Amorphous clay minerals of recent volcanic ash soils in hokkaido (ii)

In the previous study (1), it was shown that the size-fractions finer than 2 μ of the soil samples were abundant in siliceous amorphous materials. These materials are more siliceous in the specimens of the A horizons than in the C and tend to become more siliceous with increase of particle-size. Discrete amorphous silica is present in all the size-fractions finer than 2 μ and its amount is considerably greater in coarser clay fractions. Since the proportions of coarser clay fractions are greater in the whole clay fractions, it is considered that the amorphous silica of the coarser clay fractions play a significant role in determining the properties of all of the amorphous clay minerals contained. Amorphous silica exists not only in the fractions finer than 2 μ, but also in the fractions coarser than 2 μ, even in the fine sand fractions of the A horizons.     

Clay minerals of red-yellow soils derived from pleistocene sediments in Japan

Red-Yellow soils are widely developed on terraces and hilly lands in the south-western half of Japan. They do not show any evidence of bleaching in the lower part of the A horizon, and are characterized by an extremely strong acid reaction, and a very low base-status⁹⁾. There are few studies on clay mineralogy of Red-Yellow soils in Japan. Egawa et al⁴⁾. have reported on clay mineralogy of soils derived from the Pleistocene and the Tertiary sediments most of which may be regarded as Red-Yellow soils. Matsui and Katô¹⁰⁾ have described clay minerals of Red-Yellow soils derived from the Pleistocene sediment in the environs of Shinjobara, Shizuoka Prefecture. These investigations indicated that clay minerals of Red-Yellow soils derived from the Pleistocene sediments consisted mainly of kaolin minerals, whereas those of Red-Yell ow soils derived from the Tertiary sediments were of the kaolin-illite association.     

Clay minerals and humus complexes in five Kenyan soils derived from volcanic ash

Clay minerals and humus complexes and their effects on soil chemical properties were studied for five profiles around the Longonot and Susua volcanoes in Kenya. Morphologically, all profiles have some resemblance to Andepts, but there were clear-cut differences in clay contents and mineral composition as well as pH between profiles 1 through 4 and profile 5, which occur under semi-arid to subhumid and under humid climatic conditions, respectively. Profiles 1 through 4 contained embryonic halloysites with X-rays amorphous iron oxide and silica, whereas profile 5 contained a kaolin mineral, chloritized 2:1 layer silicates, gibbsite and hematite. These mineral associations probably represent early and advanced stages of weathering of volcanic ash under weak and strong leaching conditions, respectively. Embryonic halloysites consisted of more or less poorly ordered halloysite and “1.0- and 0.7-1.0-nm minerals”. The carbon contents of A horizons ranged from 100 to 112 g/kg in profiles 2, 3 and 5. Humus complexes increased in the order; Fehumus ⩽ Al-humus ⩽ Ca-humus in profiles 2 and 3, whereas Al- and Fe-humus were found exclusively in profile 5. The pH (NaF), phosphate retention and acid oxalate extractable Al qualify only profile 5 as an Andept or Andisol.     

Formation of clay minerals in soils developed from basic volcanic rocks under semiarid climatic conditions in Lanzarote,Spain

Clay formation in soils was studied in four landscapes of different ages in Lanzarote (Canary Islands). Landscape IV_B is about 250 years old, IV_A subrecent, III early young pleistocene, and landscape I, late tertiary. The soils have developed from basic to ultrabasic pyroclastic fall deposits or basalt.Clay formation in soils of landscape IV_B is in an initial stage having a clay content of 1–2 kg/m², whereas in landscape IV_A 30–40 kg/m² and in landscape III 100–200 kg clay/m² are accumulated. In polygenetic soils of landscape I more than 3000 kg clay/m² can be found. In the youngest soils IV_B, smectite is the most prominent clay mineral, whereas IV_A-soils are dominated by “amorphous” clay. The clay fraction in older soils contains illite, smectite as well as kaolinite in smaller quantities.The chemical composition of the clay fraction of soils with different ages changes significantly according to the mineral composition.     

Cation exchange properties of soils derived from lignite ashes

In Saxony‐Anhalt, Germany, an area of about 6000 ha is covered by lignite‐ash‐derived substrates. In some cases, pollutants like heavy metals or toxic organic compounds had been disposed of together with the lignite ashes. For this reason, we assessed factors influencing the cation exchange capacity (CEC) of lignite‐ash substrates exposed to natural weathering. We chose four research sites reflecting the different methods of disposal: two dumped landfills and two lagooned ashes of different ages. After determining the CEC at pH 8.1 (CEC_pot), we evaluated the influence of the content of silt and clay and the content of total organic C. As lignite‐ash‐derived substrates are rich in oxalate‐extractable Si, Al, and Fe, we performed an oxalate extraction and determined afterwards the CEC_pot to assess the contribution of oxalate‐soluble compounds to the CEC_pot. Moreover, we determined the variable charge of selected samples at pH values ranging from 4 to 7. The lignite‐ash‐derived soils had a high CEC_pot with means ranging from 25.1 cmol_c kg^–1 to 88.8 cmol_c kg^–1. The influence of the parent material was more important than the degree of weathering. The content of total organic C consisting of pedogenic organic matter and coked lignite particles together with the content of silt and clay played a statistically significant role in determining the CEC. Another property that influenced the amount of CEC in medium textured lignite ashes was the content of oxalate‐soluble silica and aluminum. After oxalate extraction, they lost about 30% of their CEC due to the dissolution of oxalate‐soluble compounds. In coarse textured lignite ashes, oxalate extraction led to higher amounts of CEC, probably due to an increase of surface area resulting either from the disintegration of particles or from etching caused by insufficient dissolution of magnetite and maghemite. Moreover, lignite‐ash‐derived substrates exhibit a high amount of pH‐dependent charge. The CEC decreased by 40% in a topsoil sample and by 51% in a subsoil sample as the pH declined from 7 to 4.     

On the volcanic ash soil in Tokachi District,Hokkaido Island,Japan

In Japan, most of the paddy fields are laid out on alluvial plain while other land crop fields are developed on plateau. The greater part of the latter ccnsists of volcanic ash soils.     

中国福建省土壤的主要粘土矿物

The clay minerals of more than 200 soil samples collected from various sites of Fujian Province were studied by the X-ray diffraction method and transmission electron microscopy to study their distribution and evolution.Montmorillonite was found in coastal solonchak,paddy soils derived from marine deposit,lacustrine deposit and river deposit,and some lateritic red soil,red soil and yellow soil with a low weathering degree.Chlorite existed mainly in coastal solonchak and paddy soil developed from marine deposit.1.4nm intergradient mineral appeared frequently in yellow soil,red soil and lateritic red soil.The content of 1.4nm intergradient mineral increased with the decrease of weathering degree from lateritic red soil to red soil to yellow soil.Hydrous micas were more in coastal solonchak,paddy soils derived from marine deposit,lacustrine deposit and river deposit.and puple soil from purple shale than in other soils.Kaolinte was the most important clay mineral in the soils iun this province.The higher the soil weathering degree,the more the kaolinite existed.From yellow soil to red soil to lateritic red soil,kaolinite increased gradually,Kaolinite was the predominant clay mineral accompanied by few other minerals in typical lateritic red soil. Tubular halloysite was a widespread clay mineral in soils of Fujian Province with varying quantities.The soil derived from the paent rocks rich in feldspar contained more tubular halloysite.Spheroidal halloysite was found in a red soil and a paddy soil developed from olivine basalt gibbsite in the soils in this district was largely“primary gibbsite” which formed in the early weathering stage.Gibbsite decreased with the increase of weathering degree from yellow soil to red soil to lateritic red soil.Goethite also decreased in the same sequence while hematite increased.     

Microbial biomass and respiration in soils derived from lignite ashes: a profile study

Microbial biomass C and soil respiration measurements were made in 17–20 yr old soils developed on sluiced and tipped coal‐combustion ashes. Topsoil (0–30 cm) and subsoil (30–100 cm) samples were collected from three soil profiles at two abandoned disposal sites located in the city area of Halle, Saxony‐Anhalt. Selected soil physical (bulk density and texture) and chemical (pH, organic C, total N, CEC, plant available K and P, and total Cd and Cu) properties were measured. pH values were significantly lower while organic C and total N contents and the C : N ratio were significantly higher in the topsoil than in the subsoil indicating the effects of substrate weathering and pedogenic C accumulation. Likewise, microbial biomass C, K₂SO₄‐extractable C, and soil respiration with median values of 786 μg biomass C g^–1, 262 μg K₂SO₄‐C g^–1, and 6.05 μg CO₂‐C g^–1 h^–1, respectively, were significantly higher in the topsoil than in the subsoil. However, no significant difference was observed in metabolic quotient between the topsoil and the subsoil. Metabolic quotient with median values of 5.98 and 8.54 mg CO₂‐C (g biomass C)^–1 h^–1 for the 0–30 cm and 30–100 cm depths, respectively, was higher than the data reported in the literature for arable and forest soils. Microbial biomass C correlated significantly with extractable C but no relationship was observed between it and total N, Cd, and Cu contents, as well as plant‐available K and P. We conclude that the presence of the remarkable concentration of extractable C in the weathered lignite ashes allowed the establishment of microbial populations with high biomass. The high metabolic quotients observed might be attributed to the heavy‐metal contamination and to the microbial communities specific to ash soils.     

Effect of intermittent drainage in reduction of methane emission from paddy soils in Hokkaido,northern Japan

ABSTRACT

Emission of methane (CH₄), a major greenhouse gas, from submerged paddy soils is generally reduced by introducing intermittent drainage in summer, which is a common water management in Japan. However, such a practice is not widely conducted in Hokkaido, a northern region in Japan, to prevent a possible reduction in rice grain yield caused by cold weather. Therefore, the effects of intermittent drainage on CH₄ emission and rice grain yield have not been investigated comprehensively in Hokkaido. In this study, we conducted a three-year field experiment in Hokkaido and measured CH₄ and nitrous oxide (N₂O) fluxes and rice grain yield to elucidate whether the reduction in CH₄ emission can be achieved in Hokkaido as well as other regions in Japan. Four experimental treatments, namely, two soil types [soils of light clay (LiC) and heavy clay (HC) textures] and two water management [continuous flood irrigation (CF), and intermittent drainage (ID)], were used, and CH₄ and N₂O fluxes were measured throughout the rice cultivation periods from 2016 to 2018. Cumulative CH₄ emissions in 2016 were markedly low, suggesting an initially low population of methanogens in the soils presumably due to no soil submergence or crop cultivation in the preceding years, which indicates a possible reduction in CH₄ emission by introducing paddy-upland crop rotation. Cumulative CH₄ emissions in the ID-LiC and ID-HC plots were 21–91% lower than those in the CF-LiC and CF-HC plots, respectively, whereas the cumulative N₂O emissions did not significantly differ between the different water managements. The amount of CH₄ emission reduction by the intermittent drainage was largest in 2018, with a comparatively long period of the first drainage for 12 days in summer. Rice grain yields did not significantly differ between the different water managements for the entire 3 years, although the percentage of well-formed rice grains was reduced by the intermittent drainage in 2018. These results suggest that CH₄ emission from paddy fields can be reduced with no decrease in rice grain yield by the intermittent drainage in Hokkaido. In particular, the first drainage for a long period in summer is expected to reduce CH₄ emission markedly.     

Clay minerals,oxyhydroxide formation,element leaching and humus development in volcanic soils

A weathering sequence with soils developing on volcanic, trachy-basaltic parent materials with ages ranging from 100–115,000 years in the Etna region served as the basis to analyse and calculate the accumulation and stabilisation mechanisms of soil organic matter (SOM), the transformation of pedogenic Fe and Al, the formation and transformation of clay minerals, the weathering indices and, by means of mass-balance calculations, net losses of the main elements. Although the soils were influenced by ash depositions during their development and the soil on the oldest lava flow developed to a great extent under a different climate, leaching of elements and mineral formation and transformation could still be measured. Leaching of major base cations coupled with a corresponding passive enrichment of Al or Fe was a main weathering mechanism and was especially pronounced in the early stages of soil formation due to mineral or glass weathering. With time, the weathering indexes (such as the (K + Ca)/Ti ratio) tend to an asymptotic value: chemical and mineralogical changes between 15,000 and 115,000 years in the A and B horizons were small. In contrast to this, the accumulation of newly formed ITM (imogolite type materials) and ferrihydrite showed a rather linear behaviour with time. Weathering consisted of the dissolution of primary minerals such as plagioclase, pyroxenes or olivine, the breakdown of volcanic glass and the formation of secondary minerals such as ITM and ferrihydrite. The main mineral transformations were volcanic glass ? imogolite ? kaolinite (clay fraction). In the most weathered horizons a very small amount of 2:1 clay minerals could be found that were probably liberated from the inner part of volcanic glass debris. The rate of formation and transformation of 2:1 clay minerals in the investigated soils was very low; no major changes could be observed even after 115,000 years of soil evolution. This can be explained by the addition of ash and the too low precipitation rates. In general, soil erosion played a subordinate role, except possibly for the oldest soils (115,000 years). The youngest soils with an age < 2000 years had the highest accumulation rate of organic C (about 3.0 g C/m²/year). After about 15,000 years, the accumulation rate of organic C in the soils tended to zero. Soil organic carbon reached an asymptotic value with abundances close to 20 kg/m² after about 20,000 years. In general, the preservation and stabilisation of SOM were due to poorly crystalline Al- and Fe-phases (pyrophosphate-extractable), kaolinite and the clay content. These parameters correlated well with the organic C. Imogolite-type material did not contribute significantly to the stabilisation of soil organic matter.     

雷州半岛玄武岩发育的时间序列土壤的发生演变

采集了广东省南部热带地区雷州半岛第四纪以来不同时代(0.58～6.12 Ma)喷发的玄武岩上发育时间序列土壤剖面样品,分析了一些理化性质的变化趋势。结果表明:(1)容重随深度增加而增大,与成土年龄有较好的相关性,说明容重在一定程度上可以指示土壤的发生层次和发育程度。(2)游离铁(Fed)、铁游离度(Fed/Fet)、活性氧化铝(Alo)和铝活化度(Alo/Ald)与成土年龄有着较好的相关性,可以指示土壤的发育程度和相对成土年龄;而无定形铁(Feo)、游离铝(Ald)和无定形硅(Sio)与成土年龄的关系不明显,尚难于指示土壤的发育程度。(3)常用的黏粒风化发育指标,如硅铝率Sa(SiO2/Al2O3)、硅铁铝率Saf(SiO2/(Al2O3+Fe2O3))、风化淋溶系数ba((Na2O+K2O+CaO)/Al2O3)、A指数((SiO2+CaO+K2O+Na2O)/(Al2O3+CaO+K2O+Na2O))、B指数((CaO+K2O+Na2O)/(Al2O3+SiO2+CaO+K2O+Na2O))、WI指数([(2Na2O/0.35)+(MgO/0.9)+(2K2O/0.25)+(CaO/0.7)]×100)、CIW指数([Al2O3/(Al2O3+CaO+Na2O)]×100)、CIA指数([Al2O3/(Al2O3+CaO+Na2O+K2O)]×100,与成土年龄之间相关性不显著,难以指示土壤的风化程度,但黏粒的Sa、Saf和元素Si迁移率的变化在一定程度上揭示了土壤中生物复硅的存在。     

Clay and iron minerals in soils of the clay plains of Central Sudan

Surface soil samples from seven profiles from the central plains of Sudan were investigated using XRD, chemical analysis and Mössbauer spectroscopy measured at 15K. Dioctahedral montmorillonite was the main constituent with different proportions of kaolinite and chlorite. Goethite was found as fine iron oxyhydroxide in all the samples, with haematite detected in two pedons. Crystallinity of the clays was observed to correlate negatively with the amount of free iron oxide in the soil. The influence of the climatic factors on the soil and its formation is discussed.     

Chemical characterization of conducive and suppressive soils for potato scab in Hokkaido,Japan

Potato common scab induced by Streptomyces scabies is a serious constraint for potato-producing farmers and the incidence of potato scab depends on the soil chemical properties. We examined the chemical characteristics of conducive and suppressive soils to potato common scab with reference to the chemical properties of nonallophanic Andosols, recently incorporated into the classification system of cultivated soils in Japan. Allophanic Andosols with a ratio of pyrophosphate-extractable aluminum (Al_p) to oxalate-extractable aluminum (Al_o) of less than 0.3–0.4 were “conducive” soils with a high allophane content of more than 3%. On the other hand, nonallophanic Andosols with a Al_p/Al_a ratio higher than this critical value were “suppressive” soils, and their allophane content was less than 2%. The concentration of water-soluble aluminum (AI) was also a useful index for separating conducive from suppressive soils as well as the Al_p/Al_a value and allophane content. The suppressive soils showed a much higher concentration of water-soluble Al at pH 4.5 to 5.5 than the conducive soils. The high concentration of water-soluble Al may be responsible for the control of the incidence of potato common scab in Andosols.     

Transformation of clay minerals in nanoparticles of several zonal soils in China

Purpose

Clay minerals significantly affect the physical, chemical, and biological processes of soils. They undergo spontaneous modification and transformation depending to the climatic conditions. Information concerning the compositions and transformation of clay minerals in nanoparticle colloids (NPs) (25–100 nm) is severely lacking. Studying clay mineral transformation is important approach to understand soil formation. This study was conducted to determine the transformation sequence of clay minerals in several zonal soil NPs.

Materials and methods

Four soils (Haplustalf, Alf-1; Hapludalf, Alf-2; Hapludults, Ult-1 and Ult-2) were collected from B horizons developed under three different climatic zones of China. Alf-1 (36° 05′ N and 117° 24′ E) was located under a warm temperate zone and Alf-2 (30° 38′ N and 115° 26′ E), Ult-1 (29° 13′ N and 113° 46′ E), and Ult-2 (19° 27′ N and 109° 17′ E) under a subtropical zone. The clay particles (<?2000 nm) (CPs) and nanoparticles (25–100 nm) (NPs) of tested soils were separated. The element composition of CPs and NPs was identified by microwave digestion method. The mineralogy and chemical bonding of clay minerals were studied by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR).

Results and discussion

With decreasing latitude, NPs and CPs showed that the molar ratio of SiO₂ to Al₂O₃ trends to diminish, indicating the phenomenon of desilication and allitization in the tested soils. XRD analysis revealed that the main clay mineral of Alf-1 NPs was illite, followed by vermiculite, kaolinite, and kaolinite interstratified minerals (KIMs). The clay minerals of Alf-2, Ult-1, and Ult-2 NPs were dominated by kaolinite (and KIMs), followed by illite, with a little content of hydroxyl-interlayered vermiculite (HIV) in Ult-1 NPs and trace content of gibbsite in Ult-2 NPs. With decreasing latitude, vermiculite and HIV decreased in NPs. When compared to CPs, smectite as well as illite-vermiculite mix-layer mineral (I-V) and illite-HIV mix-layer mineral (I-HIV) were not detected in NPs. The analysis of d060 region by XRD showed that with decreasing latitude, the main clay minerals in NPs were dioctahedral minerals (e.g., illite or kaolinite). These clay minerals resulted from the transformation of trioctahedral minerals in CPs. The disappearance of 2:1 swelling minerals and trioctahedral minerals showed that the NPs were more susceptible to weathering than CPs.

Conclusions

With decreasing latitude, the transformation of clay minerals followed the sequence of illite?→?HIV?→?kaolinite?→?gibbsite in tested NPs.

     

Swelling and mineralogy of smectites in paddy soils derived from marine alluvium,Japan

A low-swelling smectite exhibiting little intra-crystalline swelling even after saturation with Na was found in paddy soils derived from marine alluvium of Ariake Bay in Kyushu, Japan. The low-swelling smectite was considered to be beidellite-nontronite mineral containing as much as 10% Fe₂O₃. The low-swelling characteristics of this smectite are ascribed to the considerable substitution of Fe²⁺ for Al³⁺ in the ochtahedral layer which depresses the dissociation of unit layers of smectite. Transformation of low-swelling to high-swelling smectite is believed to be due to the oxidation of Fe²⁺ to Fe³⁺ in the octahedral layer.     

Genesis and characteristics of brown forest soils derived from Serpentine in Kyushu,Japan

One of the main objects of the present study is to determine what genetic soil types have been formed from ultra-basic rocks such as serpentine under humid, subtropical conditions. Red-Yellow soils, which are important zonal soils in Kyushu, have not been formed from serpentine, but Brown Forest soils have been formed, as will be described in the part 2 of this paper. This fact for the most part has been governed by the mineralogical and chemical peculiarities of serpentine. Therefore the part 1 of this paper deals with the mineralogical and chemical characteristics of serpentine as a parent rock from which Brown Forest soils have developed.     

Genesis and characteristics of brown forest soils derived from serpentine in Kyushu,Japan

Recently, micropedological investigations have become an important means to explain some pedogenetical processes. The micromorphological features of Brown Forest soils and Mountain Brown Forest soils have been reported by several investigators(12, 14–15), Braunerde can be distinguished from Braunlehm by the presence of a spongy structure, immobility of peptizable iron hydroxides, or by the absence of any fluidal structure in its (B) horizon. In his review on micropedology, osmond(16) has stated that Brown Earths do not show any evidence of illuviation of ferruginous compounds or clay formed by oxidative weathering. As a result of micromorphological investigations on soils derived from serpentine in Greece, Krause(12) has described that the dominant soil of the hillsides is rubefied Braunlehm (Rotlehm), whereas the soil developed on valley terraces is Braunerde which appears as a transformation product of Rotlehm and has a spongy fabric in the A horizon.     

Effect of nitrogen deposition on CH4 uptake in forest soils in Hokkaido,Japan

Abstract

It has been well documented by short-term artificial experiments that the CH⁴ uptake is inhibited by N input, especially NH^{4 p+}-N input. To investigate the effect of the natural N input by throughfall and other factors on the CH⁴ uptake in forest soils, we measured the CH⁴ uptake rates for 6 months during the snow-free period of the year and N input by throughfall throughout the year at 10 sites in Hokkaido, Japan, from 1997 to 2002. Water filled pore space (WFPS) and pH values in the soils varied widely among the sites (38-93% and 3.9-6.2, respectively). The rates of NH^{4 p+}-N and NH^{3 p-}-N inputs ranged from 1.3 to 6.9 kg N ha^p-1 year^p-1 and from 0.8 to 2.9 kg N ha^p-1 year^p-1, respectively. The NH^{4 p+}-N input was generally higher than the NH^{3 p-}-N input. Total N input by throughfall amounted to 2.3-9.4 kg N ha^p-1 year^p-1. The highest CH⁴ uptake rate occurred within the period from July to September (41-215 μg CH⁴ m^p-2 h^p-1) each year at most sites. CH⁴ uptake rate was relatively low (~50 μg CH⁴ M-2 h^p-1) at northern sites, while a high CH⁴ uptake rate was observed throughout the year 100 (? CH⁴ m^p-2 h^p-1) at southern sites. The mean CH⁴ uptake rates were significantly different among the sites. Cumulative CH⁴ uptake ranged from 1.4 to 6.6 kg CH⁴ ha^p-1 [184 d]^p-1 with a mean values of 3.22 ± 1.36 kg CH⁴ ha^p-1 [184 d]^p-1. Cumulative CH⁴ uptake increased with increasing temperature and decreased with an increase in precipitation (Rain), NH^{4 p+}-N input (TF^NH4) WFPS, soil total C (TC), and total N (TN). There was a quadratic relationship between the CH⁴ uptake and NH^{3 p-}-N input (TF^NO3), soil pH, and C / N ratio in soil. A regression equation was obtained as follows to predict the CH⁴ uptake in forest soils: Cumulative CH⁴ uptake = 0.47 / Rain + 0.38 / TF^NH4 + 0.34 / TC - 0.30 / TF^N03 (R ^p2 = 0.74, p = 0.0001). This equation indicates that atmospheric N input into forest soils is one of the main factors that control cumulative CH⁴ uptake with precipitation, total carbon content in soil in Hokkaido, Japan.     

Origin of cristobalite in soils derived from volcanic ash in temperate and tropical regions

Cristobalite was isolated from four Ando soils and a Podzolic soil developed on volcanic ashes in French West Indies, Indonesia and Japan by selective chemical dissolution with 6 M HCl, 0.5 M NaOH and H₂SiF₆ treatments followed by specific gravity separation. All the cristobalite isolates give X-ray diffractograms with a strong and sharp peak at 4.10–4.11 Å and associated peaks at 2.8 and 2.5 Å, which are characteristic for cristobalite of high-temperature origin. Four out of five isolates show a peak at 4.06–4.07 Å ascribed to low-temperature cristobalite. The oxygen isotopic ratios (δ¹⁸O_SMOW) of cristobalite isolates show a range of +5.3 to +11.0%. which suggests a high-temperature igneous origin. These X-ray diffraction and oxygen isotopic data indicate that the cristobalite was inherited from primary volcanic ash rather than formed during pedogenesis.     

Transformation of organic matter and clay minerals in cultivated gray forest soils

Mechanisms of the agropedogenic transformation of cultivated gray forest soils are discussed. It is shown that the loss of organic carbon from these soils upon their cultivation is mainly due to the intense mineralization of an easily decomposable fraction having a density of less than 2 g/cm³ and a characteristic size above 50 μm. Simultaneously, a certain portion of soil microaggregates is destroyed. Several additional indices of the processes of soil erosion and sediment accumulation on the soil surface (soil aggradation) are suggested. These indices take into account the character of soil clay minerals and soil organic matter. Indicative characteristics of the second humus horizon are suggested on the basis of published materials and new analytical data.