摘要:

Soil water retention measurements are relatively time-consuming and become impractical when hydrologic estimates are needed for large areas. One approach to soil water retention estimation from readily available data is based on the hypothesis that soil water retention is additive and can be approximated by summing up water retention of pore subspaces related to the soil components. Our objective was to test this hypothesis. In the 'additivity' model used in this work, the main assumptions are: (a) the additivity is applicable to gravimetric water contents, (b) water retention related to a textural fraction can be measured on packed samples consisting exclusively of this fraction's particles, (c) water retention of textural fractions contributes to total soil water retention in proportion to the volumes of pore subspaces related to each fraction. Water retention of packed cores consisting of soil textural fractions was taken from literature. The resultant model did not have fitting parameters. It was tested with samples of sandy soils that were collected in the UNSODA database from seven states of the United States and from seven other countries. The median root-mean squared error (RMSE) constituted 0.017 m3m−3, 0.023 m3m−3, and 0.028 m3m−3in coarse, medium, and fine sands, respectively. These RMSE are at the lower end of the RMSE range for the regression-based pedotransfer functions found in literature. Slopes and intercepts of the regressions of estimated water contents on measured water did not differ significantly from one and zero at the 0.05 significance level except in one case. The RMSE of the laboratory water retention estimates with additivity model was significantly less than the RMSE of the field water retention estimates from the laboratory water retention data. We discuss possible sources of errors and conclude that the attained accuracy warrants testing the additivity model with other soil data and improving this model to accommodate various types of soil structure and additional soil components.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

WETLANDS, a multidimensional model describing water flow in variably saturated soil and evapotranspiration, was used to simulate successfully 3-years of local hydrology for a cypress pond located within a relatively flat Coastal Plain pine forest landscape. Assumptions included negligible net regional groundwater flow and radially symmetric local flow impinging on a truncated conical pond, deciduous cypress trees and shallow-rooted perennial undergrowth in the pond area, and pine trees in the upland area as well as within the outer 20% of the wetland area. A minimal observed parameter set of daily rainfall, daily air temperature, soil characteristics, and pond geometry provided model input. The model described temporal patterns of daily pond water and groundwater table elevations with relatively small average signed deviations of −2 and +11 cm, respectively. Potential exists for the model to be utilized as a predictive tool for wetland hydrology, even for conditions where available empirical data for a given site is minimal and appropriate simplifying assumptions are utilized.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

In the vineyards of the Anoia-Alt Penedès region of NE Spain, erosion problems arising from a combination of soil and climate characteristics and their relief have been accelerated in the last few decades as a consequence of new cropping/planting patterns and soil management practices involving continuous tillage, which leaves the soil bare most of the time. In the rain-fed vineyards of the area, rainfall water conservationin situhas also become a problem that influences the production of high quality wine.The determination of the actual and potential erosion processes required for rational selection, design, and application of soil and water conservation practices requires identification and quantification of the hydrological properties and processes that induce runoff and soil erosion. Laboratory studies in 10 representative surface soils of the region, using simulated rainfall, proved that erosion losses were highly related to the surface sealing susceptibility of the soils, which reduced minimum rainfall water intake rates to 1 to 7 mm h−1or less after only 10 minutes of rainfall. This susceptibility was highly correlated with the high silt and low organic matter content of the soils.In most of the soils, surface protection by a cover was found to increase the minimum rainfall water intake rate 50 to 200 times. A simple laboratory test, based on the measurement of saturated hydraulic conductivity and the time to reach its minimum value, measured efficiently the sealing susceptibility of the studied soils and the relative effect of a protective cover. The minimum value of saturated hydraulic conductivity was highly correlated to runoff and soil erosion losses in soil boxes under simulated rainfall.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Heat and mass are transferred by natural convection in the ponded water of paddy fields. For a detailed study on convective fields in ponded water, we devised an improved, constant current, hot-wire method to measure the convection flow in a ponded paddy field. A long constantan wire was used as the hot wire, and two nickel (Ni) wires were used for sensing. One of the Ni wires was attached to the constantan wire to detect the temperature of the constantan wire on heating. Another Ni wire was used to detect the temperature of ambient water. These Ni wires were connected to a bridge circuit so that the output of the circuit would indicate the temperature difference between the hot wire and the ambient water. The temperature rise in the hot wire was dependent on both the power supply and the convection flow in the water. In this study, a 10-mW cm−1heat source was found to be experimentally effective. For calibration, the temperature rise was measured in stationary water by moving the sensor at very low velocities. The temperature rise decreased as the sensor velocity increased. The temperature rise was inversely proportional to the convective velocity.The convective velocity measured in a paddy field by this sensor was extremely low and fluctuated with time. The temperature difference between the boundaries of the water layer, the driving force for natural convection in the field, shows a daily fluctuation depending on the local weather conditions. The maximum velocity was 0.14 cm s−1at around noon, corresponding to the maximum temperature difference. This sensor can be employed to measure convective velocity in the field; however, a manual check of the field measurements and calibration before and after use of the sensor in the field are necessary.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Bioavailability of PAHs and PCBs in soils depends on the strength of their sorption to soil organic matter (SOM), which exists in differently reactive pools. In order to assess the fate of PAHs and PCBs in tropical soils, we examined the distribution of 20 PAHs and seven PCBs among particle-size separates (coarse sand, fine sand, silt, clay, floatables) in 10 urban surface soils (0-5 cm) of Bangkok, Thailand. The quantitatively negligible floating fraction (<1.2 g kg−1) has the significantly highest sum of PAH and PCB concentrations (on average: 2548 μg Σ PAHs kg−1, 20375 ng Σ PCBs kg−1). The average sum of PAH concentrations decreases in the order, silt (298 μg Σ PAHs kg−1) ≥ clay (275) > fine sand (174) > coarse sand (71). The highest average PAH concentration in the silt fraction may be explained by an input of PAHs associated with silt-sized particles or by a particularly high affinity of PAHs to silt-sized SOM. The latter assumption is supported by the highest average organic C (Corg)-normalized PAH concentration in silt (12.3 ± 13.2 μg Σ20PAHs (g Corg)−1) > fine sand (11.5 ± 9.6) = floatables (11.4 ± 5.8) > clay (5.9 ± 5.1) > coarse sand (5.7 ± 4.4). Average percentages of low molecular weight PAHs (<203 g mol−1) decrease from coarse to fine fractions (coarse sand: 57% < fine sand: 48 < silt: 44 < clay: 38 < floatables: 12). This indicates a degradation or volatilization of the less persistent and more volatile PAHs during turnover of SOM, which shows an increasing state of alteration with decreasing particle size. The average sum of PCB concentrations in the particle-size separates decreases in the order, clay: 2349 ng Σ PCBs kg−1> silt: 1597 > fine sand: 803 > coarse sand: 261 and is more closely related to the Corgconcentration than that of the PAHs. No indications of degradation of the low-chlorinated PCBs 28 and 52 with increasing state of SOM alteration were observed. The distribution of PAHs and PCBs among particle-size separates in the studied tropical soils is similar to that reported in the literature for temperate soils. Despite similar physicochemical properties, the markedly different distribution of PAHs and PCBs among the particle-size separates indicates different fates in soils.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

High concentrations of potassium salts added to soil have been shown to inhibit nitrification as a result of high osmotic pressures. However, low levels of added K+have also been shown to inhibit nitrification in vermiculite-clay suspensions. The latter result led to the hypothesis that K+rendered NH+4unavailable to nitrifiers by inducing NH+4fixation. Although there is evidence that K+promotes NH+4fixation in soils containing vermiculite, the potential for this mechanism to directly control nitrification in soils has not been demonstrated experimentally. The purpose of this study was to determine if adding K+inhibited nitrification in soils containing vermiculite by inducing NH+4fixation. The soils used, a Maury silt loam (fine-silty, mesic, Typic Paleudalf) and an Eden silt loam (fine, mixed, mesic, Typic Hapludalf) were first partially loaded with NH+4by equilibrating them with a 5 mM(NH4)2SO4plus 1 mMCaSO4solution. Nitrification was then studied in suspensions by adding a quantity of pretreated soil sample (equivalent to 11.45 mg adsorbed NH4-N) to flasks containing a combined solution of 1 mMCaSO4and 0, 150, or 300 ppm K+(0, 1580, or 3160 μg K+g−1soil) as K2SO4. Flasks were bubbled continuously with atmospheric air and shaken at room temperature for 15 days. Nitrification seemed to be promoted initially by the addition of K+to both soil suspension systems. Nitrification continued after solution NH+4was apparently depleted in both soil systems. Total NO2-N and NO3-N produced was highest in the Maury soil, with 300 ppm K+, whereas the most nitrification occurred in the Eden soil with 0 ppm K+. Extractions of soil suspension subsamples after the 15-day incubation period revealed a significant increase in fixed NH+4(FixNH4), with added K+in the Eden soil but not in the Maury soil. This study provided evidence that nitrification in the Eden soil was inhibited in the presence of K+because of enhanced NH+4fixation.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Phosphorus behavior in unfertilized soils of the Semiarid Argentinean Pampas is not well understood. The objective of this study was to evaluate the effect of cultivation on qualitative and quantitative phosphate concentrations as well as on P sorption of these soils. For this purpose topsoil samples (10 cm) of 10 unfertilized plain soils with different textured parent materials (Ustipsamments, Haplustolls, and Hapludolls) from five sites with increasing mean annual rainfalls (550 to 650 mm) and temperatures (15 to 16°C) were studied. Undisturbed (underProsopis caldeniaforest) and cultivated paired pedons (under rotation of annual crops and pastures for more than 30 years) were compared at each site. Cultivation decreased the inorganic P fractions (Pi) 16 to 19% in one Haplustoll and the Hapludoll, decreased the organic P fraction (Po) 7 to 54% in both Ustipsamments, and decreased the Bray-Kurtz I extractable P (Pa) 62 to 73% in one Ustipsamment, a Haplustoll, and the Hapludoll. Only one Haplustoll, cultivated during long periods with alfalfa pastures, did not show changes in any of studied P fractions. This soil lay for long periods under cattle-grazed alfalfa pastures, under which conditions P losses by wind erosion were decreased, and P recycling caused by additions of cattle and plant residues to the soil, as well as P translocation from the subsoil to the topsoil by deep alfalfa roots, were increased. Total P losses from the four other cultivated soils were larger than calculated plant removal, indicating that other processes than plant uptake decreased P amounts. The Hapludoll and one Ustipsamment were cultivated with annual crops for longer periods than with alfalfa pastures. This produced wind erosion (thinner A-horizons, and losses of easily erodable textural fractions), which depleted Piand Pain the Hapludoll and Paand Poin the Ustipsamment. A Haplustoll and an Ustipsamment, both cultivated for longer periods with alfalfa pastures than with annual crops, did not show wind erosion features but did show P decreases. The Haplustoll lost Paand Pi, probably because of plant removal without P recycling to the soil by a highly extractive production system (seed and grass-parcel production). The Ustipsamment lost Pobecause of high mineralization of organic P by tillage. Larger Polosses that occurred in both Ustipsamments were attributed to the higher mineralization rates of organic P compounds, which are known to be high in these textural soil types. The C/Pobelow 200 of all studied soils indicated that organic P compounds can supply P to the plants by mineralization. This process will be more effective in cultivated than in undisturbed soils because of their lower C/Poquotients. Solubility diagrams showed that the phosphates predominant in all studied soils were fluorapatite (FA) and hydroxiapatite (HA). Cultivation did not change phosphate forms significantly, although when pH values were increased as a consequence of lime accumulation and organic matter (OM) losses, cultivation decreased P sorption capacity of the soils, which contributed to reducing available P to plants. Cultivation with annual crops using conventional tillage systems will increase losses of inorganic P reserves by wind erosion in Hapludolls and of organic P reserves by mineralization in Ustipsamments. Conservation tillage, which leaves large amounts of plant residues on the soil surface, can reduce these losses. Cattle grazing on alfalfa pastures seems to be the best system for reducing losses of available P in any soil type because it reduces wind erosion and increases P recycling to the soil.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Long-term weathering rates are determined by comparing the chemistry and mineralogy of soil profiles of known age and the fresh source rock composition. Such calculations are based on enrichment/depletion factors determined using immobile element contents, usually Ti or Zr. For convenience, both positive and negative changes are expressed relative to a representative elementary volume. This method has been applied to several investigation areas on silicate host material. The calculation of time-averaged weathering rates of a soil profile or a corresponding soil horizon is realized with a mathematical procedure described in literature. However, this procedure contains an error that may substantially influence the results. This error is caused by an incorrect use of soil and protolith height. An amended version of the calculation procedure is presented. An example is given to demonstrate the differences between the old and the revised procedure.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Combining composts with sufficient fertilizer to meet crop requirements has several benefits over the use of compost as a sole N source, including reduced accumulation in soils of heavy metals, salts, or P. The objective of this research was to find blends of compost and fertilizer that would equal the yield and N uptake of N fertilizer. Biosolids compost-fertilizer blends made to equal 100 mg available N kg−1were compared with urea or NH4NO3fertilizer. Zero to 50% of the blend's available N came from biosolids compost; the remainder came from fertilizer.15N-labeled fertilizer was used to determine the proportion of N uptake attributed to fertilizer. Fescue (Festuca arundinaceaSchreb.) was used as the indicator plant. Compost did not increase the efficiency of NH4NO3fertilizer. The amount of available N provided by urea-compost blends was less than the amount provided by NH4NO3-compost blends. Urea-compost blends that had 17 to 50% fertilizer equivalents from compost equaled 100% urea. Blends of 50% NH4NO3and 50% biosolids compost available N or 67% NH4NO3and the equivalent of 33% biosolids compost N produced greater yields and N uptake by fescue.15N analysis suggested that NH4NO3fertilizer stimulated soil and/or compost mineralization producing more N than predicted from incubation studies of compost N mineralization rates. However, previous studies showed that soil mineralization was not affected by compost, which leads to the conclusion that fertilizer stimulated compost mineralization. Urea did not affect compost similarly, possibly because of a larger active N pool in urea compost blends compared with NH4NO3compost blends.

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