The important characteristics of soil organic matter affecting 2,4-dichlorophenoxyacetic acid sorption along a catenary sequence

Abstract

Variations in the characteristics of soil organic matter (SOM) at the field-scale are largely unknown, particularly in relation to observed variations in herbicide sorption. For the herbicide 2,4-D [2,4-dichlorophenoxyacetic acid], we found that its organic carbon-normalized sorption coefficient, Koc, varied by four-fold, from 76 to 315 L kg(-1), in the Ap-horizon along a slope transect in an undulating agricultural field in Manitoba, Canada. In order to explain the relatively large in-field variation in 2,4-D Koc values, techniques ranging from conventional chemical fractionation methods to solid state Cross Polarization and Magic-Angle Spinning (13)C-Nuclear Magnetic Resonance applied on whole soils, were used to derive SOM chemical, physical and structural parameters for correlation analyses with the measured 2,4-D Koc values on whole soils. Out of the 15 parameters considered, the 2,4-D Koc was significantly positively correlated with 1) the carbon (C) content of sodium hydroxide-extracted humic acids (r = 0.83, P < 0.01), a chemical parameter indicative of free form C in soil; 2) the molar absorptivity of humic acids at wavelength 280 nm (r = 0.81, P < 0.01), a physical parameter indicative of greater SOM aromaticity; and 3) the relatively intensity of aryl C (r = 0.92, P < 0.01) and O-aryl C (r = 0.93, P < 0.01) in whole soil, both structural parameters indicative of aromatic C. Consequently, the results suggest that in-field variations in 2,4-D Koc values are induced by variations in SOM aromaticity. Koc values are among the most sensitive parameters in herbicide fate models used in regulatory and environmental assessments. Currently, these herbicide fate models do not consider associations between SOM characteristics and Koc and hence revising model equations to include these associations may improve estimates of herbicide persistence, bioavailability and transport at the field-scale.

Environmental and Agricultural Relevance of Humic Fractions Extracted by Alkali from Soils and Natural Waters

“…For nonionic pesticides, hydrophobic bonding (partitioning) is most common, but other possible mechanisms for sorption by HS include charge‐transfer complexes, covalent bonding, hydrogen bonding, and Van der Waals attractions (Senesi and Chen, 1989; Senesi, 1992). Bonding mechanisms commonly observed for ionizable pesticides and HS include charge‐transfer (electron donor–acceptor) complexes, hydrogen bonding, ligand exchange, and Van der Waals attractions (Fusi et al, 1988; Senesi and Chen, 1989; Hermosin and Cornejo, 1993; Piccolo and Celano, 1994; Senesi et al, 1995, 1997; Ferreira et al, 2002; Sheals et al, 2002).…”

Contribution of Soil Components on the Sorption of Chlorpyrifos

“…3) in relation to the results reported by other authors. Farenhorst et al (2010) found in soil from an agricultural field in Manitoba, Canada, proportions of aromatic C (O-aryl C + aryl C) between 24 and 37% of the total area in the of 13 C NMR. Ahmad et al (2006) studied the nature of of 25 soils of Australia and Pakistan and reported proportions of aromatic C between 11 and 28%.…”

“…In the last decade, several studies have shown that K OC is a coefficient which depends not only on the adsorbate molecule but also on soil properties (Ahangar et al 2008b;Singh and Kookana 2009;Gebremariam et al 2012;Ehlers Clark et al 2010). Other authors reported that K OC values vary considerably within soillandscape (Coquet and Barriuso 2002;Farenhorst et al 2010) and both intrinsic factors (such as the natural condition of the soil) and extrinsic factors (management practices) can contribute to such variations (Farenhorst 2006). Wauchope et al 2002 analyzed K OC values from many different soils reported by several other authors finding that the coefficient of variation in K OC values was generally around 30-60% and the ratio between the maximum values and minimum values of K OC was generally around 3-10.…”

“…They also analyzed the chemistry of organic matter using 13 C NMR spectroscopy and concluded that K OC increased with aryl C content and decreased with the O-alkyl C and alkyl C content. Farenhorst et al (2010) reported that K OC values for the 2,4-D (weakly acidic herbicide) varied from 76 to 315 L kg −1 in soils obtained along a slope transect from an agricultural area in Manitoba, Canada. This study, in which SOM structure was characterized by NMR technique, showed that there is variation in field scale for K OC values of 2,4-D attributed to variation of aromaticity in the SOM given by the relative proportion of aryl C and O-aryl C. Similar results were obtained by Ahmad et al (2001) for carbaryl and phosalone (both nonionic pesticides) in soil from different regions of Australia and Pakistan.…”