How does sonication affect the mineral and organic constituents of soil aggregates?—A review

Application of ultrasound to disperse soil aggregates has been critical in enabling researchers to separate and analyze aggregate building blocks that include organic and mineral particles as well as mineral associated organic matter. But the forces generated in the process may also alter the dispersion products and, thus, potentially interfere with the interpretation of experimental results. This review summarizes present knowledge on experimental conditions that may lead to physical damage and chemical modifications of aggregate building blocks. The energy level at which physical disintegration of organic particles could be detected was as low as 60 J mL^–1. Physical damage of sand‐ and silt‐sized mineral particles was observed to commence at energy levels exceeding 700 J cm^–3. No evidence was found for the disintegration of particles within the clay‐size fraction of soils even though studies analyzing pure minerals such as kaolinite revealed particle breakage after application of energy amounts > 12,000 J cm^–3. Here we outline a strategy to minimize artifacts such as physical damage of mineral or organic particles resulting from ultrasonication by adopting a stepwise dispersion protocol involving successively higher energy levels, accompanied by a sequential separation of organic and mineral compounds.     

Evaluation of an ultrasonic dispersion procedure to isolate primary organomineral complexes from soils

Soil organic matter can be intimately associated with mineral particles of various sizes. For structural studies, soil organic matter can be isolated in particle size fractions after complete dispersion of the aggregates by ultrasonication. The ultrasonic dispersion energy necessary for complete dispersion was investigated in three A and two B horizons originating from four soils differing in pedogenesis (Gleysol, Phaeozem, Podzol, Alisol), organic C (4.2–34.5 g kg^–1) and clay content (24–294 g kg^–1). Calorimetric calibration of five probe-type ultrasonifiers revealed that the actual energy output from an instrument can depart widely from its nominal output, and that this discrepancy varies from instrument to instrument. Calorimetric calibration is therefore essential for consistency and comparisons between laboratories. Between 450 and 500 J ml^–1 of ultrasonic dispersion energy was enough to disperse completely all samples investigated. The particle size distributions obtained were close to those from standard analysis, except for smaller yields (–20 to –80 g kg^–1) of sand size fractions, which suggests that dispersion by ultrasound is more effective. Based on total C, C:N ratio and distribution of dissolved C, no detachment of soil organic matter from primary organomineral complexes and no redistribution between particle size fractions could be detected in the range 30–590 J ml^–1 of dispersion energy.     

Disruption of soil aggregates by varied amounts of ultrasonic energy in fractionation of organic matter of a clay Latosol: carbon,nitrogen and δ13C distribution in particle‐size fractions

Ultrasonic energy has been widely used to disrupt soil aggregates before fractionating soil physically when studying soil organic matter (SOM). Nevertheless, there is no consensus about the optimum energy desirable to disrupt the soil. We therefore aimed (i) to quantify the effect of varied ultrasonic energies on the recovery of each particle‐size fraction and their C, N and δ¹³C distribution, and (ii) to determine an ideal energy to fractionate SOM of a specific soil. Our results show that the 2000–100 μm particle‐size fraction was composed mainly of unstable aggregates and the 100–2 μm fraction of stable aggregates. Energies of 260–275 J ml^?1 were sufficient to disrupt most of the unstable aggregates and leave stable aggregates. The use of this threshold energy combined with particle‐size fractionation was not satisfactory for all purposes, since litter‐like material and relatively recalcitrant organic carbon present in stable aggregates > 100 μm were recovered in the same pool. An ultrasonic energy of 825 J ml^?1 was not sufficient to stabilize the redistribution of soil mass and organic matter among particle‐size fractions, but at energies exceeding 260–275 J ml^?1 relatively stable aggregates would fall apart and cause a mixture of carbon with varied nature in the clay fraction.     

Wet sieving versus dry crushing: Soil microaggregates reveal different physical structure,bacterial diversity and organic matter composition in a clay gradient

Soil microaggregates contain particles of different sizes, which may affect their potential to store organic carbon (OC). A variety of methods can be used to isolate microaggregates from the larger soil structures, among which wet sieving approaches are widely employed. We developed a novel dry crushing method that isolates microaggregates along failure planes due to mechanical stresses rather than hydraulic pressures and compared the mechanical stability, OC contents and microbial community composition between dry-crushed and wet-sieved samples with contrasting clay contents. Dry-crushed samples exhibited a higher stability and bacterial diversity compared to wet-sieved samples. As a result, the dry-crushed microaggregates had different size distributions when analysed dry and after wetting. In the dry state, dry-crushed microaggregates were larger and contained more sand-sized primary particles within the aggregate structures. The wetting of dry-crushed aggregates caused a disintegration of larger microaggregates and sand-sized primary particles into smaller microaggregates that contained finer particles. In the soils with lower clay contents, the diameter of dry-crushed microaggregates was 40 μm larger due to more sand-sized primary particles remaining within the aggregates. Depending on how much volume in microaggregates is occupied by large primary particles, the OC concentration increased in the soil with higher clay content. Wet-sieved size fractions also showed a similar pattern of OC distribution, whereas more primary particles were observed outside of aggregates. Wet sieving approaches disperse the soil into OC-rich aggregates and might be preferable if OC dynamics are investigated. Differences in bacterial community composition in dependence on clay content were more pronounced in dry-crushed microaggregates. If intact aggregate architectures are of interest for the isolation of soil structural units, the presented dry crushing method might provide an advantageous alternative that also better preserves bacterial diversity.     

施用有机肥对土壤团聚体稳定性的影响

施用有机肥是循环农业的典型措施,能够净化环境、保证食品安全、加强土壤的可持续利用。本文以中国农业大学曲周试验站长期设施蔬菜地为研究对象,试验已进行6年,共设单施有机肥、有机无机配施与无机肥3种施肥处理。结果表明:施用有机肥处理的土壤有机C含量显著高于有机无机配施和无机肥处理107.02%、171.71%;干筛分析表明有机肥处理下的土壤非水稳性团聚体的平均重量直径(WMD)和几何平均直径(GMD)值均显著高于有机无机配施与无机肥65.68%、4.18%和16.80%、8.26%;湿筛结果也表明有机肥处理下的土壤水稳性团聚体WMD、GMD值显著高于有机无机配施与无机肥41.12%、34.78%和77.78%、63.16%;0~20 cm耕层有机肥处理增加了土壤分散系数,而20~40 cm耕层有机肥处理显著降低了土壤分散系数。在蔬菜有机栽培中单施有机肥可增加土壤有机C、非水稳性团聚体、水稳性团聚体及耕层下微团聚体含量,是改良土壤结构的有效措施。     

Method of studying the degradation of soil macroaggregates based on the self-absorption of 137Cs β radiation

A method was proposed for studying the degradation of soil aggregates in time from the absorption of ¹³⁷Cs β-radiation by a soil sample. The method is based on the difference in the recorded β radiation intensities depending on the distribution of ¹³⁷Cs between the surface of the aggregates and the nonaggregated soil material. At the localization of the radionuclide on the surface of the soil aggregates, the counting rate will significantly exceed that for the samples containing a similar amount of the radionuclide uniformly distributed throughout the soil volume. This effect is due to the insignificant range of the mild β radiation in the soil (less than 1 mm depending on the radiation energy). Relatively clear calibration graphs under the selected conditions of the radiometric measurements were obtained for the aggregates whose sizes were in the range of 3–10 mm. Under natural conditions, the 7- to 10-mm aggregates of a dark gray clay loamy soil under a forest belt decomposed by 50–65% at a depth of 10 cm and by 23–32% at a depth of 30 cm. The more intense degradation of the soil aggregates of the same size was observed under the conditions of a pot experiment.     

Soil structure and management: a review

Soil structure exerts important influences on the edaphic conditions and the environment. It is often expressed as the degree of stability of aggregates. Aggregation results from the rearrangement, flocculation and cementation of particles. It is mediated by soil organic carbon (SOC), biota, ionic bridging, clay and carbonates. The complex interactions of these aggregants can be synergistic or disruptive to aggregation. Clay-sized particles are commonly associated with aggregation by rearrangement and flocculation, although swelling clay can disrupt aggregates. Organo-metallic compounds and cations form bridges between particles. The SOC originates from plants, animals and microorganisms, and their exudates. It enhances aggregation through the bonding of primary soil particles. The effectiveness of SOC in forming stable aggregates is related to its decomposition rate, which in turn is influenced by its physical and chemical protection from microbial action. Soil inorganic carbon (SIC) increases aggregation in arid and semi-arid environments, and the formation of secondary carbonates is influenced by the presence of SOC and Ca²⁺ and Mg²⁺. Soil biota release CO₂ and form SOC which increase dissolution of primary carbonates while cations increase precipitation of secondary carbonates. The precipitation of (hydr)oxides, phosphates and carbonates enhances aggregation. Cations such as Si⁴⁺, Fe³⁺, Al³⁺ and Ca²⁺ stimulate the precipitation of compounds that act as bonding agents for primary particles. Roots and hyphae can enmesh particles together while realigning them and releasing organic compounds that hold particles together, a process with a positive impact on soil C sequestration. Soil structure can be significantly modified through management practices and environmental changes. Practices that increase productivity and decrease soil disruption enhance aggregation and structural development.     

聚乙烯微塑料浓度对黑土团聚体特征及其稳定性的影响

  目的  微塑料会与土壤颗粒及团聚体相互作用而影响土壤的稳定性,探究微塑料浓度对黑土团聚体特征及其稳定性的影响,以期为农田土壤（微）塑料污染及土壤健康评价提供数据基础。  方法  通过大豆盆栽实验,研究自然条件下不同微塑料浓度（0%、0.1%、0.5%、1%、2%、5%）对黑土团聚体组成、团聚体稳定性（大团聚体含量R _{> 0.25}）、土壤团聚体特征指标（平均质量直径MWD、几何均重直径GMD、分形维数FD）的影响。  结果  不同微塑料浓度处理中,< 0.25 mm的机械稳定性团聚体含量比例最小,且 > 2 mm和 < 0.25 mm的团聚体含量比例随着微塑料浓度的增加而增加;> 2 mm的水稳性团聚体含量比例最小,< 0.25 mm的水稳性团聚体含量比例随着微塑料浓度的增加而增加;但在1%浓度时,机械稳定性和水稳性团聚体含量比例均与其他浓度的趋势相反;无植物种植的团聚体变化与种植物的相似。土壤大团聚体（R _{> 0.25}）的含量比例随着微塑料浓度的增加而显著减小,当微塑料浓度为1%时,其含量比例略低于对照试验（CK）。不同采样期,大豆成熟期的土壤机械稳定性团聚体和水稳性团聚体的MWD、GMD均比花期的小,而花期的团聚体分形维数比成熟期高,表明随着大豆生长及微塑料的作用,土壤团聚体稳定性降低。通过相关性分析表明,MWD与GMD呈极显著正相关关系,且二者均与FD值呈极显著负相关,即土壤团聚体MWD和GMD总体显著增大,FD值则显著减小,从而表征土壤颗粒团聚性下降。此外,当土壤中微塑料浓度为1%时,土壤团聚体分形维数最小,即土壤团聚作用增强。  结论  土壤中微塑料累积浓度越高,对土壤团聚体产生的破坏作用越强,导致土壤颗粒间聚合能力减弱,土壤中微塑料浓度为1%是否可作为影响黑土团聚体稳定性变化的阈值还有待后续研究,以期为全面评估微塑料对土壤质量的影响提供依据。     

Decomposition of bacterial polymers in soil and their influence on soil structure

Summary The adherence of soil particles into stable aggregates increases with the addition of monosaccharides or polysaccharde polymers to soil, either as plant residues, microbial metabolites, or as simple carbohydrates. Microbial polysaccharides are one of the most effective organic agents that promote soil aggregate stability, but the effectiveness of these polymers in stabilizing soil particles varies dramatically between microbial strains, the amount present and the prevailing environmental conditions. We conducted glasshouse and laboratory studies to determine the effectiveness of selected microbial polymers in stabilizing soil aggregates. The addition and thorough mixing of 1.0 mg microbial polymer C g^–1 soil of seven bacteria strains (Arthrobacter viscocus, Azotobacter indicus, Bacillus subtilus, Chromobacterium violaceum, Pseudomonas aeruginosa, Pseudomonas strain I, and Pseudomonas strain II), three deuteromycete strains (Cryptococcus laurentii, Hansenula holstii, and Mucor rouxii), and two reference compounds (hydroxyethyl guar and glucose) to an Arlington coarse-loamy soil resulted in stimulated soil respiration, increased aggregate stability, and decreased soil bulk density and modulus of rupture when incubated from 1 to 12 weeks. The monosaccharides present in the added polymers were rapidly decomposed and the sacchride content of the polymer-treated soil returned to the level of the soil control (with no polymer addition) after 2 weeks of incubation, while the maximum increase in soil aggregate stability was noted during the 3rd and 4th weeks of incubation. Statistical analyses showed that the glucose content of the polymers added was significantly correlated with soil aggregation [weeks 1 (r=0.78^***) and 2 (r=0.61^*)], but the extractable soil saccharides were not significantly correlated with increased aggregate stability or decreased soil bulk density during this study. When microbial extracellular polymers were added to soil only a transient increase in soil stability was measured upon decomposition of the added saccharides. This finding suggests that the stabilization of soil aggregates is a result of other microbial processes or metabolites rather than the direct binding effects of the added polysaccharides.     

降雨驱动下红壤团聚体的溅蚀特征及周转过程

[目的]研究降雨驱动作用下土壤团聚体受雨滴打击发生破碎和形成的过程,丰富土壤侵蚀研究机理。[方法]基于稀土元素示踪法,对各粒径土壤团聚体同时进行标记。在90 mm/h降雨溅蚀条件下,通过各粒径土壤团聚体(2～5 mm, 0.25～2 mm, 0.053～0.25 mm,<0.053 mm)在不同降雨特征参数(降雨历时、雨滴大小)下的质量变化和稀土元素含量变化,定量分析了团聚体间的周转路径和溅蚀颗粒特征。[结果]降雨驱动作用下,溅蚀量和溅蚀率会随着降雨动能的增加而变大,溅蚀颗粒主要分布于0.25～2 mm粒径范围内;除>2 mm的颗粒为大团聚体直接飞溅产生,<0.25 mm粒级溅蚀颗粒均主要源于大粒级团聚体破碎形成,最高可达到73.83%,其次为该粒级直接被击飞形成,同时会有小粒级颗粒吸附黏结形成;在残余团聚体的动态周转过程中,主要是相邻级别的团聚体间形成和破碎过程占比较高,其中大团聚体破碎产生小团聚体和粉黏粒团聚形成小团聚体分别对原粒级团聚体的破碎和形成方向的贡献率较高,分别达到24.06%～42.15%和36.83%～70.76%,且随着降雨时间的变化,大团聚体首先...     

A simultaneous model for ultrasonic aggregate stability assessment

Aggregate stability is a difficult to quantify, complex soil property. Ultrasonic processing of soil–water suspensions enables quantifiable and readily reproducible assessment of the level of mechanical energy applied to soil aggregates. Here, we present a method of investigating the stability and comminution of soil aggregates by simultaneously modeling the redistribution of particles throughout any arbitrarily-selected set of soil particle-size intervals as ultrasonic energy is applied to a soil–water suspension. Following model development, we demonstrate its application to 5 particle-size subgroups (0.04–2000 μm) of a Dystroxerept subject to 12 levels of ultrasonic energy between 0 and 5800 J g^− 1 (750 mL^− 1). Laser granulometry was used for particle-size distribution (PSD) analysis, providing precise, non-disruptive measurements of changes in the volume of PSD subgroups in both the microaggregate (< 250 μm; 3 subgroups) and macroaggregate (> 250 μm; 2 subgroups) fractions throughout ultrasonic treatment. Two groups of aggregates were detected exhibiting significantly (p < 0.05) different ultrasonic stability: a group composed exclusively of macroaggregates ranging 250–2000 μm in size, and a finer, relatively stable group ranging 20–1000 μm. The PSD of particles liberated from two aggregate groups significantly (p < 0.05) differed: the coarser, less-stable group liberated 13% clay (0.04–2 μm), 53% fine silt (2–20 μm), and 34% coarse silt and sand (20–250 μm); while the finer, more-stable group liberated 26% clay and 74% fine silt. The ultrasonic energy required to disrupt 25%, 50%, and 75% of all aggregates within a given PSD interval significantly (p < 0.05) differed between all selected intervals, showing a trend of declining stability with increasing particle-size. Both the flexibility of the proposed model and the extension of ultrasonic stability assessment to simultaneous analysis of both microaggregate and macroaggregate subgroups can facilitate broader application of ultrasonic methods to soil processes related research.     

The capacity of soil particles for spontaneous formation of macroaggregates after a wetting-drying cycle

The capacity of soil particles for spontaneous formation of aggregates >0.25 mm was studied in a laboratory experiment. The particles from soil aggregates (3-1 mm) (initially aggregated particles, APs) and initially free particles (FPs) of <0.25 mm in size were isolated from the soddy-podzolic and chernozemic soils under fallow and from the arable soddy-podzolic soil. The aggregates of 3-1 mm were ground and passed through a 0.25-mm sieve. Then, the aggregates and free particles were poured with water and dried, and the content of the formed aggregates and their water stability were determined; in the samples from the arable soddy-podzolic soil, the organic carbon content was also determined in the newly formed aggregates. The FPs from the untilled soils formed almost no aggregates. At the same time, the APs from these soils manifested the ability for the spontaneous formation of aggregates, including water-stable aggregates. In the arable soddy-podzolic soil, on the contrary, both FPs and APs demonstrated the capacity for spontaneous self-organization into aggregates. The water stability of the self-organized aggregates from the arable soil was similar regardless of their source (APs or FPs). It was supposed that the ability of the FPs from the arable soil to form macroaggregates reflects the mechanical degradation of the aggregates in the soil: tillage results in the degradation of the aggregates, and the particles capable of spontaneously aggregation temporarily fall in the fraction of <0.25 mm. The water-stable aggregates produced from the APs or FPs of the arable soil contained more organic carbon (1.89%) in comparison with the water-stable aggregates separated from the initial 3- to 1-mm aggregates of this soil (1.31%).     

石灰岩区土壤分形特征及其与土壤性质的关系

研究了岩溶坡地不同生态系统土壤颗粒组成和团粒结构的分形特征.结果表明,土壤颗粒组成分形维数与黏粒及物理性黏粒含量显著正相关,与砂砾含量显著负相关.团粒结构分形维数与水稳性团聚体含量显著负相关,与团聚体湿筛后的破坏率显著正相关,即分形维数愈高,>0.25 mm水稳性团聚体和水稳性大团聚体含量愈低;团粒结构的分形维数与土壤有机质有负相关趋势,与土壤阳离子交换量显著负相关,与土壤体积质量(容重)呈正相关趋势.次生灌丛岩溶生态系统退化后,土壤黏粒减少,体积质量上升,土壤水稳性团聚体含量及其稳定性下降,土壤颗粒组成分形维数降低,土壤团粒结构分形维数则呈上升趋势.颗粒组成分形维数与团粒结构分形维数对土壤质量和岩溶生态环境状况的反映是一致的.     

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.     

Erodibility of model soils with different densities

Soil erodibility as a function of the soil density was studied in a hydraulic flume. The experiments were performed with mixtures of aggregates and different-sized chernozemic soil particles with the predominance of the fraction of 0.5–2 mm, which corresponded to their proportions in the plow horizon. It was found that, when the soil density increased, the erodibility abruptly decreased and was well described by a power function with an exponent of −9.25 (R ² = 0.997). The approximate calculation of the contact area between the aggregates showed that the cohesion force between them is not a linear function of the total area of the interaggregate contacts.     

土壤原始颗粒对不同破碎机制下团聚体稳定性的影响

土壤团聚体是土壤结构的基本单元,其稳定性是描述土壤抵抗外力破坏作用的重要指标。目前常用的团聚体测定方法很少考虑到土壤原始颗粒对其不同破碎机制下稳定性的影响。以两种不同质地团聚体特征差异明显的壤质砂土和砂质黏壤土为研究对象,对土壤全样进行快速湿润(FW)、预湿润后震荡(WS)以及慢速湿润(SW)三种处理方式预处理以研究团聚体不同破碎机制,同时考虑将各粒级团聚体中的土壤原始颗粒剥离出来,消除土壤原始颗粒对各粒级团聚体含量结果的影响,研究土壤原始颗粒对不同破碎机制下团聚体稳定性的影响。结果表明:两种土壤在不同处理方式下,各粒级土壤含量存在较大差异,砂质黏壤土在三种处理模式下平均质量直径(MWD)均显著大于壤质砂土,两种土壤MWD均呈现MWD_(fw)MWD_(ws)MWD_(sw)的大小顺序,两种土壤团聚体破坏均是团聚体快速湿润时孔隙内部封闭的空气压力作用为主,其次是机械扰动作用,黏粒膨胀作用影响最小。土壤原始颗粒对各粒级团聚体的影响程度受到土壤类型和破碎机制影响,土壤原始颗粒对壤质砂土影响较大,对砂质黏壤土的影响相对较小。分散前的团聚体(0.05 mm)占总土壤的百分比(AR)值难以正确反映土壤团聚体稳定性,消除土壤原始颗粒影响后,AR能够较好体现土壤团聚体稳定性。消除土壤原始颗粒影响前后的AR比值表明土壤原始颗粒对壤质砂土的影响远远大于对砂质黏壤土的影响。     

Redistribution of particulate organic matter during ultrasonic dispersion of highly weathered soils

The use of ultrasonic energy for the dispersion of aggregates in studies of soil organic matter (SOM) fractionation entails a risk of redistribution of particulate organic matter (POM) to smaller particle‐size fractions. As the mechanical strength of straw also decreases with increasing state of decomposition, it can be expected that not all POM will be redistributed to the same extent during such dispersion. Therefore, we studied the redistribution of POM during ultrasonic dispersion and fractionation as a function of (i) dispersion energy applied and (ii) its state of decomposition. Three soils were dispersed at different ultrasonic energies (750, 1500 and 2250 J g^?1 soil) or with sodium carbonate and were fractionated by particle size. Fraction yields were compared with those obtained with a standard particle‐size analysis. Undecomposed or incubated (for 2, 4 or 6 months) ¹³C‐enriched wheat straw was added to the POM fraction (0.25–2 mm) of one of the soils before dispersion and fractionation. Dispersion with sodium carbonate resulted in the weakest dispersion and affected the chemical properties of the fractions obtained through its high pH and the introduction of carbonate. The mildest ultrasonic dispersion treatment (750 J g^?1) did not result in adequate soil dispersion as too much clay was still recovered in the larger fractions. Ultrasonic dispersion at 1500 J g^?1 soil obtained a nearly complete dispersion down to the clay level (0.002 mm), and it did not have a significant effect on the total amount of carbon and nitrogen in the POM fractions. The 2250 J g^?1 treatment was too destructive for the POM fractions since it redistributed up to 31 and 37%, respectively, of the total amount of carbon and nitrogen in these POM fractions to smaller particle‐size fractions. The amount of ¹³C‐enriched wheat straw that was redistributed to smaller particle‐size fractions during ultrasonic dispersion at 1500 J g^?1 increased with increasing incubation time of this straw. Straw particles incubated for 6 months were completely transferred to smaller particle‐size fractions. Therefore, ultrasonic dispersion resulted in fractionation of POM, leaving only the less decomposed particles in this fraction. The amounts of carbon and nitrogen transferred to the silt and clay fractions were, however, negligible compared with the total amounts of carbon and nitrogen in these fractions. It is concluded that ultrasonic dispersion seriously affects the amount and properties of POM fractions. However, it is still considered as an acceptable and appropriate method for the isolation and study of SOM associated with silt and clay fractions.     

土粒表面电场对土壤团聚体破碎及溅蚀的影响

团聚体是土壤结构的基本单元,其稳定性是评估土壤抗侵蚀能力的重要指标。土壤团聚体破碎是降雨溅蚀发生的关键一步。土粒表面电场对团聚体稳定性具有重要影响,必然也会深刻影响降雨溅蚀过程。该文以黄土母质发育的黄绵土和塿土为研究对象,采用不同浓度的电解质溶液定量调控土粒表面电场,研究不同电场强度对团聚体破碎及溅蚀的影响。结果发现:1)随电解质浓度的降低,土粒表面电位升高,表面电场增大,黄绵土和塿土团聚体平均重量直径减小,团聚体稳定性降低,降雨溅蚀量增大。2)电解质浓度小于10-2 mol/L,黄绵土和塿土表面电位绝对值分别高于202.0和231.6 mV,此时团聚体稳定性和溅蚀量变化不明显,表明表面电位202.0和231.6 mV分别是影响黄绵土和塿土团聚体稳定性及溅蚀的关键电位。3)随着土粒表面电场的减弱,团聚体破碎后释放的<0.15 mm微团聚体含量减小,>0.25 mm大团聚体含量增加,团聚体倾向于破碎为更大粒级的团聚体。4)电场作用下团聚体的破碎特征对降雨溅蚀具有重要的影响,溅蚀量与团聚体破碎释放的<0.15 mm微团聚体含量呈显著正相关,与>0.25 mm大团聚体含量呈显著负相关。上述结果表明,当降雨进入土壤后,对于干燥的土壤而言,土壤溶液电解质浓度被迅速稀释,土粒表面产生强大的电场,该电场通过影响团聚体破碎程度进而影响降雨溅蚀。该研究有助于加深对降雨溅蚀的科学认识,同时也为土壤团聚体稳定性及降雨溅蚀的人为调控提供了一定的理论依据。     

Eine Methode zur Ermittelung der wasserspannungsabhängigen Änderung des Sauerstoffpartialdruckes und der Sauerstoffdiffusion in einzelnen Bodenaggregaten

A method to determine oxygen partial pressure and oxygen diffusion in single soil aggregates as a function of soil moisture tension Anaerobic zones occur even in unsaturated soils of silty or clayey texture, that are aerated sufficiently in their macropore system. These zones can be related to the inner parts of soil aggregates. To describe the oxygen balances in soils it is necessary to measure not only in soil profiles but as well in single soil aggregates within a range of soil matrix potentials. Therefore oxygen partial pressure in single soil aggregates of different texture was measured continuously as a function of soil matrix potential. For that purpose we developed an oxygen sensitive microelectrode with a tip diameter of 0.5 mm, that is sturdy enough to measure even in sandy soils. One microtensiometer (diameter of the tip < 0.5 mm) and one oxygen microelectrode were placed in water saturated soil aggregates. Soil water potential and oxygen partial pressure were measured continuously during soil drying. The results show an aeration of primarily anoxic soil aggregates at different soil matrix potentials due to different texture and structure. The clayey polyhedral aggregates of the Vertisol were aerated at significantly lower soil matrix potentials than the loamy prisms of the Fluvisol. These show higher values of oxygen partial pressure even at soil water potentials less than 150 hPa. In the aggregates of the Vertisol, that have a fine texture, values of rel. aparent diffusion D_s/D_o were in the range of 1 · 10^?3 at soil water potentials < ?     

Influence of Artificial Zeolite and Hydrated Lime Amendments on the Erodibility of an Acidic Soil

In this study, the effectiveness of artificial zeolite and hydrated lime, as amendments, to reduce surface runoff and soil loss from acidic soil taken from Yamaguchi prefecture in Japan was assessed. Air‐dried soil aggregates (?2 mm) were amended with zeolite at 10% and with lime at 0.5%. The amended aggregates were packed to an average dry‐bulk density of 1.30 Mg m^?3 in small soil trays and subjected to simulated rain intensities of 30 and 60 mm h^?1. Two pretreatments were used before subjecting them to simulated rainfall: (a) soil incubated for 2 weeks and (b) soil kept for 5 months, irrigated every two days. The data indicated that surface runoff was characterized by three phases. The amendments' impact was most significant during the first two phases as the amendments induced the formation of particles sized ? 106 µm, which are mostly responsible for the crusting. Amendments decreased soil losses, and the magnitude of reduction became higher when the irrigation pretreatment was applied before simulated rainfall. The amendment led to increase in soil organic carbon, wet aggregate stability, saturated hydraulic conductivity, and the large particle size in the sediment. The results suggest that surface runoff and soil loss in acidic soils can be substantially reduced by application of zeolite or lime, and this is attributed to the increase in wet aggregate stability and the large particle size in the sediment because of the amendments, and in this regard zeolite is more effective than lime.