摘要:

SummaryBeneath a layer of artificially deposited peat debris, an eluvial E horizon and thin iron pan (Bsm) has developed in the upper part of the Aphorizon of a brown podzolic soil (Haplorthod) near Castletownbere, Ireland. The thickness of the E horizon and the depth of the pan are directly related to the thickness of the layer of peat debris.The original soil (Haplorthod) was strongly podzolized with a significant accumulation of organic carbon, iron and aluminium in the spodic Bs. The iron pan of the upper sequum, on the other hand, is rich in iron. This iron pan seems to have developed as a result of reduction of iron in the E horizon, transport of divalent Fe2+cations and precipitation in the Bsm as Fe2(OH)3, after the peat layer was deposited. We conclude that podzolization and iron pan development were fundamentally different processes.The study suggests a fundamental change in the iron pan‐blanket peat development sequence previously postulated by palaeoenvironmentalists in Irelan

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01401.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummarySoil aggregates are known to protect organic matter against microbial attack, and thereby stabilize it. The scale at which aggregation is effective and the degree of control it exerts on organic matter transformations are not well known. I investigated whether rapid and slow pools of organic nitrogen bound to the<2‐μm clay fraction could be related to different locations of the clay fraction in the soil structure. Dry aggregates were separated from soil to which15N had been added for 4 years in a field experiment. Soil samples were at maximal immobilization of a labelled inorganic N dressing, and after 1 and 3 years of the beginning of the remineralization phase. Clay‐sized particle fractions (<2 μm) were.separated from the different aggregate‐size classes, and their total C and N contents, and15N isotopic excess, were determined. It seems that recently immobilized N associated with the clay fraction was rapidly sequestered in microaggregates (0.1 mm) of soil

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01402.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryRice systems in Asia have intensified rapidly in the past 30 years, and significant areas of irrigated lowland rice are now supporting two or three rice crops per year. Our objective was to compare the chemical composition of soil organic matter (SOM) from four fields with different histories of rice cropping intensity and soil submergence: (i) a single‐crop rainfed, dryland rice system without soil submergence, (ii) an irrigated rice and soybean rotation, and irrigated (iii) double‐ or (iv) triple‐crop rice systems in which soil remains submerged during much of the year. In all four soils, extracted mobile humic acid (MHA) and calcium humate (CaHA) fractions were of modern age by14C‐dating, and represented about 20% of total N and organic C. The MHA was enriched in N and hydrolysable amino acids (AA) compared with CaHA in all soils. With increased frequency of irrigated rice cropping, however, there was a large increase in phenolic content of SOM. We speculate that slower lignin decomposition caused by deficiency of O2in submerged soil leads to incorporation of phenolic moieties into young SOM fractions. The increased phenolic character of these fractions may influence N cycling and the N supplying capacity of lowland soils supporting two or three annual crops of irrigat

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01403.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryDynamic simulation models of the nitrogen cycle are an important aid for predicting the effect of land management on nitrate concentration in ground and surface waters. These models often neglect, or treat in a highly empirical way, the process of denitrification. The solute leaching model SLIM, suitable for use in aggregated soil, was linked to two alternative denitrification submodels ('simple structure'and ‘aggregate assembly’ treatments). The linked models can be used in dynamic simulation of the fate of nitrate in soil. The models were calibrated using water release and soil respiration data from two field sites (a clay loam and a sandy loam). The fate of Br−1and 99 atom percentage15NO−3tracers on drained plots was measured at these sites. Both models predicted less denitrification from plots receiving small inputs of fresh organic residue than was actually lost; both did better where residue inputs were large. The ‘aggregate assembly’ model predicted short‐term losses immediately after incorporation of residues more successfully than did the ‘simple structured’ model. The simulated dynamics of denitrification during the autumn and winter in the two models were also very different. Overall N balances were better predicted in the clay loam than the sandy loam. Possible reasons for discrepancies between model output and real data include immobilization (especially when large inputs of residues were incorporated), denitrification in the subsoil, and an inadequate description of the way in which degradation of organic matter depen

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01404.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryTwo different humic materials, one from a forest soil and the other from wormcasts, were used to study the influence of mineral and organic acids on the conformational properties of humic substances. The macromolecular changes were followed by low pressure gel permeation chromatography after titrating humic material to low pHs with acids. All organic acids (mono‐, di‐, tri‐carboxylic, and oxy‐acids), added to humic solution prior to a gel permeation in an alkaline buffer, were able to shift the totality of absorbance of the humic chromatographic peak from high to low molecular sizes. Mineral acids, phenol, alcohols, dipolar aprotic solvents, could not produce the same shift and gave total absorbance at the column void volume as in the case of humic substances alone. The chromatographic peak shifted back to elution volumes proper of higher size molecules when the humic‐organic acid mixture was back‐titrated to highpHs before gel permeation. Elution in a much stronger alkaline buffer did not change the overall macromolecular behaviour.These results suggest that humic substances behave as micelles in solution and that hydrophobic bondings play an important role in holding humic molecules together. The organic acids enter the interior of the humic micelle‐like aggregates and alter the stereochemical hydrophobic arrangement of the humic material. In alkaline conditions the negative charges developed disrupt the apparent high molecular size configuration and disperse the humic aggregates into small micelles. Such conformational properties of humic substances appear to be a function of pH and of the concentration of

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01405.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryA multidomain goethite was analysed with atomic force microscopy (AFM) to obtain information about the size and shape of pores on the surface, which are important for sorption processes of cations and anions. Wedge‐shaped pores>100 nm long at the surface occurred at the domain boundaries. They were 20‐30 nm wide, becoming narrower towards the interior of the crystals to 2 nm and less. The AFM showed these pores to be at least 4–6 nm deep. The shape of the pores suggests that smaller pores continue much further into the interior of the crystals. Besides the irregularities between domains, randomly distributed steps of<1 nm occurred. Hydrothermal treatment did not change crystal size, but made the surfaces smoother and reduced both the size and number of the pores and also the BET surface area (from 75.3 to 35.1 m2g−1). The existence of meso‐ (2–30 nm width) and micropores (<2 nm width) in goethite crystals explains the slow diffusion of heavy metals, phosphate and other ions into goethite particles. Therefore, the external and internal surfaces of goethite crystals and their spatial heterogeneity have to be taken into account in models of ion adsorption

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01406.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryThe aggregative effect of organic matter on soil particles is conditioned by its composition and concentration and the degree to which it is associated with metal ions and oxides in the soil. We treated suspensions of kaolin or quartz substrates at pH 5 with metal (Fe or Al;c.3 to 23 mg g−1of substrate) or humic acid (HA; 7 to 53 mg g−1of substrate) or both. In the absence of added Fe or Al, the proportion of HA retained by the substrate decreased from 98.5 to 50.8% for kaolinitic samples, and from 35.8 to 23.4 for quartz‐based samples, as the HA dose increased. In the presence of Fe or Al, the proportion of HA removed from solution increased and was always>91%.Untreated kaolin flocculated between pH 2.0 and 4.5. This flocculation interval narrowed as the dose of HA increased, widened as the dose of HA‐A1 increased, but was barely affected by HA‐Fe. Similarly, the aggregative effect of HA‐Fe associations on kaolin was smaller than that of HA‐A1 associations, which induced formation of aggregates>100 μm that were essentially absent in the untreated substrate. There was a similar but less marked difference between the aggregative effects of HA‐Fe and

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01407.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryCation exchange reactions in clays and soils have repeatedly been shown in the literature to be hysteretic during typical laboratory experiments. Yet all existing models of solute transport in soils and aquifers assume these reactions to be thermodynamically reversible. To remedy this situation a mathematical description of cation exchange hysteresis is needed. In the present research isotherms were obtained for a K‐Mg exchange on Camp Berteau montmorillonite. Scanning curves were also measured, corresponding to arbitrary reversals of the direction of the exchange reactions. Three different Preisach‐hysteresis models were used to describe the scanning curves. The fit of each hysteresis model to the experimental observations was analysed in terms of the features of the model itself and, from a physical viewpoint, in terms of a recently proposed conceptual model of cation exchange hystere

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01408.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryThe influence of soiľs solid phase on the dielectric constant of the soil over a range of moisture contents has been studied. Samples of soil, soil‐like, and also other porous materials were analysed using Time Domain Reflectometry (TDR) to determine the contribution of bulk density and porosity to the function that relates dielectric constant to water content.The study showed that bulk density, and thus also porosity, substantially affects the relation between dielectric constant and water content. Two equivalent, empirical, normalized conversion functions were found, one accounting for bulk density and the other for porosity. Each of them reduced the root mean square error of the dielectric TDR determinations of moisture to 0.03, regardless of the materialľs bulk density and poros

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01409.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryMoisture content and bulk density largely characterize physical and mechanical soil status and behaviour. A nondestructive determination of these soil properties is essential. Time domain reflectometry (TDR), although widely accepted for determination of volumetric water content,θ, has its limitations, and recently a frequency domain (FD) sensor has been developed and tested. An equation relating relative permittivity, ɛ′, to gravimetric water content,w, and bulk density,p, was established for three soil types (sand, sandy loam and clay). If ɛ′ andware known, our model can be used to calculate bulk density and associated volumetric water content,θ, keeping in mind thatθ= pw.Utilization is found in long‐term monitoring of moisture fluctuations or short‐term detection of traffic‐induced s

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01410.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY