The influence of tillage practice and dissolved organic carbon on the soil sorption and desorption of fluometuron and norflurazon

Author

Jennifer S. Dore

Date of Award

12-1996

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant, Soil and Environmental Sciences

Major Professor

Michael E. Essington

Committee Members

Thomas C. Mueller, Mike Mullen

Abstract

Fluometuron (FLMT) and norflurazon (NORF) are preemergence herbicides applied to cotton in the southeastern U.S. Residual levels of both herbicides have been detected in soils long after application, potentially posing risks to subsequent crops and groundwater resources. Herbicide sorption is an important mechanism influencing their fate and behavior. Sorption by soil organic matter (SOM), which is influenced by tillage practice, is the dominant mechanism by which herbicides are removed from the aqueous phase. Dissolved organic carbon (DOC) concentrations greater than 100 mg L^-1 have been shown to reduce the sorption of some trace organic compounds in soil systems.

The objectives of this study were to 1) determine the effect of tillage, (SOM content) on the sorption and desorption of FLMT and NORF, 2) determine the effect of DOC source (poultry litter, dairy manure, and no-till (NT) surface soil) and concentration on the sorption of FLMT and NORF, and 3) examine the effect of NT DOC on subsoil sorption of FLMT and NORF.

Sorption of FLMT and NORF to NT and conventional tillage (CT) managed surface soil and subsoil [Lexington silty loam (fine-silty, mixed, thermic, Typic Paleudalf)] was characterized using batch sorption isotherms. Herbicide sorption experiments were conducted in the presence and absence of DOC extracted from poultry litter, dairy manure and NT surface soil. Desorption of FLMT and NORF was analyzed in the absence of DOC, with DOC in the desorption solution only, and with DOC in both the sorption and desorption systems. Dissolved organic carbon was extracted from poultry litter, dairy manure, and NT surface soil. The DOC extracts were analyzed for organic carbon content. The E₄/E₆ ratio and gel permeation chromatography were employed to characterize the DOC extracts.

The increased SOM content of NT soil enhanced the sorption of FLMT and NORF. The retention of FLMT and NORF by Lexington silt loam soil decreases in the following order: NT > CT > subsoil. Herbicide sorption conformed to the L-curve and C-curve isotherms, with the C-curve being more representative of sorption by the high organic carbon, NT surface soil. Sorption of FLMT and NORF was generally reduced with DOC in the sorption equilibration solution, regardless of source or concentration. The greatest reduction in FLMT and NORF sorption occurred in the presence of poultry DOC. Dairy DOC ranked second in decreasing the amount of herbicide sorbed, followed by NT DOC. Spectroscopie E₄/E₆ ratios of the DOC molecules indicate that the poultry, dairy, and NT DOC extracts are composed of predominately low molecular weight compounds. According to gel chromatography, poultry DOC had the greatest distribution of large DOC molecules, while dairy DOC had a significant distribution of both smaller and larger molecules. No-till DOC was composed of a single size distribution of molecules, close in size to FLMT and NORF. The spectroseopic characterization of the DOC molecules coupled with the sorption results suggests that the large DOC molecules were responsible for the reduced sorption of the herbicides.

The concentration of DOC in the sorption systems also influenced the sorption of FLMT and NORF. In general, sorption decreased with increasing DOC concentration. Recent literature suggests that DOC concentrations below 100 mg DOC L^-1 do not effect the sorption of the herbicides. However, this study shows that DOC concentrations as low as 75 and to a lesser extent, 37.5 mg L^-1 result in reduced herbicide sorption.

The aqueous association of herbicide with DOC can be quantified by the K_DOC. Solid SOM is known to be at least twice as effective a sorbent for herbicides compared to soluble DOC. Therefore, SOM should exert a much greater influence on the sorption of FLMT and NORP. The organic carbon normalized distribution coefficients (K_OC values) indicate the contribution of SOM to organic compound sorption. Computed K_DOC values greatly exceeded the K_OC values calculated in this study. Thus, DOC complexation of the herbicides in solution is improbable in these systems. Reduced herbicide retention in the presence of DOC may result from competition between the herbicide and DOC molecules for limited sorption sites is at work in these systems. Conversely, DOC may coat the soil surfaces and present a hydrophobic surface that forces the herbicides to remain in aqueous solution.

Desorption of FLMT and NORF was hysteretic in all soils. Hysteresis was greater with NORF, compared to FLMT. No effect of SOM content on desorption hysteresis was evident. Desorption of FLMT and NORF from soil was enhanced with the addition of DOC to the soil systems. Poultry DOC enhanced desorption the greatest followed by dairy and NT DOC, respectively. The addition of DOC at the desorption step appeared to reverse herbicide sorption when compared with the no-DOC isotherm. However, the addition of DOC at the desorption step changed the chemistry of the system. Thus, desorption should revert to a DOC sorption isotherm. Some poultry DOC isotherms showed reversible herbicide sorption. However, in general, desorption was incomplete with DOC in the desorption solution only. In systems with DOC concentrations greater than 20 mg L^-1, desorption reversibility was approached with equilibration time. However, in systems with 10 mg DOC L^-1, desorption became more hysteretic with equilibration time. Thus, a threshold concentration of DOC (regardless of source) exists between 20 and 10 mg DOC L^-1, influencing the sorption and desorption of the herbicides.

Desorption of FLMT and NORP sorbed in the presence of DOC, occurred at a much faster rate than in the systems where DOC was absent during sorption. Poultry DOC promoted the greatest decrease in sorption and the greatest desorption, followed by dairy and NT DOC. The desorption of FLMT and NORP from SUB soil with 1150 mg poultry DOC L^-1 in the desorption solution resulted in less herbicide sorption than at the onset of the desorption process, with only 920 mg poultry DOC L^-1 in the sorption system. Perhaps, the desorption of herbicides in the presence of 1150 mg poultry DOC L^-1 followed an sorption isotherm consistent with the presence of 1150 mg poultry DOC L^-1 in the sorption solution. Dairy DOC facilitated the desorption of FLMT and NORP adsorbed in the presence of lesser concentrations of dairy DOC. Similarly, NT DOC resulted in less sorption of FLMT and NORP, and greater desorption of the herbicides.

The greater SOM content of no-till systems results in greater sorption of FLMT and NORP. The addition of DOC to the systems results in reduced sorption of the herbicides. Desorption of FLMT and NORP exhibits hysteresis without added DOC. However, DOC concentrations greater than 10 mg L^-1 enhance the desorption of FLMT and NORP from soil.

Recommended Citation

Dore, Jennifer S., "The influence of tillage practice and dissolved organic carbon on the soil sorption and desorption of fluometuron and norflurazon. " Master's Thesis, University of Tennessee, 1996.
https://trace.tennessee.edu/utk_gradthes/6843