ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

More than 30 years have passed since the first application of nuclear magnetic resonance (NMR) spectroscopy to soil organic matter (SOM). Since then, there has been an explosion of applications using1H,13C,31P, and15N NMR on both solution and solid-state samples. These have greatly enhanced our understanding of all areas of SOM research, including understanding the nature of decomposition, and the effects of cultivation, characterization of organic forms of N and P, and interactions of SOM with xenobiotics and metals. However, in many ways, NMR remains underutilized, and applications continue to be severely hampered by the lack of instrumentation, expertise, and opportunities for graduate training. This article attempts to give a broad overview of accomplishments as well as suggestions for new opportunities.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

Unsaturated hydraulic properties of field soils are needed for water flow and solute transport calculations in variably saturated soils. The purpose of this study was to quantify the spatial variability and spatial crosscorrelation of estimated parameter values of a flexible retention model that was fitted to measured retention data. Moisture retention characteristic (MRC) curves were measured on 100-cm3undisturbed soil cores collected at 180 locations along a 31-m-long transect in a three-layered soil profile at depths of 0.1, 0.5, and 0.9 m. Sampling intervals in the horizontal direction were, alternately, 0.1 and 0.9 m. Saturated hydraulic conductivity (Ks) was determined on the same soil cores using a constant head permeameter. The drying part of the MRC curves was described by the four-parameter retention model of van Genuchten with fitting parameters, namely &thetas;r, &thetas;s, α, andn. Spatial variability of the five parameters, &thetas;r, &thetas;s, α,n,andKs,was investigated for the three horizons using conventional statistics and geostatistical techniques. Maximum coefficient of variation (CV) was found forKs,i.e., 599%, 322%, and 897% for the 0.1-, 0.5-, and 0.9-m soil depths, respectively. Macropores and small sampling volume contributed to this large variability ofKs. When all three soil depths are considered, residual water content (&thetas;r) and shape factor α showed moderate heterogeneity with a maximum CV of 156 and 53%, respectively. Small spatial heterogeneity was observed for shape factornand saturated water content &thetas;s, with a maximum CV of 22 (for 0.1-m depth) and 8% (for 0.9-m depth), respectively. Most hydraulic parameters at different layers exhibited convex experimental semivariograms that could be described by means of spherical models, with a spatial range between 4 and 7 m. The correlation scales for cross-semivariograms for pairs of cross-correlated parameters were found to be of similar magnitude to those pertaining to the direct semivariograms of correlated variables.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

A pot experiment was conducted to investigate the boron (B) bioavailability to corn seedlings (Zea mays L) in an artificial soil mix produced from alkaline coal fly ash and sewage sludge under greenhouse conditions. Sludge was amended with ash at an application rate of 0, 5, 10, 35, and 50% (w/w), and each mixture was then mixed with a loamy soil at either 1:1 or 1:5 (v/v). Both water soluble B (WS-B) and hot water soluble B (HWS-B) increased with an increase in the fly ash amendment rate. Plant tissue B contents also increased according to the rate of ash amendment, and at high ash application rates, more than 70% of the tissue B content was accumulated in leaves. Among the plant organs, B contents of young leaves showed a better correlation with HWS-B contents than with WS-B, and HWS-B gave a better indication on soil B availability. Boron toxicity symptoms in leaf margins were observed in 50% and 35% ash-amended sludge at 1:1 soil-mixing ratio, with shoot [B] reaching 225 mg kg-1. However, a significant yield reduction was observed only at 50% ash-amended sludge at 1:1 v/v, indicating that factors in addition to B might affect plant growth.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

Recent concern about rising levels of atmospheric CO2has directed attention to the stores of organic carbon in soils and to changes caused by human activities. In southwest France, thick, humic, loamy soils have developed from Quaternary silty alluvial deposits. Most forest lands on these soils have been converted to continuous intensive corn cropping. The spatial distribution of C stores under forest and the loss of C upon this conversion have been modeled in previous studies. The objective of this study was to determine if a spatial and temporal analysis of this conversion makes possible reliable estimates of the organic carbon fluxes attributable to this conversion. Forest clearing showed a sigmoïd temporal progression. The analysis of changes in land use provided a useful tool to improve the prediction of soil organic carbon fluxes to the atmosphere. From 1948 to 1984, carbon release to the atmosphere was about 2.2 Tg, that is 6.35 kg.m-2. Temperate soils may be an important source of atmospheric carbon when they are high in carbon content initially and then cultivated intensively.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

Cypress/pine flatwoods are a dominant landscape of the lower coastal plain of the southeastern U.S. It is clear that the cypress swamps are wetlands, but it is not clear that the intervening pine ecosystems would meet hydrologic criteria of a wetland definition. Hydrology, as defined by the periodicity of the fluctuating water table and its effect on the redox environment of the soil, will determine whether these areas are wetlands. The objectives of this study were (i) to evaluate the relationship between water table depth and redox environment for a typical pine flatwoods/cypress swamp landscape and (ii) to use this relationship to contrast the amount of the study area that would fit a specific wetland hydrology definition where that definition was based on either water table depth or the development of a reduced environment in the soil surface. One hundred twenty water table wells were installed in a grid system over a 42-ha study site, and water table depth was measured approximately every 2 weeks. At selected locations, redox electrodes were installed at a 5-cm depth in pine planations growing on Spodosols and landscape positions that were in transition between these Spodosols and the adjoining cypress swamps. A Geographical Information System was used to calculate the area of the study site meeting different criteria related to water table depth or soil reduction. Results suggest that 20 to 56% of the study area would meet a hydrologic definitions of wetland, depending on the hydrologic criteria used. In these landscapes, water tables between 15 and 20 cm below the soil surface promoted reduced conditions in the surface. This criteria could be used for evaluating redox conditions relative to hydrophytic vegetation and soil redoximorphic features.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID