An integrative approach of organic matter stabilization in temperate soils: Linking chemistry,physics, and biology

With this topical issue, we present the work of the Priority Program 1090 of the German Research Foundation (“Deutsche Forschungsgemeinschaft DFG”): “Soils as a source and sink for CO₂ – mechanisms and regulation of organic matter stabilisation in soils”. This introduction gives an overview on the sites investigated and the major research approaches, including a glossary of major terms used in the field of soil organic matter research. We point out the advantages of integration of data from a broad field of different soil‐science disciplines and the progress achieved by application and combination of new analytical methods describing the quality and turnover of soil organic matter.     

Partitioning characteristics of PAHs between sediment and water in a shallow lake

Goal, Scope and Background

Distribution of hydrophobic organic contaminants in abiotic compartments is essential for describing their transfer and fate in aquatic ecosystems. Taihu Lake is the third largest freshwater lake in China. Water quality of Taihu Lake has deteriorated greatly during the last decades and has threatened the water supply. The aim of the present study was to investigate the partitioning of polycyclic aromatic hydrocarbons (PAHs) among overlying water, suspended particulate matter (SPM), sediments, and pore water in Meiliang Bay, Taihu Lake and to provide useful information for the ecological engineering in this area.

Materials and Methods

Overlying water and surface sediment were sampled from six sites in Meiliang Bay, Taihu Lake, China. Within 72 h of sampling, sediments were centrifuged to obtain the pore water. Overlying water samples were filtered to separate dissolved and SPM samples. After extraction, samples were purified following a clean-up procedure. PAH fraction was obtained by elution with a mixture of hexane: DCM (7:3, V/V) and analyzed by GC/MS.

Results

PAHs concentrations in overlying water varied from 37.5 ng/L to 183.5 ng/L. Concentrations of PAHs in pore water were higher than those in overlying water. The total concentrations of 16 priority PAHs in sediments ranged from 2091.8 ng/g-dw to 4094.4 ng/g-dw. PAHs concentrations on SPM were decreased with suspended solid concentrations (SSC). Total PAHs concentrations on SPM varied in the range of 3369.6 ng/g-dw to 7531.1 ng/g-dw. The partition coefficients between sediment and overlying water (log K _oc) for PAHs with log K _ow<5 were positively correlated with their octanol-water partition coefficients (log K _ow) (n=39, r=0.79, p<0.0001). Partition coefficients between sediment and pore water (log K _oc′) for all PAHs were also significantly correlated with their log K _ow values (n=48, r=0.82, p<0.0001).

Discussion

In general, PAHs derived from combustion sources tend to bind strongly to soot particles in natural sediment. Consequentially, K _oc values observed in the natural environment could be orders of magnitude higher than those predicted by linear correlation relationships under laboratory conditions. In the present study, the ratio of log K _oc values to log K _ow values falls consistently above 1, indicating that the sediment soot carbon in the bay was more attractive for PAHs than n-octanol. The log K _oc′ was also higher than that predicted under laboratory conditions, suggesting that the measured pore water PAH concentrations were lower than those predicted. That is to say, not all the sediment PAHs can be available to partition rapidly into sediment pore waters. A variation in soot content is a possible reason. Furthermore, concentrations of PAHs on SPM were higher than those in sediments. The compositions of PAHs on SPM and in sediments were similar, indicating the importance of re-suspension process of sediments in the partitioning process of the shallow lake.

Conclusions

The results indicated the equilibrium partitioning model could be used to predict PAHs distribution in various phases of a shallow lake in the stagnation period, but re-suspension processes should be considered to modify the relationship between log K _ocs and log K _ows.

Recommendations and Perspectives

Concentration, particle size and composition of resuspended particles could affect the relationship between log K _ocs and log K _ows. Further work should be done under field conditions, especially where a steady thermodynamic equilibrium state could be assumed.

     

添加活性炭对土壤有效磷测定的影响研究

针对NY/T 148-1990测定方法中,加活性炭目的是降低由于浸出土壤有机质所产生的颜色对有效磷测定结果的影响,但是却限制了土壤有效磷的批量测定,为此进行了不加活性炭条件下测定土壤有效磷的改进试验。试验结果表明,不加活性炭条件下,浸出土壤有机质所产生的颜色对土壤有效磷测定结果在测定误差范围内可以忽略不计;两种有效磷测定方法的平均值均在误差范围之内,但改进方法所测结果更接近于NY/T 148-1990标准值,测值间离散度低,稳定性强,且测定速度快、结果准确度及精密度高、成本低。     

Soil chemical properties and microbial biomass after 16 years of no-tillage farming on the Loess Plateau, China

Data from a 16-year field experiment conducted in Shanxi, on the Chinese Loess Plateau, were used to compare the long-term effects of no-tillage with straw cover (NTSC) and traditional tillage with straw removal (TTSR) in a winter wheat (Triticum aestivum L.) monoculture. Long-term no-tillage with straw cover increased SOM by 21.7% and TN by 51.0% at 0–10 cm depth and available P by 97.3% at 0–5 cm depth compared to traditional tillage. Soil microbial biomass C and N increased by 135.3% and 104.4% with NTSC compared to TTSR for 0–10 cm depth, respectively. Under NTSC, the metabolic quotient (CO₂ evolved per unit of MBC) decreased by 45.1% on average in the top 10 cm soil layer, which suggests that TTSR produced a microbial pool that was more metabolically active than under NTSC. Consequently, winter wheat yield was about 15.5% higher under NTSC than under TTSR. The data collected from our 16-year experiment show that NTSC is a more sustainable farming system which can improve soil chemical properties, microbial biomass and activity, and thus increase crop yield in the rainfed dryland farming areas of northern China. The soil processes responsible for the improved yields and soil quality, in particular soil organic matter, require further research.     

Response of an agricultural soil to pentachlorophenol (PCP) contamination and the addition of compost or dissolved organic matter

The response of a fresh, agricultural soil when contaminated with pentachlorophenol (PCP) and supplemented with compost (C) or dissolved organic matter (DOM) was studied in the laboratory. The concentration of PCP and the changes in various functionally related properties (i.e. microbial biomass, basal respiration, soil hydrolase and oxidoreductase activity) were measured over 150 d. Variations in the main physical and chemical properties of the soils were also monitored. Two different doses of compost (C1 = 0.27% and C2 = 0.83%, corresponding to 10 and 30 t ha⁻¹, respectively) or DOM (D1 = 0.07% and D2 = 0.2%) equivalent to the carbon content of the two compost doses C1 and C2 were used and the following five systems were investigated: soil (S), soil–compost (S-C1 and S-C2) and soil–DOM (S-D1 and S-D2). PCP concentrations declined progressively and significantly with time. This effect was most pronounced for the soils amended with the lower compost dose C1 (S-C1) and with the two DOM (S-D1 and S-D2) amounts. Significantly reduced amounts of PCP were extracted after its 500-d residence in the various systems. Higher amounts of the residual PCP were extracted from the humic acids (HA), fulvic acids (FA) and humin–mineral (HU) fractions of the 500 d aged samples than from the same unfractionated samples, indicating that the residual PCP preferentially accumulated in the organic fractions of soil. The soil showed an endogenous microbial activity as indicated by basal respiration, microbial biomass and all the enzymatic activities tested (dehydrogenase, glucosidase, phosphatase, arylsulphatase and urease). Addition of the PCP severely depressed some of the tested biochemical properties suggesting an inhibitory effect on microbial activity. Conversely, higher basal respiration, and similar β-glucosidase and phosphatase activities were measured in comparison with the controls. No significant effects were observed following the addition of two doses of the compost or the DOM. Fungal colonies belonging to the taxonomic group of Ascomycetes and identified as Byssochlamys fulva developed with time in all the PCP-contaminated samples. Growth of B. fulva in vitro in the presence of PCP showed that the isolate was tolerant to 12.5 and 25 mg l⁻¹ PCP and degraded 20% of its initial concentration in 8 d. Overall, the results indicate that many complex processes occurred in the contaminated soil and combinations of these determined the response to PCP contamination. The sorption of PCP to the soil matrix (which increased with time) and its degradation/transformation by indigenous soil microbial activity were likely involved. Both the processes appeared to be favoured by the presence of dissolved organic matter.     

Atmospheric CO2 enrichment and nutrient additions to planted soil increase mineralisation of soil organic matter, but do not alter microbial utilisation of plant- and soil C-sources

Plants link atmospheric and soil carbon pools through CO₂ fixation, carbon translocation, respiration and rhizodeposition. Within soil, microbial communities both mediate carbon-sequestration and return to the atmosphere through respiration. The balance of microbial use of plant-derived and soil organic matter (SOM) carbon sources and the influence of plant-derived inputs on microbial activity are key determinants of soil carbon-balance, but are difficult to quantify. In this study we applied continuous ¹³C-labelling to soil-grown Lolium perenne, imposing atmospheric CO₂ concentrations and nutrient additions as experimental treatments. The relative use of plant- and SOM-carbon by microbial communities was quantified by compound-specific ¹³C-analysis of phospholipid fatty acids (PLFAs). An isotopic mass-balance approach was applied to partition the substrate sources to soil respiration (i.e. plant- and SOM-derived), allowing direct quantification of SOM-mineralisation. Increased CO₂ concentration and nutrient amendment each increased plant growth and rhizodeposition, but did not greatly alter microbial substrate use in soil. However, the increased root growth and rhizosphere volume with elevated CO₂ and nutrient amendment resulted in increased rates of SOM-mineralisation per experimental unit. As rhizosphere microbial communities utilise both plant- and SOM C-sources, the results demonstrate that plant-induced priming of SOM-mineralisation can be driven by factors increasing plant growth. That the balance of microbial C-use was not affected on a specific basis may suggest that the treatments did not affect soil C-balance in this study.     

Effects of additions of organic matter on the penetration resistance of glacial till for the entire water tension range

Hardsetting soil properties are undesirable in agricultural soils because they hamper crop production by limiting seedling emergence and root growth via increased mechanical soil resistance at low moisture contents. The objective of this study was to determine the effect of additions of organic matter on the penetration resistance of a hardsetting soil for the entire water tension range. Investigations were carried out on Saalian glacial till, which is used as a reclamation substrate in post-lignite-mining reclamation. Proportions of 0%, 1%, 2%, 3% and 4% by mass of organic matter (OM) were used. The remoulded samples were saturated under a constant load of 2.4 kPa to achieve bulk densities equivalent to a soil depth of 15–20 cm via water-induced consolidation. Subsequently, the mixtures were adjusted to water tensions between 10⁰ and 10⁷ hPa and penetrated using a small cone penetrometer. Compared to 0% OM, the addition of 1% OM led to a very small but significant (P < 0.01) increase in the bulk density, while between 1% and 4% OM bulk density was seen to decrease in a linear fashion. At moisture contents greater than field capacity, penetration resistance values were consistent with the observed changes in bulk density, leading to an increase in the samples containing 0–1% OM to critical values for root-growth and a decrease for samples containing 2% and more organic matter reaching to values non-critical for roots. At moisture contents smaller than field capacity, penetration resistance values were inversely related to the bulk density, supporting the concept that the type of organic matter added contributed to soil cohesion. Modeling the relation between water tension and penetration resistance using a sigmoidal equation showed a high consistency between the observed data and the model.     

Mechanisms of real and apparent priming effects and their dependence on soil microbial biomass and community structure: critical review

The number of studies on priming effects (PE) in soil has strongly increased during the last years. The information regarding real versus apparent PE as well as their mechanisms remains controversial. Based on a meta-analysis of studies published since 1980, we evaluated the intensity, direction, and the reality of PE in dependence on the amount and quality of added primers, the microbial biomass and community structure, enzyme activities, soil pH, and aggregate size. The meta-analysis allowed revealing quantitative relationships between the amounts of added substrates as related to microbial biomass C and induced PE. Additions of easily available organic C up to 15% of microbial biomass C induce a linear increase of extra CO₂. When the added amount of easily available organic C is higher than 50% of the microbial biomass C, an exponential decrease of the PE or even a switch to negative values is often observed. A new approach based on the assessment of changes in the production of extracellular enzymes is suggested to distinguish real and apparent PE. To distinguish real and apparent PE, we discuss approaches based on the C budget. The importance of fungi for long-term changes of SOM decomposition is underlined. Priming effects can be linked with microbial community structure only considering changes in functional diversity. We conclude that the PE involves not only one mechanism but a succession of processes partly connected with succession of microbial community and functions. An overview of the dynamics and intensity of these processes as related to microbial biomass changes and C and N availability is presented.     

Use of pyrolysis/GC–MS combined with thermal analysis to monitor C and N changes in soil organic matter from a Mediterranean fire affected forest

In Mediterranean ecosystems, forest fires are a common phenomenon. They involve the transformation of vegetation and litter, leaving charred residues and so influencing the carbon cycle by changing (a) the amounts of soil organic matter and (b) the proportions within it of pools with differing stability. In addition to affecting C cycles, fires also affect the amounts of N within soil organic matter, and its availability.     

东北黑土活性有机碳、氮分布研究

通过对东北黑土区不同地点、不同层次的土壤样品的分析,探索活性有机碳(轻组有机C,N、颗粒有机C,N和易氧化有机C,N)在东北地区的分布情况。结果表明,随着纬度的增加,土壤活性有机C、N含量增加;随着深度的增加,土壤活性有机C、N含量减少。说明纬度低的地区,活性有机碳库转化较快,库容较小,而耕作措施相近的情况下,植物残体和根残体在有机C积累方面应予以重视。