摘要:

Prediction of pesticide volatilization from soils also requires accurate prediction of water transport. One objective of this study was to assess the differences in predicting water content and water evaporation from soil when water vapor diffusion is and is not included in the water transport simulation. Another was to evaluate if the water transport model can predict water transport adequately in both dry and wet soils and if it is suitable to be incorporated into a pesticide fate and transport model. These two objectives are achieved through numerical simulation and soil column experiments. Two levels of initial water content were used in the experiments. Wet air and dry N2were passed alternately across the soil surface. A Time Domain Reflectometry system and a hygrometer were used to monitor changes of water content and relative humidity at various depths during the experiments. The modeled soil-water content and evaporative fluxes agreed reasonably well with measurements. The water contents near the soil surface, simulated with and without water vapor diffusion, were different enough to significantly affect diazinon vapor concentration at the soil surface. Therefore, water vapor diffusion should be included in the water transport model in order to simulate the pesticide sorption process in dry surface soils more accurately.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Volatilization of pesticides from soil is one of the key processes causing transport of chemicals to non-target areas. This study focuses on how water transport in the surface soil affects volatilization of pesticides. Volatilization of soil-incorporated diazinon [O, OdiethylO-(2-isopropyl-4-methyl-6-pyrimidinyl) phosphorothioate] was measured under different initial soil-water contents and soil-water evaporation conditions. These conditions were varied by passing wet air and dry N2alternately across the soil surface. When dry sweep gas (N2) was used in high initial water content columns, diazinon volatilization was accelerated as a result of appreciable upward water flow. When upward water flow was insufficient to meet evaporative demand, soil-water content at the soil surface was quickly reduced below a threshold, and diazinon volatilization started to decrease exponentially because of diazinon adsorption on the dry soil surfaces. When the sweep gas was switched to humidified air, the soil surface adsorbed water and diazinon volatilization increased dramatically. A numerical model was developed to simulate pesticide transport in soil for isothermal conditions. The calculated and measured diazinon volatilization fluxes agreed well, but the model overestimated diazinon concentrations in the deeper depths of the soil significantly.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Glyphosate [N-(phosphonomethyl)glycine] is a broad-spectrum herbicide that is used widely in agriculture, forestry, and urban settings. It is one of the most heavily applied pesticides in the US, with an annual use greater than 12 million kg. When added to soil glyphosate is readily sorbed, losing both its efficacy as an herbicide and its potential for translocation. Despite such tenacious binding to soil colloids, the adsorbent surface offers little protection from biodegradation because the glyphosate decomposes so rapidly, with a half-life as short as several days. Attempts to elucidate the sorption mechanism of glyphosate, thereby providing a mechanistic explanation for this biolability, have thus far been unsuccessful. Therefore, the goal of this research was to derive the architecture of an important model system, the Cu-glyphosate-gibbsite surface complex, in which Cu serves as a probe of the local bonding environment and gibbsite serves as a representative soil mineral adsorbent. Copper K-edge extended X-ray absorption fine structure (EXAFS) spectra of Cu-glyphosate complexes in aqueous solution at pH 5.5 revealed Cu complexation by the phosphonate moiety. Introduction of microcrystalline gibbsite resulted in the loss of P from the coordination sphere of Cu and the formation of a ternary complex through chemisorption of the phosphonate group to the oxide surface. The EXAFS spectra of Cu in the Cu-glyphosate-gibbsite surface complex matched spectra for Cu-glycine and Cu-methylglycine complexes in aqueous solution, thereby providing evidence for Cu complexation by the carboxyl O and amine N. Despite the chemisorption of glyphosate to oxide surfaces via the phosphonate group, the distal end of the glyphosate molecule extends into aqueous solution where it may be biodegraded rapidly.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

A parameter estimation approach that combines a numerical code for unsaturated flow with a nonlinear optimization method was used to analyze horizontal infiltration data for Columbia silt loam and Hesperia sandy loam. The data were previously investigated byNielsen et al. (1962)using the analytical method ofBruce and Klute (1956). As with the original analysis, water content profiles at the different times could be analyzed accurately in a simultaneous fashion only when the applied pressure head at the column boundary was close to saturation (−0.02 m). For much lower boundary pressures (−0.50 and −1.00 m), water content profiles for the different times had to be analyzed independently. Excellent agreement was obtained between diffusivities calculated from the same water content profiles using either the analytical method of Bruce and Klute or the numerical parameter estimation technique. However, in addition to diffusivities, the numerical parameter estimation analysis also provided estimates of the entire soil-water retention and hydraulic conductivity functions. Numerical analysis of the experimental data produced functional forms of the hydraulic properties, compared with point values, when using the analytical analysis.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

This study compares eight models for scaling soil-saturated hydraulic conductivity,Ks, using a database of 402 data sets collected from 25 sources in the literature. The database includes data regardingKs, particle size distribution, soil bulk density, and soil particle density, with textures ranging from sand to heavy clay. The results showed that models based on the nonsimilar media concept and the Kozeny-Carman model performed best for estimating soilKsusing scaling. In contrast, Campbell and Poulsen-Saxton models were not suitable for scalingKs. The group of models that gave the second best estimation ofKsincluded Saxton, Cosby, Vereecken, and the complex Brakensiek models. All eight of the models had smaller estimation deviations for sandy soils than for clayey soils. Moreover, the results showed that the sample that had an average of the characteristics of all the samples should be taken as a reference point when scalingKs, whereas for soil samples with identical texture, it would be better to employ the sample with minimum bulk density as the reference point. The NSMC and Kozeny-Carman models are recommended for scalingKs.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Little is known regarding the impact of land use on the isotopic signature of dissolved organic matter (DOM), although such impacts should occur first in this most mobile part of soil organic matter. The aim of the present study was to investigate the effects of both land use and change on the isotopic composition of DOM. We extracted water-soluble fulvic acids (FA) as the main fraction of DOM from the topsoil, groundwater, and surface water of six sites differing in land use in a natural fen area, and we analyzed13C,14C, and15N. Long-term intensive land use (of at least 50 years) resulted in the enrichment of water-soluble FAs with13C. Enhanced peat decomposition caused by conventional crop farming resulted in the highest14C age of water-soluble FAs. The radiocarbon age of the water-soluble FAs in the topsoil and surface water was as much as 1700 years, much older than previously assumed. Long-term agricultural land use with high fertilizer input resulted in14N enrichment of water-soluble FAs, probably because of the incorporation of light N isotopes derived from mineral fertilizers.14C and15N isotopes indicated a distinct change of C and N turnover after changing from conventional crop farming to unimproved pasture. The N and C isotopes of water-soluble FAs are a useful tool to indicate changing C and N cycles caused by long-term (50 to 200 years) differences in land use. Short-term changes in land use (<5 years) are reflected only in the14C and15N signature of water-soluble FAs.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

The influence of the nature and properties of oxides in soil and sediment environments on the characteristics of polyphenol-derived humic polymers formed by abiotic catalysis is not fully understood. The objective of this study was to investigate the catalytic power of Mn(IV), Fe(III), Al, and Si oxides in the transformation of pyrogallol and the development of carboxylic and aliphatic groups in the resulting humic polymers. The metal oxides differed significantly in their effectiveness for catalyzing pyrogallol humification. The results indicate that the carboxylic group of humic polymers developed as a result of ring cleavage of pyrogallol catalyzed by the oxides. The ability of short-range ordered oxides of Al, Si, and especially Fe and Mn, in enhancing the formation of aliphatic and carboxylic groups in humic polymers derived from polyphenol humification may, in part, contribute to the origin of aliphaticity of humic substances and the variation of their characteristics, including carboxylic group contents with soils from different climatic zones.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

There has been increasing concern over nutrient loss from agricultural practices that may contribute to the accelerated contamination of groundwater and surface waters. This concern is greater in sandy soils, which have minimal nutrient retention capacity. Both field- and column-leaching studies were conducted to examine the leaching of NO−3, PO3−4, and K in a Riviera fine sand (loamy, siliceous, hyperthermic, Arenic Glossaqualf) under grapefruit production that received 0 to 168, 0 to 30, and 0 to 168 kg ha−1yr−1of N, P, and K, respectively. The concentrations of NO3-N, PO4-P, and K were measured in soil solution sampled using suction lysimeters installed above (120 cm) and below (180 cm) the hardpan. Column leaching was conducted using soil collected from 0 to 30, 30 to 60, 60 to 90, 90 to 120, 120 to 150, and 150 to 180 cm of the profile, and the amounts of N, P, and K applied to the leaching column were equivalent to the application rates in the field. The concentrations of NO3-N, PO4-P, and K in soil solution at both 120- and 180-cm depths increased with increasing fertilizer rates. Fertigation tended to enhance leaching of NO3-N, PO4-P, and K compared with dry soluble granular application. The concentrations of NO3-N and PO4-P in soil solution were much higher at the 120 cm depth than at the 180 cm depth, whereas the reverse was true with respect to K solution concentrations. The average concentrations of NO3-N in soil solution at both the 120- and 180-cm depths over 3 years were well below 10 mg L−1, the U.S. Environmental Protection Agency (U.S. EPA) drinking water quality standard, even at the highest rate of fertilizer application. Solution K concentrations at the 180-cm depth were close to or slightly higher than 12 mg L−1, the maximum level for drinking water set by the European Community. Solution PO4-P concentrations at the 120-cm depth (average 0.25 to 0.70 mg L−1over 3 years for plots receiving various amounts of fertilizer) were much higher than the U.S. EPA criteria for fresh waters (0.025 mg L−1in lakes and 0.05 mg L−1in streams) and may constitute a P source for surface waters as soil solution above this depth is likely to seep into water furrows through lateral movement and be discharged into drainage water. A column leaching study demonstrated that the argillic horizon in the Riviera fine sand effectively reduced downward movement of PO4-P and K because it had a much greater sorption capacity for phosphate and K, but it had much less effect on the vertical transport of NO3-N along the soil profile. The results indicate that best management practices in this sandy soil region should be directed to minimizing the leaching of PO4-P and K in addition to NO3-N.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID