摘要:

Air permeability can be used to describe the structure of the soil but may also be used to predict saturated hydraulic conductivity. This raises the question of whether the two parameters exhibit the same degree of scale dependency. In this study the scale dependency of water permeability (saturated hydraulic conductivity,Kw) and air permeability (ka, at a matric water potential of −50 cm H2O) was tested at four different sites (three horizons at each site), by using two measurement scales (100 cm3and 6280 cm3). No clear effect of scale on variability was observed. Air and water permeability displayed higher variabilities for two structured loamy soils compared with two sandy soils. For the more structured soils, the variability between measurements was lower for air compared with water permeability. Both air and water permeabilities were higher at the large scale compared with the small scale, but this scale-dependent difference was less pronounced in sandy soils, suggesting a smaller representative elementary volume. For three of the four soils, a highly correlated relationship betweenKwandkaon both small and large soil samples was observed. For the fourth soil, water retention data revealed that the samples were not sufficiently drained at −50 cm H2O to validate a comparison between the two parameters. PredictiveKw(ka) relations for the remaining three soils at the two scales compared favorably with a generalKw(ka) relation proposed byLoll et al. (1999). This study supports the use of a general predictive relation betweenkanear field capacity (at around −50 to −100 cm H2O) andKw, but caution should be taken if the soil has a large content of pores that will drain at or close to a matric water potential of −50 cm H2O.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

Phenolic compounds originating from plant residue decomposition or microbial metabolism form humic-like polymers in the presence of various phenoloxidases or metal oxides. Enzyme-mediated reactions were reported to result in the decarboxylation or demethoxylation of substrate molecules; decarboxylation was also observed with metal oxides. To obtain more information on these phenomena, several humic precursors were incubated with various phenoloxidases (peroxidase, laccase, tyrosinase) or birnessite (δ-MnO2) and monitored for CO2evolution and methanol production. Additionally, some reaction mixtures were analyzed for methane evolution. By using the test compounds labeled with14C in three different locations (carboxyl group, aromatic, or aliphatic chain), we demonstrated that14CO2evolution (ranging from 4.6 to 63.5% of the initial radioactivity) was mainly associated with the release of carboxyl groups. Minimal mineralization of14C-labeled aromatic rings or aliphatic carbons occurred in ferulic or p-coumaric acids (0–5.6%). Demethoxylation ranged from 0.5 to 13.9% for 2,6-dimethoxyphenol and syringic acid, respectively. The methyl groups in 2-, 3-, and 4-methylphenol resisted release, as indicated by the lack of methane or methanol production. In previous studies, chlorophenols incubated with various phenoloxidases or birnessite were subject to dehalogenation. It appears that dehalogenation, decarboxylation, and demethoxylation of phenolic substrates are controlled by a common mechanism, in which various substituents are released if they are attached to carbon atoms involved in coupling. According to the experimental data, electron-withdrawing substituents, such as –COOH and –Cl, are more susceptible to release than electron-donating ones, such as –OCH3and –CH3. The release of organic substituents during polymerization of humic precursors may add to CO2production in soil.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

The distribution of polycyclic aromatic hydrocarbons (PAH) and polychlorinated biphenyls (PCB) among differently reactive soil organic matter (SOM) pools may influence their mobility and bioavailability. Our objective was to examine the distribution of PAH and PCB among density fractions of urban soils of Bangkok. We determined the concentrations of 20 PAH and 7 PCB in bulk samples and in density fractions <1.6 g cm−3, 1.6–2.0 g cm−3, 2.0–2.2 g cm−3, 2.2–2.4 g cm−3, and >2.4 g cm−3of five urban surface soils (0–5 cm) from Bangkok. In the bulk soil, the sum of PAH concentrations ranged from 47 to 140 μg kg−1and that of the PCB concentrations ranged from 0.10 to 1.20 μg kg−1. The two fractions with a density >2.2 g cm−3contained more than 90% of the total soil mass. The C and N concentrations increased with increasing density; the C/N ratio decreased, indicating increasing SOM alteration with increasing density. In the sum of the five density fractions we found, on average, 158% of the sum of PAH concentrations and 246% of the sum of PCB concentrations in the bulk soil. This indicated that density fractionation provided better access to sorption sites for the solvent. In general, the concentrations of PAH and PCB in the density fractions decreased with increasing density. The organic C concentration in the density fractions was significantly correlated with the sum of PAH concentrations (r= 0.84), with the sum of PCB concentrations (r= 0.64), and with the concentrations of each individual PAH (r>0.60) and PCB (r>0.54). There were no significant differences in the contributions of low-molecular weight PAH (<203 g mol−1) and PCB (Nos. 28 and 52) to the sum of PAH and of PCB concentrations between the density fractions, indicating that the mixture of PAH and PCB was not altered in the same way as SOM. Our results suggest that PAH and PCB are partitioned to SOM in density fractions independent of SOM composition. We conclude that density fractions of soils do not represent specific PAH and PCB pools.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

Cover crop residues on no-till soil will intercept a portion of applied herbicides. Thus, herbicide efficacy in no-till systems depends, in part, on rainfall to wash the herbicide onto the soil. Tillage and cover crop residue may also influence sorption and degradation of a herbicide in soil. This series of studies examined fluometuron [N,N-dimethyl-N′- [3-(trifluoromethyl)phenyl]urea] wash-off from native vegetation, hairy vetch (Vicia villosa), and wheat residue (Triticum aestivum), related wash-off to sorption on these residues, and compared fluometuron sorption and degradation in soil from long-term native, vetch, and wheat cover crop plots used with either conventional or no-till cotton (Gossypium hirsutum). A rainfall simulator was used to wash spray-applied fluometuron from plant material. Through-flow was analyzed for fluometuron by high-performance liquid chromatography. More fluometuron was washed off native vegetation than vetch or wheat residues, which retained fluometuron about equally. Fluometuron sorption on these residues was determined in a batch study. Sorption was least with native vegetation (KD= 11 L kg−1), and there was minimal difference between vetch and wheat in sorption (KD= 17 L kg−1). Fluometuron wash-off could be modeled from the batch sorption data. Sorption of fluometuron in surface soil from each tillage by cover crop combination was determined in a batch experiment. Sorption was adequately described by the Freundlich model with N ∼ 0.9 for all soils but K values of no-till soils (average ∼ 2 L kg−1) nearly twice that of corresponding conventional-till soils. Degradation of fluometuron in these surface soils was determined by incubating fortified samples for 6, 15, 30, and 60 days. Soil extracts showed that degradation was more rapid in any no-till soil than in its conventional-till analog. Within either tillage treatment, degradation was slowest in vetch soil. Half-lives ranged from 7 and 8 days (no-till native and wheat soils, respectively) to 51 days (conventional-till vetch soil). Fluometuron half-lives in conventional-till wheat and native (19 and 27 days, respectively) and no-till vetch (31 days) soils were intermediate. Microbial activity was higher in no-till soil, consistent with faster degradation.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

Soils with andic properties have a high anion retention capacity, which is mainly related to the large amounts of active Al and Fe compounds that they contain. The phosphate (PO4) and sulfate (SO4) sorption capacity of soils with andic properties from Galicia, NW Spain, was investigated. Phosphate sorption experiments were conducted by adding 1.6, 16.1, and 48 mMKH2PO4to soils, and those of sulfate by adding 0.4 and 1.6 mMK2SO4(ratio of soil:solution, 1:10). The pH of the extracts was measured, as were the levels of PO4(only in the PO4experiments), SO4, Si, and dissolved organic carbon. The soils retained much higher amounts of PO4than of SO4. Surface horizons displayed a higher capacity than subsurface horizons for retaining PO4but not for retaining SO4. The PO4retention capacity was related to the sum of contents of oxalate extractable-Al (Alo) and -Fe (Feo), which tended to increase with soil organic carbon content and was mainly attributed to the presence of Al-humus complexes. Sorption of PO4primarily produced displacement of dissolved organic carbon, Si, and SO4into solution and increases in solution pH, whereas sorption of SO4only generated increases in solution pH. The low ability of SO4anions to displace sorbed organic C mainly explains why surface horizons retained similar or even smaller amounts of SO4than subsurface horizons, despite the higher Aloand Feocontents of the former. The results emphasize the different affinities of PO4and SO4for reactive surfaces and the interaction of organic C in sorption processes.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

Protons, weak acids, and anions present in the soil solution because of biotic activity or because of fertilizer additions may affect the dissolution of minerals, depending on their ability to weaken the metal-oxygen bonds and consequently detach the metal from the surface. Among the anions able to form inner-sphere complexes, phosphate has been shown to be effective in promoting dissolution of oxides and allophane clays, but no information is available about its effect on Mg-silicates, even though the presence of the dissolution products of serpentine in soils may be of concern for plant nutrition. The interaction of phosphate with serpentinite and its effect on the dissolution of the rock in the presence of different electrolytes was studied. A serpentinitic rock was shaken for 24 h with 0.01MKCl or CaCl2and increasing concentrations of KH2PO4(P). The pH and the contents of P, Mg, and Si in the solutions were determined, and the dissolution data compared with those obtained by computer chemical simulations. At low P additions, adsorption predominated both in KCl and in CaCl2, and the ligand exchange reactions buffered the acidity caused by the weak acid dissociation. The release of Mg was not different from that obtained with the electrolyte alone, whereas Si in solution probably derived also from sorbed forms on the serpentine surface. In the presence of CaCl2, however, the Mg concentration was higher, probably because of the presence of exchangeable forms. When the initial P concentration increased, the situation was different with the two electrolytes; adsorption continued with KCl without reaching a plateau, whereas precipitation of Ca-phosphates occurred with CaCl2. In KCl, the ligand exchange reactions were not sufficient to neutralize the protons from acid dissociation, the pH decreased, and the mineral dissolution was enhanced. However, the elemental release in the presence of phosphate was lower than what was expected, probably because of some shielding effect of the active metal sites by the anion. In the presence of CaCl2, instead, the precipitation of insoluble salts enhanced the H+concentration and consequently the dissolution of serpentine. At the highest P additions, the amounts of elements released into the solution were lower than the calculated ones and independent of P concentration, probably because of the precipitation of Ca phosphates on the mineral surface that might protect serpentine from further dissolution. A different situation was observed for KCl, with which the dissolution increased, probably because of the destabilization of active sites.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID