摘要:

The maintenance of an appropriate soil structure and favorable soil hydraulic conductivity (HC) is critical for the sustainability of irrigated soils. Clay dispersion and/or swelling satisfactorily explain the observed reductions in HC at relatively low electrolyte concentrations (C) and/or relatively high sodium adsorption ratios (SAR). However, uncertainties still remain with regard to the mechanisms responsible for observed reductions in HC at relatively high C and/or low SAR values. Our objective was to determine the effect of a range of C and SAR values on clay dispersion and HC of four calcareous, micaceous, arid-zone soils and to ascertain the contribution of the osmotic explosion effect to the observed HC reductions at high electrolyte concentrations.We determined the saturated HC and clay dispersion in the leachates of soil and soil:sand (1:2 w/w) columns equilibrated at successively diluted electrolyte concentrations of 500, 100, 50, 10, 5, 3, and 1 meq L−1of SAR 0 and 10 (meq L−1)0.5, followed by deionized water (C < 0.01 meq L−1). The observed decreases in HC at C levels below the flocculation value (FV, i.e., minimum C without clay dispersion) of these soils were attributed to clay dispersion and partial plugging of conducting pores. A total dispersed clay of less than 2 g kg−1soil (i.e., less than 0.8% of the total soil clay in the column) was responsible for a cumulative 72% reduction in HC, indicating that very small quantities of dispersed clay had a dramatic effect on the water-conducting properties of these soils.Significant decreases in HC also occurred at C levels well above those at which dispersed clay appeared in the leachate. The estimated threshold electrolyte concentrations (Ct at which HC starts to decrease) were higher than the corresponding FV of these soils, indicating that clay dispersion was not responsible for the observed reductions in HC. The steepest reductions in HC originated in systems where the steepest concentration gradients developed between the micropores and the macropores (i.e., in the soil:sand columns, close to the advancing front of the displacing solution and for changes in electrolyte concentrations from 500 to 100 meq L−1). We concluded that an osmotic explosion effect (i.e., the osmotic water movement into, macroscopic swelling within, and the destruction or slaking of the aggregate) was responsible for the observed HC reductions at these high electrolyte concentrations.

ISSN:0038-075X    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

Experimental evidence suggests that nitrogen (N) mineralization proceeds differently under fluctuating temperature conditions than it does at constant temperature. Although N mineralization is believed to follow first-order kinetics, and the mineralization rate constant is believed to follow Q10temperature dependence, no effort has been made to use this information to predict the effect of fluctuating temperature on N mineralization. In this paper, we present solutions to the first-order N mineralization equation for a rate coefficient with Q10temperature dependence. Solutions are presented for a number of simple patterns of temperature fluctuation in time. Example calculations show that the nonlinear temperature dependence of the Q10relationship causes mineralization under fluctuating temperature conditions to exceed that occurring at constant temperature. Sensitivity analysis shows that the Q10constant and the amplitude of the temperature fluctuation strongly influence the difference in mineralization obtained for the two temperature patterns. These results can be used to improve the design of experiments conducted to study the effect of-temperature fluctuations.

ISSN:0038-075X    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

Soils containing appreciable amounts of 2:1 clay minerals frequently contain a substantial portion of their total N as nonexchangeable NH+4trapped within the interlayers of these clay minerals. Moreover, NH+4applied as fertilizer to these soils may be rapidly fixed. A laboratory study was conducted to assess the role of soil microorganisms in the release of clay-fixed NH+4and its eventual accumulation in forms suitable for plant uptake in two alluvial soils of the lower Mississippi floodplain. Experiments involving15N-labeled NH+4, initially present solely as nonexchangeable15NH+4, showed that indigenous heterotrophic microorganisms readily assimilated this N when the availability of a C substrate created demand. When Commerce silt loam initially containing 180 mg of clay-fixed15NH+4-N kg−1was incubated for 28 days, 48% of fixed15N was recovered as microbial biomass N (12%) and as exchangeable NH+4-N (36%). The15NH+4released was not rapidly refixed by clays, even though only small amounts were nitrified. Fumigation to inhibit soil biological activity resulted in a substantial reduction in the release of fixed15NH+4. In contrast, the addition of 4000 mg of mannitol-C kg−1to stimulate biological activity enhanced the release of fixed15N and stimulated its subsequent nitrification. Experiments using two C-amended soils containing 90 or 180 mg of fixed15NH+4-N kg−1showed that 64 to 96% of this N was released in 28 days. Measures of microbial respiration indicated that recently fixed NH+4was 23 to 46% as available as NH+4added directly to soil as a neutral salt and considerably more available than native fixed NH+4. The conclusion that heterotrophic microorganisms play a principal role in the release of recently fixed NH+4was supported by experiments showing that addition of N in this form stimulated microbial activity and growth. These findings suggest that the activity of heterotrophic microorganisms may conserve soil N by promoting the release of recently fixed NH+4, primarily near the surface of roots that supply C to these organisms.

ISSN:0038-075X    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

Field variability of element concentrations in crops is important to consider for nutrient application as well as for evaluation of environmental hazards. The paper gives an analysis of spatially sampled wheat plant tissue in the field to determine spatial properties of 20 elements for a partly sludge-applied agricultural soil in northern Tunisia. A comparison is made with the spatial variability of geochemical elements in the soil. A geostatistical analysis shows that both soil and plant metal concentrations have a spatial structure with a range of 10 to 30 m for the sampling scale considered. Compared with the soil, element variability in the plant is almost an order of magnitude larger. Consequently, the study indicates that field variability of element concentrations in plants can only partially be explained by the corresponding soil variability.

ISSN:0038-075X    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

Variability in crop stands over very short distances is a widespread problem on the sandy soils of the Sahelian zone of Africa, and this phenomenon may be related to soil variability. The objectives of this study were to: (i) elucidate patterns in the variability of some soil chemical properties using classical statistics and geostatistics, (ii) compare the chemical properties and their variation in two dunal soils, and (iii) identify the implications of variability patterns in dunal soils for sampling technique. Two soils (Psammentic Paleustalfs) in Niger, West Africa were sampled extensively on a macro-scale (3500 m2) and on a micro-scale (0.2 m2). Soil pH, exchangeable acidity, available P, exchangeable bases, and organic matter were determined. Coefficients of variation (CVs) and sample number requirements were calculated, and semivariograms were constructed. On the macro-scale, the CVs for exchangeable Mg (74–114%) were highest. Exchangeable Ca (32–77%) and Al (20–100%) and available P (25–55%) also had high CVs on both soils. Semi-variograms revealed that the sill, range, and nugget variance of available P were greater than those for soil pH, exchangeable Al, and exchangeable H. Variance was slightly greater in the North-South direction than in the East-West direction for soil pH, exchangeable Al, and exchangeable H. The anisotropy is probably related to the predominant East-West direction of the axis of sand dunes in this region. Available P was significantly positively correlated with micro-elevation, which may be related to the spatial distribution of bushes and trees in the previous fallow period. Low P contents are widespread in the soils of Niger, and large responses to fertilizer P have been observed. Depth to acid subsoil was more variable in these soils than in those described in the literature, and variation in rooting depth could be reflected in above-ground plant growth. However, the sill for available P was considerably higher than for pH or exchangeable Al at all sampling depths, and spatial variability in crop stands is likely to be related to soil P levels as they vary with micro-elevation.

ISSN:0038-075X    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

The growing demand for the determination of several analytes in a large number of soil samples explains the increasing interest in developing automatic methods of soil analysis. This paper reports the use of flow injection analysis (FIA) to perform automatically the simultaneous determination of the pairs K/Ca, Ca/Mg, Na/K, and Na/Mg in soils.A dialysis unit was used to divide the sample plug into two different streams and also to create large dilution capacity. After appropriate treatment inside the manifold, the resulting plugs were directed to two detectors placed in parallel, thus allowing bicomponent analysis. Atomic absorption and flame emission spectrometry were used as detection processes.The results obtained for an average of 25 soil samples by the developed FIA systems were in good agreement with those provided by the reference methods, i.e., the relative deviations were no larger than 3%. Sampling rates from 80 to 120 determinations per hour, and relative standard deviations below 3%, were obtained.

ISSN:0038-075X    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

Shelterbelt planting is often recommended as a means to reduce wind erosion. Six sheltered and three unsheltered fields representing moderately coarse to moderately fine textured soils were sampled along transects to determine the effects of the belts on soil wind erosion and soil properties.137Cs measurements indicated that shelterbelts about 6 m tall and 200 m apart considerably reduced off-field soil losses from moderately coarse and medium textured soils. Off-field soil losses from unsheltered moderately fine textured soils were low and similar to those from sheltered fields. Shelterbelts did not prevent in-field redistribution, and soil deposition was evident out from the shelterbelts for about 20 m. Organic C and 15-bar water content were increased, and bulk density decreased, in a strip about 10 m wide adjacent to the belts. Increased biomass production from the shelterbelt and its grass edge are probably the main cause for the increased organic C content. Decreased bulk electrical conductivity indicated increased leaching out to about 15 m from the belt. Leaching has also decreased the pH of the surface soil but has not affected the depth to carbonates.

ISSN:0038-075X    

出版商:OVID    

年代:1995

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1995

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1995

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1995

数据来源: OVID