191,229 sequences which ranged from 15,710 to 22,730 per sample were obtained and affiliated to 13 phyla, 30 classes and 155 genera. The peat forest soils showed high bacterial diversity and species richness. The tree species seems to have more strong impact on the bacterial diversity than the type of peat soil, which drives the changes in bacterial community structure. The dominant taxonomic groups across all soils (>1% of all sequences) were Proteobacteria, Acidobacteria, Actinobacteria, Bacteroidetes, Planctomycetes and Verrucomicrobia. The relative abundance of bacteria phylum and genus differed between soil types and between vegetation. Significant differences in relative abundance of bacteria phyla were only found for Gemmatimonadetes and Cyanobacteria between the pristine and the drained peat forest soils. At genus level, the relative abundance of several genera differed significantly between the peat soils with same or different tree species, including Burkholderia, Caulobacter, Opitutus, Mucilanginibacter, Acidocella, Mycobacterium, Bradyrhizobium, Dyella and Rhodanobacter.     

Additions of maize root mucilage to soil changed the structure of the bacterial community

The organic compounds released from roots (rhizodeposits) stimulate the growth of the rhizosphere microbial community. They may be responsible for the differences in the structure of the microbial communities commonly observed between the rhizosphere and the bulk soil. Rhizodeposits consists of a broad range of compounds including root mucilage. The aim of this study was to investigate if additions of maize root mucilage, at a rate of 70 μg C g⁻¹ day⁻¹ for 15 days, to an agricultural soil could affect the structure of the bacterial community. Mucilage additions moderately increased microbial C (+23% increase relative to control), which suggests that the turnover rate of microorganisms consuming this substrate was high. Consistent with this, the number of cultivable bacteria was enhanced by +450%. Catabolic (Biolog^® GN2) and 16S-23S intergenic spacer fingerprints exhibited significant differences between control and mucilage treatments. These data indicate that mucilage can affect both the metabolic and genetic structure of the bacterial community as shown by a greater catabolic potential for carbohydrates. We concluded that mucilage is likely to significantly contribute to differences in the structure of the bacterial communities present in the rhizosphere compared to the bulk soil.     

Microbial community composition and rhizodeposit-carbon assimilation in differently managed temperate grassland soils

Rhizodeposit-carbon provides a major energy source for microbial growth in the rhizosphere of grassland soils. However, little is known about the microbial communities that mediate the rhizosphere carbon dynamics, especially how their activity is influenced by changes in soil management. We combined a ¹³CO₂ pulse-labeling experiment with phospholipid fatty acid (PLFA) analysis in differently managed Belgian grasslands to identify the active rhizodeposit-C assimilating microbial communities in these grasslands and to evaluate their response to management practices. Experimental treatments consisted of three mineral N fertilization levels (0, 225 and 450 kg N ha⁻¹ y⁻¹) and two mowing frequencies (3 and 5 times y⁻¹). Phospholipid fatty acids were extracted from surface (0-5 cm) bulk (BU) and root-adhering (RA) soil samples prior to and 24 h after pulse-labeling and were analyzed by gas chromatography-combustion-isotope ratio mass spectrometry (GC-c-IRMS). Soil habitats significantly differed in microbial community structure (as revealed by multivariate analysis of mol% biomarker PLFAs) as well as in gram-positive bacterial rhizodeposit-C uptake (as revealed by greater ¹³C-PLFA enrichment following pulse-labeling in RA compared to BU soil in the 450N/5M treatment). Mowing frequency did not significantly alter the relative abundance (mol%) or activity (¹³C enrichment) of microbial communities. In the non-fertilized treatment, the greatest ¹³C enrichment was seen in all fungal biomarker PLFAs (C16:1ω5, C18:1ω9, C18:2ω6,9 and C18:3ω3,6,9), which demonstrates a prominent contribution of fungi in the processing of new photosynthate-C in non-fertilized grassland soils. In all treatments, the lowest ¹³C enrichment was found in gram-positive bacterial and actinomycetes biomarker PLFAs. Fungal biomarker PLFAs had significantly lower ¹³C enrichment in the fertilized compared to non-fertilized treatments in BU soil (C16:1ω5, C18:1ω9) as well as RA soil (all fungal biomarkers). While these observations clearly indicated a negative effect of N fertilization on fungal assimilation of plant-derived C, the effect of N fertilization on fungal abundance could only be detected for the arbuscular mycorrhizal fungal (AMF) PLFA (C16:1ω5). On the other hand, increases in the relative abundance of gram-positive bacterial PLFAs with N fertilization were found without concomitant increases in ¹³C enrichment following pulse-labeling. We conclude that in situ¹³C pulse-labeling of PLFAs is an effective tool to detect functional changes of those microbial communities that are dominantly involved in the immediate processing of new rhizosphere-C.     

Microbial activity and community structure in two drained fen soils in the Ljubljana Marsh

Fen peatlands are specific wetland ecosystems containing high soil organic carbon (SOC). There is a general lack of knowledge about the microbial communities that abound in these systems. We examined the microbial activity and community structure in two fen soils differing in SOC content sampled from the Ljubljana Marsh under different seasonal conditions. Substrate-induced respiration and dehydrogenase activity were used as indicators of total microbial activity. Both methods indicated higher microbial activities in the fen soil with the higher SOC content on all dates of sampling. To determine whether the differences in microbial activity were associated with differences in the microbial community structures, terminal restriction fragment length polymorphism (T-RFLP) of bacterial 16S rRNA genes was performed. Comparison of the T-RFLP profiles revealed very similar community structures in both fens and in the two seasonal extremes investigated. This suggested a stable community structure in the two fens, which is not affected by the SOC content or seasonal variation. In addition, a bacterial 16S ribosomal RNA gene based clone library was prepared from the fen soil with the higher SOC content. Out of 114 clones analysed, approximately 53% belonged to the Proteobacteria, 23% to the Acidobacteria, 21% to a variety of other taxa, and less than 3% were affiliated with the Firmicutes.     

Aggregation and organic matter in subarctic Andosols under different grassland management

Quantity and quality of soil organic matter (SOM) affect physical, chemical, and biological soil properties, and are pivotal to productive and healthy grasslands. Thus, we analyzed the distribution of soil aggregates and assessed quality, quantity, and distribution of SOM in two unimproved and improved (two organic and two conventional) grasslands in subarctic Iceland, in Haplic and Histic Andosols. We also evaluated principal physicochemical and biological soil properties, which influence soil aggregation and SOM dynamics. Macroaggregates (>250 µm) in topsoils were most prominent in unimproved (62–77%) and organically (58–69%) managed sites, whereas 20–250 µm aggregates were the most prominent in conventionally managed sites (51–53%). Macroaggregate stability in topsoils, measured as mean weight diameter, was approximately twice as high in organically managed (12–20 mm) compared with the conventionally managed (5–8 mm) sites, possibly due to higher organic inputs (e.g., manure, compost, and cattle urine). In unimproved grasslands and one organic site, macroaggregates contributed between 40% and 70% of soil organic carbon (SOC) and nitrogen to bulk soil, whereas in high SOM concentration sites free particulate organic matter contributed up to 70% of the SOC and nitrogen to bulk soil. Aggregate hierarchy in Haplic Andosols was confirmed by different stabilizing mechanisms of micro- and macroaggregates, however, somewhat diminished by oxides (pyrophosphate-, oxalate-, and dithionite-extractable Fe, Al, and Mn) acting as binding agents for macroaggregates. In Histic Andosols, no aggregate hierarchy was observed. The higher macroaggregate stability in organic farming practice compared with conventional farming is of interest due to the importance of macroaggregates in protecting SOM and soils from erosion, which is a prerequisite for soil functions in grasslands that are envisaged for food production in the future.     

Microbial community utilization of added carbon substrates in response to long-term carbon input manipulation

The chemical composition and quantity of plant inputs to soil are primary factors controlling the size and structure of the soil microbial community. Little is known about how changes in the composition of the soil microbial community affect decomposition rates and other ecosystem functions. This study examined the degradation of universally ¹³C-labeled glucose, glutamate, oxalate, and phenol in soil from an old-growth Douglas-fir (Pseudotsuga menziesii)—western hemlock (Tsuga heterophylla) forest in the Oregon Cascades that has experienced 7 y of chronic C input manipulation. The soils used in this experiment were part of a larger Detritus Input and Removal Treatment experiment and have received normal C inputs (control), doubled wood inputs, or root and litter input exclusion (no inputs). Soil from the doubled wood treatment had a higher fungal:bacterial ratio, and soil from the no inputs treatment had a lower fungal:bacterial ratio, than the control soil. Differences in the utilization of the compounds added to the field-manipulated soils were assessed by following the ¹³C tracer into microbial biomass and respiration. In addition, ¹³C-phospholipid fatty acids (PLFA) analysis was used to examine differential microbial utilization of the added substrates. Glucose and glutamate were metabolized similarly in soils of all three litter treatments. In contrast, the microbial community in the double wood soil respired more added phenol and oxalate, whereas microbes in the no inputs soil respired less added phenol and oxalate, than the control soil. Phenol was incorporated primarily into fungal PLFA, especially in soil of the double wood treatment. The addition of all four substrates led to enhanced degradation of soil organic matter (priming) in soils of all three litter treatments, and was greater following the addition of phenol and oxalate as compared to glucose and glutamate. Priming was greater in the no inputs soil as compared to the control or doubled wood soils. These results demonstrate that altering plant inputs to soil can lead to changes in microbial utilization of C compounds. It appears that many of these changes are the result of alteration in the size and composition of the microbial community.     

Evolution and qualitative modifications of humin-like matter during high rate composting of pig faeces amended with wheat straw

During a 4-week period of composting of wheat straw-amended pig faeces, humin (HU)- and core-HU-like matter were isolated by NaOH-Na₄P₂O₇ treatment of the compost bed, respectively, without and with previous extraction by organic solvent and by H₂SO₄. The changes in the content and elemental composition of both fractions in the compost bed were monitored. Evidence of the compositional changes was also obtained by NMR spectroscopy and by pyrolysis-GC / MS studies. The results indicated that core-HU-like matter was mainly aromatic, while HU contained both core-HU-like and other types of easily degradable organic matter. Correlation of the data found in this study with data from previous studies on humic acid (HA)- and core-HA-like matter in the same composting process indicated that in the time range from 2 to 4 weeks, the weight loss of the core-HU-like matter amounted to 788 g, whereas the weight of total core-HA-like matter increased by 87 g. In spite of the high weight loss, the NMR and pyrolysis-GC / MS spectra failed to reveal significant changes in the chemical nature of the core-HU-like residue. However, the chemical composition of the core-HA-like matter changed significantly and tended to become similar to that of the core-HU-like matter when the composting time increased. The data suggest that, during composting, core-HU-like matter undergoes both conversion to new core-HA-like soluble matter and biodegradation to volatile products.     

Environmental and spatial characterisation of bacterial community composition in soil to inform sampling strategies

Soil physicochemical properties and microbial communities are highly heterogeneous and vary widely over spatial scales, necessitating careful consideration of sampling strategies to provide representative and reproducible soil samples across field sites. To achieve this, the study aimed to establish appropriate sampling methodology and to determine links between the variability of parameters, utilising two sampling strategies. The first (design 1) involved extracting 25 cores from random locations throughout the field and pooling them into five sets of five cores. The second (design 2) involved a further 25 cores within five 1 m² sub-plots. Sub-samples from each sub-plot were pooled in order to determine between and within sub-plot variability. All samples were analysed independently and as pooled sub-samples. Results indicate that pooling spatially separated samples significantly reduced the variability in pH, compared to individual samples. Pooling samples from a small area resulted in lower within sub-plot variability than between sub-plots for pH and bacterial community composition assessed by terminal-restriction fragment length polymorphism analysis. Following multivariate statistical analysis, a large amount of variation in community composition was explained by soil pH, which is remarkable given the relatively small size of the sampling area and minor differences in pH. Moisture content was also important in determining bacterial communities in the random design (design 1). In the 1 m² sub-plot design (design 2), the spatial location of the plots explained a large degree of the variation in bacterial community composition between plots, which was due to spatial autocorrelation of pH and possible additional environmental parameters. This study emphasises the importance of sampling design for obtaining representative samples from soil.     

Post-harvest residue management effects on recalcitrant carbon pools and plant biomarkers within the soil heavy fraction in Pinus radiata plantations

Forest soils contain about 30% of terrestrial carbon (C) and so knowledge of the influence of forest management on stability of soil C pools is important for understanding the global C cycle. Here we present the changes of soil C pools in the 0-5 cm layer in two second-rotation Pinus radiata (D.Don) plantations which were subjected to three contrasting harvest residue management treatments in New Zealand. These treatments included whole-tree harvest plus forest floor removal (defined as forest floor removal hereafter), whole-tree, and stem-only harvest. Soil samples were collected 5, 10 and 15 years after tree planting at Kinleith Forest (on sandy loam soils) and 4, 12 and 20 years after tree planting at Woodhill Forest (on sandy soils). These soils were then physically divided into light (labile) and heavy (stable) pools based on density fractionation (1.70 g cm⁻³). At Woodhill, soil C mass in the heavy fraction was significantly greater in the whole-tree and stem-only harvest plots than the forest floor removal plots in all sampling years. At Kinleith, the soil C mass in the heavy fraction was also greater in the stem-only harvest plots than the forest floor removal plots at year 15. The larger stable soil C pools with increased residue return was supported by analyses of the chemical composition and plant biomarkers in the soil organic matter (SOM) heavy fractions using NMR and GC/MS. At Woodhill, alkyl C, cutin-, suberin- and lignin-derived C contents in the SOM heavy fraction were significantly greater in the whole-tree and stem-only harvest plots than in the forest floor removal plots in all sampling years. At Kinleith, alkyl C (year 15), cutin-derived C (year 5 and 15) and lignin-derived C (Year 5 and 10) contents in the SOM heavy fraction were significantly greater in stem-only harvest plots than in plots where the forest floor was removed. The analyses of plant C biomarkers and soil δ¹³C in the light and heavy fractions of SOM indicate that the increased stable soil C in the heavy fraction with increased residue return might be derived from a greater input of recalcitrant C in the residue substrate.     

Chemical changes and phosphorus release during decomposition of pea residues in soil

To study C chemistry and nutrient dynamics in decomposing residues and P dynamics at the residue-soil interface, young pea (Pea-Y) and mature pea (Pea-M) residues were incubated in a sandy soil with low P availability. The study was conducted in microcosms in which the residues were separated from the soil by a nylon mesh. Controls consisted of microcosms without residues. Residues and the soil in the immediate vicinity of the nylon mesh were sampled after 5, 15, 28, 42 and 61 days. Residue chemistry was studied by ¹³C nuclear magnetic resonance (NMR) spectroscopy and determination of C, N and P concentrations. Compared to Pea-M, Pea-Y was characterised by higher N and P concentrations, higher percentage of proteins, esters, fatty acids and sugars, and was more easily decomposable in the first 15 days. Pea-M residues had a greater percentage of cellulose and other polysaccharides than Pea-Y and showed a more gradual loss in dry weight. Differences in C chemistry and N and P concentration between the residues decreased with time. The decomposition of Pea-Y and Pea-M residues resulted in an increase in microbial P in the residue-soil interface compared to the control, but available P was increased only in the vicinity of Pea-Y residues.     

固态13C和15N核磁共振法研究15N标记土壤的腐殖质组分

Five humic fractions were obtained from a uniformly ^15N-labelled soil by extraction with 0.1 mol L^-1 Na4P2O7,0.1mol L^-1 NaOH ,and HF/HCl-0.1 mol L^-1 NaOH,consecutively,and analyzed by ^13C and ^15N CPMAS NMR (cross polarization and magic angle spinning nuclear magnetic resonace).Compared with those of native soils humic fractions studied as a whole contained more alkyls ,methoxyls and O-alkyls,being 27%-36%,17%-21%and 36%-40%,respectively,but fewer aromatics and carboxyls(bein 14%-20% and 13%-90%,respectively),Among those humic fractions ,the humic acid(HA)and fulvic acid(FA) extracted by 0.1 mol L^-1 Na4P2O7 contained slightly more carboxyls than corresponding humic fractions extracted by 0.1 mol L^-1 NaOH ,and the HA extacted by 0.1 mol L^-1 NaOH after treatment with HF/HCl contained the least aromatics and carboxyls.The distribution of nitrogen functional groups of soil humic fractions studied was quite similar to each other and also quite similar to that of humic fraction from native soils.More than 75% of total N in each fraction was in amide from,with 9%-13% present as aromatic and /or aliphatic amines and the remainder as heerocyclic N.     

植烟土壤提取物质对烟株生长及根际土壤细菌多样性的影响

对盆栽烟草外源添加不同浓度植烟土壤提取物质(T1:40μg·mL-1;T2:120μg·mL-1;CK:蒸馏水对照),探讨植烟土壤提取物质对烟草生长及土壤细菌多样性的影响。结果表明,植烟土壤提取物处理使烟株生长受抑制,且随处理浓度的增加受抑制程度显著提高,具体表现为烟株变矮,叶面积变小,光合作用能力降低,且烟草的保护酶系统受到破坏,丙二醛含量随处理浓度加大而增加,T2处理的丙二醛含量是对照的3.44倍。对外源添加物质处理后烟草根际土壤微生物T-RFs分析发现,在对照检测到17个门24个纲,T1处理有14个门21个纲,T2有10个门17个纲;丰富度指数的变化也和门纲的变化一致,随着处理浓度的增加而显著降低。可见外源添加物质处理后,根际土壤细菌群落减少,多样性水平下降。对各处理的根际土壤微生物T-RFs变化与烟株生长变化进行相关性分析表明,在外源添加物质处理的土壤中存在较多的负相关T-RFs片段,且这些片段中较多为病原菌;而正相关的T-RFs片段主要存在于对照土壤中,其中有较多与土壤营养元素循环相关的微生物。本研究结果显示,在外源添加植烟土壤提取物质处理下,烟草的生长受抑制,烟草根际土壤的微生态受到破坏,且随浓度的提升而加重。因此,连作土壤中自毒物质的富集是造成烟草连作障碍的主要原因。关键词烟草连作障碍根际细菌自毒作用T-RFLP     

Antagonistic and synergistic effects of fungal and bacterial growth in soil after adding different carbon and nitrogen sources

The effect of adding easily available and more complex carbon sources, with and without nitrogen, on fungal and bacterial growth and activity in soil were studied in the laboratory. Total microbial activity was estimated by measuring respiration, fungal growth with the acetate-in-ergosterol incorporation technique and bacterial growth with the thymidine and leucine incorporation techniques. The substrate additions consisted of glucose and cellulose, with and without nitrogen (as ammonium nitrate), and gelatine. The microbial development was followed over a 2-month period. The respiration rate increased within a few days after adding glucose, with and without nitrogen, and gelatine, initially by more than 10 times, but after 2 months no differences were seen compared with the control. Bacterial growth estimated with the thymidine and leucine incorporation techniques gave similar results. Adding glucose with nitrogen, or gelatine, increased bacterial growth within a few days up to 10 times, but even after 2 months of incubation bacterial growth rates were still about 5 times higher than in the control. Adding only glucose increased bacterial growth rates by about twice over the whole incubation period. Fungal growth rates especially increased after adding cellulose and nitrogen, although a minor increase was found after adding cellulose alone. Fungal growth rates started to increase after 10 days of incubation with cellulose. There were indications of synergistic effects in that bacterial growth increased after the fungi had started to grow after adding cellulose. Treatments resulting in high bacterial growth rates (adding easily available carbon sources) led to decreased fungal growth rates compared with the control, indicating antagonistic effects of bacteria.     

Qualitative modifications of humic acid-like and core-humic acid-like during high-rate composting of pig faeces amended with wheat straw

Humic acid-like (HA-like) and core-humic acid-like (core-HA-like) were characterized during the high-rate composting process by CP-MAS ¹³C NMR, pyrolysis-gas chromatography (GC)/mass spectrometry (MS), and elemental analysis. Results obtained indicated that humification proceeded through a relative concentration of aromatic fractions due to the faster degradation of the O-alkyl and alkyl fractions. Core-HA-like, after purification of the parent material, showed a large reduction of the O-alkyl fraction in terms of HA-like. We concluded that HA-like consisted of refractory organic molecules, such as lignin and biopolymers, which formed a stable structure (core-HA-like) coated with degradable material associated with the core by weak physical association, ether or ester bounds.     

Microbial community abundance and structure are determinants of soil organic matter mineralisation in the presence of labile carbon

Altered rates of native soil organic matter (SOM) mineralisation in the presence of labile C substrate (‘priming’), is increasingly recognised as central to the coupling of plant and soil-biological productivity and potentially as a key process mediating the C-balance of soils. However, the mechanisms and controls of SOM-priming are not well understood. In this study we manipulated microbial biomass size and composition (chloroform fumigation) and mineral nutrient availability to investigate controls of SOM-priming. Effects of applied substrate (¹³C-glucose) on mineralisation of native SOM were quantified by isotopic partitioning of soil respiration. In addition, the respective contributions of SOM-C and substrate-derived C to microbial biomass carbon (MBC) were quantified to account for pool-substitution effects (‘apparent priming’). Phospholipid fatty acid (PLFA) profiles of the soils were determined to establish treatment effects on microbial community structure, while the ¹³C-enrichment of PLFA biomarkers was used to establish pathways of substrate-derived C-flux through the microbial communities. The results indicated that glucose additions increased SOM-mineralisation in all treatments (positive priming). The magnitude of priming was reduced in fumigated soils, concurrent with reduced substrate-derived C-flux through putative SOM-mineralising organisms (fungi and actinomycetes). Nutrient additions reduced the magnitude of positive priming in non-fumigated soils, but did not affect the distribution of substrate-derived C in microbial communities. The results support the view that microbial community composition is a determinant of SOM-mineralisation, with evidence that utilisation of labile substrate by fungal and actinomycete (but not Gram-negative) populations promotes positive SOM-priming.     

Fate of humic acids isolated from natural humic substances

Composition of humic acids (HA) is a function of plant-derived inputs, degradation processes regulated by microorganisms, organo-mineral interactions and age. Characterization of different origin humic substances is important for evaluation of their contribution to stabile and labile carbon pool in the environment. The relative abundance of chemical components in HA isolated from soils, compost, commercial lignohumates, alginite, acadiane and lignite was studied with aim to quantify content of important biomarkers such as amino acid, lipids and polyphenols. HA were considered as a heterogeneous complex and high concentration of peptides, polyphenols and lipids was determined in acadian-HA to compare with soil-HA. Compost-HA contained much more amino acids to compare with soil-HA samples. Alginite-HA and lignite-HA were similar in biomarkers content to soil-HA. Fourier transform infrared spectroscopy confirmed that chemical composition and functional groups content differs with the origin, humification degree and the age of studied samples. Soil-HA are typically composed of a variety of ?OH, COOH?, C–O, C–H₂, (aliphatic and aromatic) groups, quinines, lignin fragments, polysaccharide, monosaccharide and proteins fragments, which are linked together by ?O?, ?NH?, ?H=, >C=O, metal ions and –S? groups. ¹³C NMR spectroscopy showed that aromatic carbon content was the highest in lignite-HA and soil-HA.