摘要:

SummaryThe occurrence of greigite (Fe3S4) in soils is reported for the first time. It forms irregularly‐shaped aggregations within plant cells in the Gr2horizon of a gley soil developed from colluvial material. Greigite was identified by X‐ray diffraction and magnetic measurements and was investigated by optical and transmission electron microscopy. Biogenic formation is proposed, based on the elongated shape of single greigite crystals, and sulphur isotope analyses, which showed a depletion in34S relative to the soil‐water sulphate. The cell‐edge length of 0.98639±0.00003 nm is significantly smaller than values reported for sedimentary greigite. The mean coherence length of 27 nm agrees with TEM observations and indicates that the single greigite crystals lie in the superparamagnetic region. However, the fine aggregates show magnetically single‐domain behaviour. Greigite is the only carrier of a stable magnetic remanence in the soil profi

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00490.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryHigh spatial resolution is obtained from a gamma‐ray transmission density gauge by restricting the measured counts to a narrow band of the energy spectrum, close to the emission energy peak. The effect on measurement accuracy of any movement of this measurement window relative to the energy peak was investigated. The findings were related to anticipated energy peak movements in a proposed LED‐based gain‐stabilization system.Movements of the energy peaks during recording of unstabilized spectra prevented direct comparisons of spectra at different positions. A simulation procedure was, therefore, developed in which movements of the measurement window relative to sets of stable calibration spectra were examined.When analysing spectra, recorded using a gauge with a different gain‐stabilization system, accuracy was found to be unaffected by simulated peak movements of up to 0.03 MeV in the direction of increasing energy. However, movements of stabilized spectra in the direction of decreasing energy, and of unstabilized spectra in either direction, increased measurement errors to twice the level of inherent measurement errors within 0.02 MeV, with errors in bulk density of up to 0.7 Mg m−3for movements of 0.1 MeV. The spectra of the new LED‐based stabilization system are expected to behave in a manner similar to the unstabilized system, therefore requiring regular monitoring of the pe

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00491.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryA slightly modified critical‐state model was formulated in order to account for the volume‐change behaviour at yield and failure observed in triaxial tests on unsaturated soils. Model parameters were specified for two soils (a sandy loam and a loam), each at three different soil‐moisture contents. Maximum shear strain was integrated numerically for 36 cylindrical load paths with constant confining pressure (type I) or constant mean normal stress (type II). Predicted stress‐strain relationships for load paths bringing soils from a normally‐consolidated to a critical state were compared with stress‐strain relationships observed for identical load paths in triaxial tests of the lubricated ends variety.The agreement between predicted and observed maximum shear strain depended on type of load path and soil‐moisture content. The model failed to predict maximum shear strain at stress states close to critical. The absolute difference between observed and predicted strain was on average ≤0.05 for deviatoric stresses smaller than 90% of the critical‐state values. The comparable maximum differences were 0.11 and 0.07 for load‐path types I and II, respectively. The largest differences were found for the largest soil‐moisture contents. The type of load path had a considerable effect on sample distortion, type I giving rise to larger (predicted and observed) maximum shear strain at

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00492.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryThe Green&Ampt infiltration analysis is applied to the problem of the water uptake by aggregates when they are surrounded by water. Two situations are analysed, namely, when there is free escape of the displaced air and when there is no escape of the air. These extreme situations provide bounds for estimating the water uptake for the practical case when some air escapes through the aggregate's surface in the form of bubbles as the aggregate wets up. It is shown that the rate of water uptake is directly proportional to the square of the sorptivity of the aggregate material and inversely proportional to the square of the final water uptake. Experiments on spherical stabilized clay aggregates of different radii were in agreement with the theoretical analysis that predicted the observed very rapid wetting up. The analysis also showed that when there was free escape of air, the rate of advance of the wetting front into cylindrical and spherical aggregates decreased from an initially infinite value to a minimum value and then increased to an infinitely large value when the front reached the centre of the aggregate, in contrast to the continually decreasing rate into plate‐like aggregates. This was demonstrated in experiments on the radial water movement into a fine sand contained in a cone.The analysis and experimental results indicate that preferential macropore flow in aggregated soils can be initiated very rapidly when air entrapment occurs within the aggregate

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00493.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryA simple model was employed to interpret the results of a series of measurements of gas diffusion in soil cores. The model divides air‐filled porosity into three functional categories: arterial, marginal and remote. Diffusion along the axis of the core occurs through arterial pores; marginal pores do not contribute to axial diffusion; remote pores are isolated from gas transport. Simulations based on the model closely resembled data acquired from real cores. Optimizing the fit between real and simulated data gave estimates of the three functional pore fractions which generally made sense (compaction or wetting of cores resulted in reduced arterial and increased marginal porosities, for example). Dividing the pores into the different classes specified by the model was functionally equivalent (i.e. observable results were identical) to the introduction of a tortuosity factor to represent pore convolution. In order to account for observed diffusion rates in terms of pore convolution alone it is sometimes necessary to invoke implausibly high tortuosities; the introduction of marginal porosity renders this unnecessary without in any way compromising the ability of the model to simulate real diffusion dat

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00494.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryThe consequences of slow diffusion of strongly adsorbed solutes into soil aggregates are not fully understood. The distribution of32P after diffusing down a soil column cannot be explained from a consideration of liquid‐phase impedance factor and isotopic exchange alone, as can that of36CI.A model was developed that considers the soil to provide linked parallel intra‐aggregate and inter‐aggregate pathways. With this geometry, simulations agreed fairly well with experimental data when the intra‐aggregate impedance factor was 0.001, using other model parameters that had been determined in independent equilibration studies. With this intra‐aggregate impedance factor, the model yielded straight‐line plots for chloride and the overall impedance factor derived agreed closely with the experimental one. The intra‐aggregate impedance factor for32P also agreed with the rate of reaction of the rapidly‐exchanging P fraction previously determined. The slowly‐exchanging P fraction has little influence on the P concentration profiles up to 10 d, but the small exchanged amount does have an effect at run times of 57 d.The slow intra‐aggregate diffusion of strongly adsorbed solutes decreases the amount adsorbed from a surface source of supply, and it also decreases the amount that is taken up by a surface sink. However, the interaggregate solution concentration remote from a source of supply is considerably increased, which may be important if the solute is a poll

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00495.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryWe have determined electrical conductivity (Ee) and total dissolved salts (S) in saturation extracts from 39 soil samples from the Baza basin (Province of Granada, south‐east Spain).Eeranged from 2.8 to 110dS m−3, andSfrom 2 to 444 8 dm−3.The relationship betweenSandEewas not linear. When the saturation extracts were diluted with progressively larger quantities of distilled water and their electrical conductivity calculated (Eec) with the equationwhereEdandEware the conductivity of the diluted extract and the distilled water andfis the dilution factor, the relationship betweenSand Eectended to become linear.The highest linear correlation coefficient relatingS(mg dm−3) andEec(dS m−1) was reached whenEec, values were calculated for dilutions with a conductivity (Ed) between 0.1 and 0.3 dS m−1(E*ec). The regression equation wasS= 490E*ecwith r2= 0.999. This relationship can be used in all saturation extracts, regardless of the concentration and type of i

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00496.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryA pot experiment was conducted to study changes over time of Cd and Zn in soil solution and in plants. Radish was grown in a soil which had been contaminated with heavy metals prior to 1961. Constant amounts of a fertilizer solution (NH4NO3, KNO3) were added daily. Soil solution was obtained at intervals by displacement with water.The cumulative additions of small amounts of fertilizers were made equal to the plants' requirements at the final harvest but were found to exceed them during most of the experiment. Excess fertilizers caused substantial increases of major (K, Ca, Mg) and heavy‐metal (Cd, Zn) ions in soil solutions and a decrease in soil pH, probably due to ion‐exchange mechanisms and the dissolution of carbonates.Uptake of Cd and Zn into leaves was correlated with the mass flow of Cd (adjusted r2= 0.798) and Zn (adjusted r2= 0.859). Uptake of K, Ca and Mg by the plants was independent of their concentrations in solution.It is concluded that, in order to study effects of plants on heavy‐metal availability and obtain soil solution that has not been altered by fertilizer ions, nutrients must be added according to the needs and growth of the plants. This could be achieved by linking fertilizer additions to the rate of transpiration, as nutrient uptake and transpiration were closely correlated in this exper

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00497.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryMeasurements of total organic C, δ13C and δ14C are given for topsoils taken from six experimental sites in southern England. At each site, some of the samples were collected before and some after the thermonuclear tests of the early 1960s, so that pre‐ and post‐bomb samples could be compared for radiocarbon content. The current Rothamsted model for the turnover of organic C in soil gave an acceptable fit to the data from five of the sites, apart from one aberrant radiocarbon measurement. The annual input of C to the topsoil was calculated for the five sites from these fits; the values obtained were: 0.15 t C ha−1a−lfor a site on silty clay loam, kept bare by hand weeding since 1870; 0.2 for unmanured spring barley growing on a sandy loam; 2.95 for a fertilized all‐arable rotation on a loamy sand; 1.9 for the same fertilized all‐arable rotation on a silty loam and 2.5 from this rotation on a calcareous silty loam. The corresponding values for Net Primary Production at the five sites were 0.15, 0.76, 5.16, 5.71 and 5.46 t C ha−1a−l. In fitting the model to the radiocarbon data it was necessary to postulate that all these sites contained substantial quantities of biologically‐inert organic matter, ranging from 2

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00498.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryThe monoacid part of soil lipids was studied in a hydromorphic sandy podzol under pine trees (Pinus maritimasp.). The undecomposed forest‐litter layer (L), the fragmented mycelium‐invaded litter layer (F) and the A1 soil horizon were sampled, and analysed for total lipid and total monoacid contents. Total monoacids were separated into straight‐chain components and terpenic components.Straight‐chain monoacids were determined as free acids and esters of fatty alcohols and of glycerol. Among these components, branched alkanoic acids occurred in the A1 soil horizon. Alkenoic acids were mainly determined as glycerides. Free n‐alkanoic acids were mainly produced in soil from terminal oxidation of plant n‐alkanes and plant n‐alkanols. Free alkanoic acids with the longest carbon chains and alkanoic acids esterified as glycerides were concentrated in the A1 soil horizon.Terpenic monoacids were mainly diterpenic components from pine resin. Their concentration decreased markedly during the decomposition of

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00499.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY