摘要:

Microporous properties of soil materials are considered important to the physical sequestration processes of contaminants and the influence on risk assessment for chemicals in the environment. We studied the microporous properties of five organic soil materials and two agricultural topsoils and their size separates. The methods used were gas adsorption with N2, CO2, C2H6, C2H4, and C2H2, as well as retention of the polar liquid EGME (ethylene-glycol-dimethyl-ether). Measured surface areas and micropore volumes determined with gas adsorption were strongly dependent on the molecular dimensions of the adsorbent, revealing that dried organic and mineral soil materials behave like rigid structures in the micropore range. A precipitated humic acid behaved differently, revealing a more open and/or homogeneous micropore structure. The EGME-derived surface areas of organic soil materials had a magnitude similar to the CO2-derived surface areas. The microporous structure of the more rigid soil organic matter (SOM) materials does not seem to be affected strongly by EGME penetration. The SOM rigidity seems to be positively correlated to SOM polarity and negatively correlated to sorption coefficients of naphthalene under wet conditions. The surface areas of mineral soils and size-separates measured with CO2and EGME were of comparable size in the absence of swelling clay minerals.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Identification of soil hydraulic parameters is an important step for modeling soil water flow and transport behavior. Parameter fitting by inverse modeling is becoming increasingly attractive as a way to avoid experimental difficulties. In this study, we linked the inverse program SUFI (Sequential Uncertainty domain FItting) with the one-dimensional flow model HYDRUS5 to investigate the applicability of inverse modeling to the water regime of a layered field soil. Measured pressure head and water content data from an irrigation and drainage experiment were used. The soil profile at the experimental site consisted of at least five soil layers, including a compacted pan. We formulated a total of eight inverse scenarios in an effort to explain the observed pressure head and water content data. Important processes in the experiment included, among others, air entrapment and preferential flow. We obtained a relatively precise simulation of pressure head and water content only after accounting for a pan layer and treating the irrigation and the drainage phases separately. We found that fitting alone, even with 20 to 30 parameters, could not account satisfactorily for the soil hydraulic behavior. Rather, correct accounting of the soil hydraulic processes as well as the soil system proved to be essential.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Colloids in soil water are a constituent of natural geochemical fluxes and have the potential to facilitate contaminant transport, but few data are available on their composition and concentration. This study addresses how the composition and concentration of groundwater solids relate to hydrological and soil morphological variables of the Florida flatwoods landscape. Groundwater from saturated soil horizons was sampled biweekly for 1 year along an Aquod/Udult boundary using piezometers designed specifically to minimize disturbance and to permit the valid assessment of suspended solids. Readily dispersible clay from core samples of soil horizons was collected and quantified. Groundwater and soil colloids were analyzed physically, chemically, and mineralogically. Aquod groundwater had consistently lower pH, higher electrical conductivity, and more total solids (TS) and organic carbon (OC) than did Udult groundwater. Significant decreases in both TS and OC concentrations in groundwater occurred with depth for both soils. In contrast, the mineralogy of groundwater colloids was insensitive to soil and horizon differences. Quartz dominated inorganic colloid fractions in groundwater samples from all horizons, even in argillic horizons where clay fractions contained little or no quartz. No statistical correlations were found between masses of groundwater colloids and soil water-dispersible clay. However, the proportion of organic carbon was higher in groundwater than in soil matrices. Results are consistent with carbon and colloidal quartz movement in shallow groundwater of the soils studied and document that natural colloid and solute fluxes can be highly soil specific.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Subsoil A1 can restrict deep root growth which, under dryland conditions, can cause lower yields during periods of water deficit. Soils that have high levels of subsoil Al must be identified so that appropriate management strategies, such as selection of deep rooting, Al tolerant cultivars or chemical amelioration of the subsoil, can be implemented. Thirteen soils representing three landscape positions from the Southern Mississippi Valley were sampled to 75 cm at 15-cm depth increments. Short-term bioassays were conducted with wheat (Triticum aestivumL.), cotton (Gossypium hirsutumL.), and soybean (Glycine max(L.) Merr.) at each depth increment to identify soils with potentially phytotoxic concentrations of A1. Subsoil pH values generally decreased with depth to a low of 4.1 at the 45- to 60-cm depth increment. Values of 1NKCl-extractable Al were as high as 1010 mg kg−1in the 60- to 75-cm depth increment, whereas exchangeable Ca concentrations dropped as low as 207 mg kg−1. Concentrations of 1NKCl-extractable A1 in soils of the Loessial Plains (326 mg kg−1) were nearly twice those measured in soils from the Loessial Hills (180 mg kg−1) and more than three times greater than soils from the Bottomlands and Terraces (99 mg kg−1). Aluminum (KCl-extractable) concentrations that resulted in a 30% reduction in root length (a threshold value considered to reduce crop yields) varied among crop species and ranged from 2 mg kg−1to more than 667 mg kg−1. Subsoil Al is a problem for a significant percentage of hectarage in the Southern Mississippi Valley and should be taken into consideration, especially under dryland farming.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Conservation tillage results in the concentration of plant-available P near the soil surface. We studied the effects of conservation tillage on P speciation by examining the distribution of P in inorganic and organic chemical pools. Depth-incremented soil samples were collected from long-term (9- and 10-yr) no-till (NT) and disk tillage (DT) systems cropped in corn (Zea maysL.) with a wheat (Triticum aestivumL.) cover crop. Rates of P were 0, 20, and 60 kg P ha−1yr−1. Total P (PT), organic P (PO), and available P (Mehlich-3, M3-P; Olsen NaHCO3-pH 8.5, Olsen-P) were determined. P was also extracted from the following chemical pools: non-occluded Al-bound (Al-P), non-occluded Febound (Fe-P), occluded-reductant-soluble (CBD-P), and Ca-bound (Ca-P). Total P did not vary with depth, but was greater in NT than in DT and increased with P rate. Organic P increased with P rate in the 0- to 8-cm depth. Organic P was greater in NT plots in the 8- to 60-cm depths, averaging 75 mg kg−1for NT and 48 mg kg−1for DT plots. Mehlich 3-P and Olsen-P were greatest in the surface 4 cm and in the 60-kg P ha−1plots, with higher levels observed in NT plots. On average, the forms of P (as a % of total P) in NT soil was 6.2% Al-P, 33.9% Fe-P, 33.9% CBD-P, and 4.7% Ca-P. Average P distribution in DT soils was 5.4% Al-P, 35.6% Fe-P, 31.3% CBD-P, and 5.1% Ca-P. The influence of tillage on P distribution was primarily limited to the soil surface, with the exception of Al-P, which was greater in the 8- to 30-cm depths of the NT plots. Because the impact of tillage was limited to a thin, soil surface layer (<4 cm), soil P-test rating would not be affected by tillage practice. However, the improper collection of soil samples from NT (i.e., too shallow) for P-testing may provide erroneous P-test results and fertilizer recommendations.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

We incubated intact peat cores from depth intervals of 5-15, 15-25, 25-35, and 35-45 cm from ombrotrophic bog, poor fen, and beaver pond margin sections of a cool-temperate peatland. CO2production was measured over 12-day incubation periods at 4 and 14 °C and under oxic and anoxic conditions. Rates ranged from 0.06 to 0.66 mg CO2g−1dry peat d−1under oxic conditions and from 0.002 to 0.098 mg CO2g−1d−1under anoxic conditions, and rates generally decreased with depth in the profiles. When expressed on a volumetric basis, production rates ranged from 0.3 to 23.4 g CO2m−3d−1, and there was much less variation in CO2production rates within profiles because the bulk density of peat increased with depth. The Q10quotient, between 4 and 14 °C, ranged from 1.0 to 7.7, depending on sample and incubation conditions, with an average of 2.0 for oxic and 2.7 for anoxic conditions. Oxic:anoxic ratios averaged 7:1, 16:1, and 12:1 for the bog, poor fen, and beaver pond margin samples, respectively. Degree of decomposition (von Post index) was the substrate property most strongly correlated with CO2production. Based on temperature and incubation data for the peat profiles to a depth of 45 cm, annual decomposition values (k) ranged from 0.016 to 0.060 yr−1under oxic conditions and from 0.001 to 0.007 yr−1under anoxic conditions. A model of CO2emission from the three sites, based on the incubation data and thermal and water table regime, gave good agreement with measuredin situCO2emission rates (r2= 0.72,n= 18), although summer emission rates were underpredicted, possibly because of the absence of a root production component in the incubations or because of underestimation of CO2production rates in field conditions above the water table.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Field evaluation of nutrient release from composts is important to estimate nutrient contribution to crops, potential leaching of nutrients, and, ultimately, to determine optimum application rates, timing, and placement of composts. Field incubation and laboratory analyses were conducted to evaluate the mineralization rate and transformation of N in biosolids (BSD), yard waste (YW), and West Palm Beach co-compost (WPCC). Each of the composts or biosolids was packed into PVC columns (8 cm height, 5 cm id) and inserted vertically into the upper layer of an Oldsmar fine sand (sandy, siliceous, hyperthermic Alfic Arenic Haplaquods) of raised citrus beds. The top end of the PVC column was capped to prevent excessive leaching of nutrients from the columns. The moisture equilibrium between the incubated sample and the soil in the field was attained through the bottom and four side holes of each column, which were separated from the contacting soil by 400-mesh nylon screen. A set of the incubated columns was removed at monthly intervals, and the soil underlying each column (a core of 20 cm height and 5 cm diam) was sampled to analyze for KCl-extractable NH4-N and NO3-N. Total C and N of the incubated samples were determined at the end of the 1-year incubation. Organic N mineralization rates during the 1-year incubation were 23.3, 23.5, and 48.4% of the total organic N in the YW, WPCC, and BSD, respectively, as estimated by the organic N decrease method. The mineral N (NH4-N plus NO3-N) recovered from both the compost and the underlying soil by KCl extraction accounted for, at best, 36, 43, and 57% of the total mineralized N determined from the organic N difference before and after incubation for the WPCC, YW, and BSD, respectively. During the first 6 months of incubation, NH4-N was the dominant form of mineralized N, but NO3-N accounted for more than 50% of the mineral N during the later part of the incubation for the YW and BSD. NO3-N was the dominant mineral N throughout the whole incubation period for the WPCC. Application rates, timing, and placement of composts similar to the BSD, which contain high N concentration, should be adjusted for high N release to minimize the risk of NO3-N leaching into groundwater.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Soil discontinuities, defined by significant texture and mineralogical, and/or age differences, are usually verified from a small number of pedons without producing information about the local distribution of properties that define the discontinuity. A discontinuity has geomorphic significance as well as important implications for interpretations. Along the western footslopes of the Virginia Blue Ridge, Typic Paleudults and Typic Fragiudults seem to have two depositional sequences in the sola: soil texture differences and abrupt color changes are present. Our objective was to show the discontinuity occurring in the soils using data analysis and multivariate statistics. Stratified systematic unaligned sampling within a 50-m2area, the Wilcoxon Signed-Rank test, principal component analysis, and discriminant analysis were used to verify the discontinuities at four locations. Twenty-five soil properties were evaluated by the Wilcoxon Signed-Rank test. Nineteen significantly different properties (P= 0.10) were detected at Site 1, 17 at Site 2, 13 at Site 3, and 15 at Site 4. The first principal component (PC) loadings showed most properties contributed about equally to the first PC, with the exception of clay-free properties at Site 2 and Site 4. The second component loadings also identified clay-free properties as the best indices for defining the discontinuities. Discriminant analysis of scores from the first five principal components showed the components contributed from 91 to 96% to the discriminant functions. Discriminant functions clearly separated Unit A from Unit B at each site. Hotelling'sT2test showed the distances were significant (P≤ 0.001) between Unit A and Unit B multivariate means. We conclude discontinuities are present at the four locations, and they can be differentiated spatially by textural differences and clay-free indices.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID