摘要:

Potential applications of air pocket type tensiometers in measuring hydraulic head profiles in deep vadose zones are discussed. Advantages of this method include (i) the ability to obtain tensiometer measurements far beyond the approximately 9 mdepth often associated with the limit of conventional tensiometry, (ii) ease of regular gauge calibration, and (iii) low cost. Advantages relative to buried, dedicated pressure transducer tensiometers are gained at the expense of substantial losses in gauge sensitivity, S*. In view of this compromise, an analysis was performed to determine the optimal fractional waterfilled length, F, for air pocket tensiometers. It is shown that the critical ratio governing the nature of S*‐optimization is approximated by (II*— IIo)z*, where II*represents the absolute matric head, IIois the vapor pressure of water expressed in head units, and z*is the depth of the tensiometer tip. When (II*— IIo)/z*> 1, S*is optimized when F→1. However, when (II*— IIo)/z*< 1, S*is optimized as F→0. The central role of (II*— IIo)/z*arises from the fact that S*= Pa/Vg, where Parefers to the absolute pressure of all tensiometer headspace gasesexcludingwater vapor, and Vgrefers to the volume of the gas phase within the tensiometer headspace. When (II*— IIo) is less than z*, S*goes to zero because the absolute pressure in the tensiometer headspace approaches the vapor pressure of the tensiometer water (Po) when attempts are made to fill the tensiometer column with liquid water. In the more familiar case of II*— IIobeing larger than z*, the dominance of Paover Poassures that S*increases as the instrument is filled. To test the predicted nature of S*, laboratory experiments were performed on 1.11‐6.36‐, and 11.91‐m long tensiometers over a range of values of (II*— IIo) and F sufficient to provide three orders of magnitude variation in S*. Measured S*agreed well with predicted values, and supports the conclusion that response times are minimized with F→0, in situations where (II*— IIo)/z*< 1.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

The effect of pH on copper desorption from a sandy soil and on complexation by dissolved organic fractions was studied, using a soil, with pH ranging from 4.3 to 5.6, that had been polluted with copper a decade ago. For the desorption of exchangeable copper we used an unbuffered electrolyte solution (0.0033MCa(NO3)2; ionic strength 0.01M). Free copper was determined using an ion exchange resinmethod. Copper bound by dissolved organic matter was divided into a humic and a fulvic fraction using ultrafiltration. The Cu desorption isotherms were nonlinear and dependent on pH. The (desorbed) exchangeable copper was less than 3% of the total copper in the soil. About 1% of exchangeable copper was complexed with nitrate, and free copper varied from 2% (pH 5.7, 0.3 mmol/kg total Cu) to 9% (pH 4.4, 3 mmol/kg total Cu). The remainder of exchangeable Cu was bound by dissolved organic matter. The amount of Cu bound by solid organic carbon (CuSOC) was almost equal to Cu bound by dissolved organic carbon (CuDOC) when both are expressed in mmol/g C, although there was a pH dependency. At low pH (4.4) Cu was predominantly bound by fulvic fraction, and at pH 5.7 most was bound by humic fraction. Desorption of copper from soil and copper binding by humic and fulvic fraction (all different pH dependent) could be described adequately with the Two Species Freundlich (TSF) equation. With this equation the competition between soil, fulvic fraction, and humic fraction for free copper (Cu2+) was shown.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Serpentinitic soils are of great ecological and environmental interest, yet pedologic studies of such soils are relatively rare, particularly concerning heavy metal solidphase speciation. This study examined the surface association and solid‐phase speciation of metals, the mineralogy and the surface chemistry of serpentinitic soils. Surface association and solid‐phase speciation were assessed by selective wet chemical methods. Mineralogy was determined by X‐ray diffraction and surface chemistry by potentiometric titration. The morphology of clay minerals was studied by transmission electron microscopy. Surface‐as‐sociated metals were divided into exchangeable and nonexchangeable fractions. Adsorbed Mg and Ca were predominantly in exchangeable form. More than 10% of adsorbed Mn and Ni were present in exchangeable form, but generally <1% of adsorbed Al and Fe were exchangeable. Serpentine and Fe oxides were the major soil mineral components, along with lesser amounts of quartz, whereas gibbsite and smectite were not detected. Iron oxides contained more than 10 mole‐% of non‐Fe elements (Al, Cr, Ni, and Si). The soils also contained Mn oxides that appeared to incorporate Co and Ni.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

The possible occurrence of reducing microsites in synthetic soil aggregates and their influences on the distribution of selenium species with redox‐dependent mobilities was tested using the synchrotron X‐ray fluorescence microprobe (SXRFM). Synthetic, effectively two‐dimensional soil aggregates of diameters ranging from 10 to 30 mm were constructed, with and without inclusion of sections ofScirpus robustusandS. californicusroot sections. Each aggregate was uniformly wetted with a saline solution containing 240 g m‐3Se [98% as Se(VI), and 2% as Se(IV)]. Gasphase porosities varied between individual aggregates from 0.00 to 0.40 and were maintained relatively constant during the incubation period of up to 17 days. Exchanges of soil gases with atmospheric air occurred only along the periphery of the aggregates. Scanning of the aggregates using SXRFM demonstrated that Se was essentially homogeneously distributed in soils withoutScirpusroot sections, suggesting that Se remained primarily as the soluble Se(VI) species. The SXRFM results revealed large accumulations oftotalSe in regions surrounding embedded sections ofScirpusroots. As much as 20‐fold local enrichment with respect to total Se was measured in water‐saturated soils within 1 to 4 mm of decomposing roots. These observations provide support for a model of localized reducing zones in which Se(VI) is reduced to less mobile Se(IV) and to insoluble Se(0), resulting in local accumulation of total Se. The measured Se accumulation in one microsite compared reasonably well with a simple transient Se(VI) diffustion model. It is postulated that such mechanisms may account for similar heterogeneities observable in some Se‐contaminated soils at Kesterson Reservoir. Such heterogeneities in concentrations of Se and other constituents within individual soil aggregates have important implications with respect to reactivity and need to be included in any detailed mechanistic modeling of chemical cycling within soils. This work also provides an example of the substantial capabilities of SXRFM in studies of soils.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

There have been several recent reports of cotton lint yield response to potassium (K) fertilization from areas east and west of Arizona in the Cotton Belt. However, there is no documentation of the K status in southern Arizona within these Sonoran Desert soils. The physical, chemical, and mineralogical properties affecting the K status of 10 soils (six Entisols, three Aridisols, and one Mollisol) common to cultivated soils of the Sonoran Desert were studied to determine which soils might respond to K fertilization. The dominant clay minerals included mica, vermiculite, smectite, and palygorskite. All the soils contained some vermiculite but, none contained more than 5% vermiculite in the clay fraction. Mica was common in the clay fraction of the soils, except for the Superstition sand, which contained the lowest levels of exchangeable K (≤100 mg kg‐1) throughout the 1.2 m of the soil profile. A minimum of 210 mg K kg‐1was contained in the top 0.6 m of the soils. These levels of exchangeable K are much greater than levels of exchangeable K in other areas that have reported K deficiencies. In an attempt to quantify the K status of these soils, the K sufficiency level for the surface 0.3 m was calculated and compared with the exchangeable K level. Only the Superstition soil had a K deficit (—20 mg kg‐1) when compared with the exchangeable K. The other soils contained > 110 mg kg‐1more exchangeable K than the calculated sufficiency level. Additionally, the K desorption rates were calculated for the surface 0.3 m of each soil, and each had the capacity to desorb > 10 mg K kg‐1per typical irrigation event. With the exception of Superstition sand, none of these soils was determined to have exchangeable K levels or clay mineralogy that would indicate that fertilizer K would be required for crop production. Rotations that could potentially remove large amounts of K in consecutive years from any soil will require that exchangeable K levels be monitored by soil‐testing procedures.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

The release of P from two phosphate rocks (PRs) of different reactivity with equimolar concentrations (1 or 10 mmol/L) each of 19 low‐molecular‐weight (LMW) organic acids was studied. The results were compared with those released with a similar concentration of one of three mineral acids (H2SO4, HNO3, and HCI). The PRs were Kodjari PR (low reactive PR) and North Florida PR (medium reactive PR). The LMW organic acids included two monocarboxylic aliphatic acids (pyruvic and glycolic acids), nine dicarboxylic aliphatic acids (oxalic, tartaric, malonic, fumaric, malic, &agr;‐ketoglutaric, maleic, oxalacetic, and succinic acids), two aliphatic tricarboxylic acids (citric andcis‐aconitic acids), and six aromatic acids (phthalic, salicylic, vanillic, gallic, protocatechuic, andp‐hydroxybenzoic acids). In these studies, a 1‐g sample of <100‐mesh‐sieved PR was equilibrated with 25 ml of each acid at 25°C for 24 h. The P in the filtrate was determined. Results showed that the release of P from the PRs with the acids was affected by the chemical structure, type, and position of the functional groups of the ligands, and the concentrations of the acids. The ratios of the P‐released/protons‐added varied among the acids, phosphate rocks, and the concentration of the acid used. These ratios for most of the organic acids were greater than those for the mineral acids, suggesting chelation of the metals associated with P in the PRs. In general, the aliphatic acids with &bgr;‐hydroxyl and &agr;‐carboxyl groups were more effective than other aliphatic or aromatic acids in releasing P from PRs. At 1 mmol/L acid, citric and oxalic acids were more effective than H2SO4in releasing P from the two PRs; the P released from Kodjari PR was greater than that released from North Florida PR. Increasing the acid concentration from 1 to 10 mmol/L corresponded to almost a 10‐fold increase in the P released from the rocks. At both concentrations of acids, the amounts of P released from the rocks with the organic and mineral acids were significantly correlated with the amounts of Ca and Mg released. The amounts of P released were negatively correlated with the pK1values and positively correlated with the log KAIvalues of the organic acids.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Controls on dissolved organic carbon (DOC) concentrations were examined through field and laboratory measurements of a Typic Haplaquod in Michigan. Average DOC concentration in the soil solution at 30‐cm depth was 32 mg/L, and groundwater DOC concentration at 2‐m depth was 18 mg/L. Oxidation‐reduction (redox) potentials measured in the upper 30 cm of soil ranged from —220 mV to +500 mV, indicating the presence of both reduced and oxidized conditions at the site. Mineral soil organic carbon (SOC) ranged from 0.47% in the Bg horizon to 2.70% in the Bhs horizon. Citrate‐dithionite extractable iron (Fec/d) ranged from 5.9 &mgr;g/g in the E horizon to 85.0 &mgr;g/g in the Bhs horizon. Citrate‐dithionite extractable aluminum (Alc/d) ranged from 2.1 &mgr;g/g in the Bg horizon to 15.3 &mgr;g/g in the Bhs horizon. Mineral SOC, Fec/d, and Alc/dconcentrations were generally in the low range of those reported for Spodosols in Canada and United States. DOC concentration in both the soil solution and groundwater was strongly negatively related to Alc/dand Fec/d. Null‐point DOC (DOCnp) concentrations (point at which no net DOC sorption occurs) obtained from sorption studies ranged from greater than 90 mg/L in the E horizon to 18 mg/L in the Bs horizon. This indicated that DOC sorption in the mineral subsoil potentially increased with depth. DOCnpwas also negatively related to Fec/dand Alc/d. Although DOC concentrations were strongly correlated with Fe and Al in the mineral soil, concentrations of those elements were so low that little DOC was retained in the mineral soil.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Many of the micaceous soils of the Indo‐Gangetic alluvial plain of northwestern India are sodic and have clay‐enriched textural B horizons. These soils sometimes lack identifiable clay skins. In order to determine the genesis of the textural B horizons of these soils, both with and without identifiable clay skins, one Natrustalf (Sakit soil) and one Aeric Halaquept (Rahamabad soil) were selected for study.The results indicate that (i) the parent material and the mineralogy of the clay fractions are uniform, thus discounting stratification; (ii) the clay in the B horizon is not formed in place, as evidenced by increase in the K2O content of the sand and silt fractions with depth; (iii) the fine clay (<0.2 &mgr;m) resulting substantially from weathering of biotite has preferentially translocated in a deflocculated form, causing an apparent decrease of clay mica with depth; (iv) the void argillans are typically of the type “impure clay pedofeatures,” which seem to have resulted from the impairment of the parallel orientation of the clay platelets induced by dispersion of both clay and silt size layer silicates in sodic environment; and (v) the decrease in the clay mica (<2 &mgr;m) with depth could be a sure test of clay illuviation, even when clay skins identifiable in the field are absent, as a step toward precise and unambiguous definitions of soil taxa.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID