1,721,416 research outputs found
Electrophoretic mobility and clay mineralogy of some fragipan and non-fragipan soil horizons.
No full textThe physical properties of the horizons above and below the fragipan are different from those of the fragipan itself. Indeed, fragipans are characterized by massive, dense, and brittle consistence when moist, hard when dry, but their clods are completely disrupted when submerged in water. These physical characteristics have been related to the presence of clay and weak chemical binding, but there is no general agreement on the main process causing the development of this typical behaviour. We recently found that the low permeability and high bulk density of fragipans are linked to specific arrangements of particles: an open packing of the clay phase is associated to an extremely dense packing of silt and sand. This combination was not present in any other horizon of some Typic Fragiudalfs.
Since the packing arrangement of soil colloids depends on the flocculation/dispersion behaviour of clay particles, we evaluated the electrophoretic mobility (EM) and the mineralogical composition of the water dispersible clay fractions of fragipans and non-fragipan horizons. The EM was determined in a pH range from 4 to 6.5, and the hydrodynamic diameter was recorded. The mineralogy of the clay fraction was assessed by XRD.
The clay fraction was composed of a complex suite of minerals: vermiculite prevailed but, illite, chlorite, kaolinite and several interstratified minerals were also present. When clay was separated using only deionised water, the proportion of minerals varied. The EM data of non fragipan samples ranged between -1.2 and -3.3 μm cm s-1 V-1 as a function of pH, whereas fragipan samples had less negative values and varied from -0.7 to -2.3 μm cm s-1 V-1. In both cases, the permanent charge prevailed over the pH dependent, but the contribution of oxides to EM was greater in non fragipan. At the highest pH, when the electrostatic repulsion was higher, the hydrodynamic diameter was similar or slightly smaller in Bx than in non-Bx (0.7 μm and 0.7-0.9 μm, respectively). With decreasing pH, the changes in the electrophoretic mobility were accompanied by an increase in particle size in Bx horizons, up to 0.9-0.8 μm. At the lowest pH, the clay extracted from non-fragipans showed a smaller hydrodynamic diameter, suggesting repulsion within the clay-sized aggregate and confirming the role of oxides.
As Bx horizons were richer in pedogenic Fe oxides than non-fragipans, our results indicate that the open packing of the clay phase typical of fragipans is the result of water stable aggregation among layer silicates and oxides, combined with a poorly aggregated water dispersible clay
Carbon and nitrogen pools in Alpine forest soils: the effect of coarse fragments on variability
No full textRelationships between pedogenetic features and clay mineralogy in Entisols from two alluvial plains of southern Italy
No full textThe relationships between the southern Italy pedoclimatic environment features, the soil properties, and the composition of the soil clay fraction were investigated. Representative Entisols from either Volturno River (pedon V37) or Sibari (pedon S90) River alluvial plains were collected. Both soils exhibited vertic features, indicating the presence of swelling clay minerals. However, the X-ray diffraction (XRD) analysis provided evidence for difference in the mineralogical composition of the clay fractions. The comparison among the XRD patterns of V37 samples ascertained occurrence of smectite. Differently, chlorite minerals with expansible component were identified by comparing XRD patterns for S90 samples. In both cases, "open' illite and kaolinite were also recognized. -from Author
Pore-size distribution and particle arrangement in fragipan and non-fragipan horizons
No full textFragipan is a widely distributed subsoil horizon that induces severe limitations to plant growth and land use, mainly because of its high bulk density. In this work, we evaluated the pore-size distribution through the analysis of the cumulative curve of intruded mercury volume in some soils with fragipan horizons. This approach provides information also about the arrangement of particles, thus we compared the results obtained for fragipan and nonfragipan horizons to relate porosity and particle arrangement with the specific physical properties of fragipans. The total volume of intruded mercury did not allow to discriminate between fragipan and nonfragipan horizons. However, from the variation of the pore volume as a function of the radius, two modal classes of pores were found, coarse and fine, respectively. The fine-pore class arose from the arrangement of clay particles, and its volume was correlated to clay contents (r = 0.787) and to clay packing density (r = –0.621). The clay fractions in fragipans were less densely packed than in the other B horizons, even if they had similar clay contents. The coarser-pore modal class is known to arise from the interactions between clay, silt, and sand particles, and its volume was different among horizons. Fragipan had a low volume of this modal pore class. In addition, a packing density for the coarser phase of 0.74, corresponding to a rhombohedral packing, was found only in fragipans. Thus, the low permeability and high bulk density of fragipans are linked to specific arrangements of the particles: an open packing of the clay phase is associated to an extremely dense packing of silt and sand. This combination is not present in any other soil horizo
Destabilization of aggregates in some Typic Fragiudalfs
No full textSeveral mechanisms are responsible of the destabilization of soil aggregates in water: slaking, swelling, and dispersion of the clay and mechanical breakdown by abrasion. The aggregate resistance differs in relation to the dominant disruptive phenomenon, and by applying different methods to assess aggregate stability, a specific physical susceptibility of different horizons may be evidenced. Therefore, we evaluated the relative importance of the mechanisms of breakdown in some Typic Fragiudalfs, taking into account the specific horizon characteristics, to understand if the fragipan brittleness is related to a specific mechanism, and how the soil properties affect the resistance to fast wetting. The standard wet sieving test, which evaluates all destabilization mechanisms together, indicated a clear role of soil organic matter in protecting the aggregates, with greater losses in the deeper horizons (about 70%), but did not allow us to discriminate between fragipans and other horizons. When the losses caused by water abrasion were separated from the breakdown due to wetting phase using a kinetic approach, a high breakdown caused by fast wetting was found in deep horizons, but still no difference between fragipan and nonfragipans were visible, even though fragipan clods are known to be particularly sensitive to fast wetting. By excluding the effect of organic matter and prewetting the samples with ethanol, differences between these horizons appeared within the water saturation phase and fragipans and non-fragipans were found to be sensitive to different mechanisms. In fragipans, the relative percentage of slaking was higher, always above 30%, whereas for non-fragipans clay dispersion and swelling weighted more heavily. The complexity of the clay fraction did not allow us to relate the mineralogy to swelling or dispersion, but slaking was instead clearly related to clay arrangement and the consequent porosity characteristics (r2 = 0.62), and not to clay content only
Effect of humic acids on the stability and porosity of re-formed soil aggregates
No full textErosion processes destroy soil aggregates and, in some extreme cases, may completely remove the topsoil. A rapid re-formation of water-stable aggregates should then occur to avoid further losses and to restore soil functionality, but aggregation would depend on field conditions and on soil cementing agents. In this work, we investigated the effects of the addition of humic acids and of one cycle of wetting and drying on aggregation and on the physical properties of soil re-formed aggregates. The first horizons of two alpine soils were discarded to simulate a total topsoil removal, and the second ones of each profile, similar in organic matter amounts, but different in clay, were incubated for two weeks. At the end of the incubation period, the samples were fractionated by dry sieving into the <2, 2-5 and >5mm diameter classes, and the stability and porosity were determined. The wet-dry cycle alone induced aggregation, and the higher the clay contents, the larger the re-formed aggregates. This effect was however amplified by the addition of humic acids that enhanced the formation of the largest units. the >5mm fraction was, nevertheless, extremely unstable and contained up to 57% of water-stable microaggregates; lower contents were instead found in the smallest size aggregate fractions. After 10 minutes of wet-sieving the percentage of stable aggregates ranged from 61 to 86.5 in the different size classes. The breakdown caused by slaking was low: 1.7 and 6.2% for the <2mm and 2-5mm aggregates, respectively, indicating that the major loss-inducing process was water abrasion, and water stability was found to decrease with increasing aggregate size. No differences caused by humic acids were found. The pore size distribution was in the aggregate size classes and influenced by the sample treatment: the <2mm aggregates showed a lower presence of >3μm voids, and a higher amount of <0.1μm pores than the 2-5mm aggregates, and the addition of humic acids increased the average size of the pores. In laboratory conditions, we observed a fast formation of aggregates, influenced by the properties of the soils and by the addition of humic acids. The tendency of soil particles to cohere into aggregates in the presence of humic acids induced a more open soil structure with larger aggregates and larger sized pores, but with a lower resistance to rupture. In short time term, therefore, humic acids played an important role in the process of aggregation, but resulted less effective in stabilising the re-formed aggregates
Relationship between clay properties and some physical properties in two alpine soils in piedmont (italy)
No full textSoil water erosion is influenced by several properties and is normally explained by a two step process: the breakdown of soil aggregates caused by raindrop impact and the removal of material through losses with water flow.
The role of clay in the global process depends on the soil type: it may decrease aggregate breakdown by slaking and increases that by differential swelling (microcracking), but it may also affect the amount of particles in the dispersed phase, enhancing therefore the second step of the erosion process.
The dispersibility of fine suspensions depends on a number of properties of the solid phase including the relative amounts of cations on the exchange sites, the mineralogical composition, the surface properties and the size of the particles, which are in turn affected by soil characteristics such as organic matter content.
Most of the studies on the effect of clay on aggregate stability are related to clayey soils. The aim of this work was to evaluate the dispersibility of the clay fraction and the stability of the aggregates on two Alpine soils (Entisol and Alfisol) with low clay contents.
Samples are similar for pH, subacid, and organic matter content. CEC value is 20.0 and 15.7 cmol(+)kg-1, the base saturation is however very different in the two soils and the dominant exchangeable cations are calcium in the Entisol and magnesium in the Alfisol. The Alfisol has more clay and it shows a higher ratio water to chemically-dispersible clay and flocculation value. These findings indicate that the colloidal fraction is more dispersible and more stable in the Alfisol than in the Entisol. In the aggregates the total pore volume and the pore radius decrease as a function of clay content. Both soils show a good stability of the aggregates but differences were found in the kinetics of breakdown.
In Alpine soils, clay properties seem therefore to affect the breakdown caused by the compression of air entrapped inside the aggregates during wetting, but also the mobility of particles in water
Assessing the origin of carbonates in a complex soil with a suite of analytical methods
No full textStable isotope C analysis is the most reliable method used for the distinction and understanding of soil carbonates origin. However, in soils with a complex geological setting the carbonate δ13C signature could lead to incorrect interpretations if used alone. Thus coupling this technique to other methods may be necessary. In this work we evaluated advantages and disadvantages of several methods, some of which are well known while others are still unused, to distinguish among carbonates of different origins in a soil developed on “Valle Versa Chaotic Complex”, a marly geological formation in North-western Italy. For a better evaluation of their potentialities the methods were also applied to simpler situations used as a reference for carbonate of pedogenic and lithogenic origins. Thin sections analysis revealed the presence of three kinds of carbonates in the investigated complex soil: one was pedogenic, while two showed clear lithogenic origin. The lithogenic carbonate that showed a low δ13C (about −9‰) was interpreted as freshwater while isotopic signature increased up to −4‰ with the presence of marls, thus no evidence of pedogenic precipitations could be obtained with isotopic analysis. The mean crystallite domain (L104) was highly variable and related to the amounts of co-precipitated impurities in the carbonates. Thus these methods provided important information about the formation environment. These rarely used techniques permitted to distinguish between pedogenic and lithogenic material in simple systems, but did not adequately support the presence of pedogenic carbonates in the complex soil. Surface areas and porosity evaluated by N2 adsorption are particularly influenced by the processes occurring during calcification such as the development of coatings and pore infillings. The comparison between additive models and measured specific surface area, indeed allowed us to observe the effect of pedogenic carbonate on the physical properties, although it did not permit any quantification. These results indicated that, although all the methods were able to distinguish between pedogenic and lithogenic origins in simple systems, only micromorphology and N2 adsorption techniques allowed for the identification of pedogenic carbonate in a more complex soil system
Rimozione di carbonati, sostanza organica e ossidi di ferro
No full textPer studiarne la mineralogia, la frazione terra fine (diametro equivalente delle particelle < 2 mm) del suolo viene normalmente suddivisa nelle classi granulometriche sabbia, limo ed argilla, cui corrispondono notevoli differenze di composizione mineralogica riferibili ai processi di alterazione della roccia madre alla superficie terrestre. Nella sabbia, ed in modo particolare nella frazione più grossolana della sabbia (2-0.2 mm), predominano, infatti, i minerali primari ereditati dal parent material, nella frazione argillosa (< 0.002 mm) i minerali secondari, mentre nella frazione limosa (0.02-0.002 mm) entità mineralogiche primarie e secondarie sono presenti in quantità intermedie. La frazione granulometrica argilla, ed ancor più l’argilla fine (< 0.001 mm), sono quindi le più adatte ad essere studiate con lo scopo di accertare la formazione nel suolo di nuovi minerali, in seguito a fenomeni di neogenesi (dissoluzione di minerali e precipitazione di prodotti dalla soluzione) o di trasformazione di fasi preesistenti.
La scarsa cristallinità dei minerali argillosi del suolo rispetto a quelli litogenici ne renderebbe difficile l’identificazione nel campione non separato in classi granulometriche. Inoltre, le differenze in composizione mineralogica delle diverse classi vanificherebbero i tentativi di interpretazione delle variazioni mineralogiche a fini pedogenetici.
Nel suolo le particelle minerali sono generalmente presenti non come particelle singole, ma unite a formare aggregati in seguito all’azione cementante di sostanze quali la sostanza organica, i carbonati, gli ossidi di ferro e di alluminio.
L’uso di agenti chimici che selettivamente rimuovono i cementi liberando le singole particelle prima della separazione del campione di terra fine in classi granulometriche è considerato un prerequisito necessario ai fini di una corretta identificazione delle fasi mineralogiche presenti.
E’ importante tenere conto che i trattamenti finalizzati all’eliminazione delle sostanze cementanti possono, tuttavia, provocare modificazioni nella organizzazione strutturale e nelle proprietà chimiche dei fillosilicati, con conseguente variabilità delle fasi identificate in funzione dei metodi adottati. A tale proposito sono reperibili in letteratura diversi lavori che, valutando i meccanismi di azione degli agenti chimici maggiormente utilizzati nei trattamenti preliminari alla separazione in classi granulometriche, indicano quali fasi argillose sono maggiormente vulnerabili. In tutti i casi, l’elevata variabilità che caratterizza i minerali della frazione argillosa del suolo non consente una valutazione a priori dell’effetto del pre-trattamento. Anche il trattamento di rimozione della sostanza organica mediante ossidazione con perossido d’idrogeno non è esente da controindicazioni. L’acidità che si libera durante il processo, con conseguente abbassamento del valore di pH, può indurre la parziale solubilizzazione di fasi minerali a scarso ordine cristallino. La non completa ossidazione della componente organica può portare ad accumulo nel residuo minerale di ossalati di calcio.
In questo capitolo vengono considerati e descritti i procedimenti analitici comunemente impiegati per rimuovere dalla frazione terra fine del suolo i carbonati, la sostanza organica e gli ossidi di ferro amorfi e cristallini. Alla luce di quanto esposto precedentemente e tenendo sempre presente che non esistono tecniche di dissoluzione selettiva, ma solo tecniche di dissoluzione preferenziale, si consiglia di non eseguire pre-trattamenti di routine sui campioni da sottoporre ad analisi mineralogica, ma di valutarne di volta in volta l’applicabilità sulla base del tipo di indagine che si intende effettuare
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