The influence of maize residues on the mobility and binding of benazolin: Investigating physically extracted soil fractions

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Abstract

The amount of non-extractable residues and the distribution of benazolin and its metabolites were evaluated three months after herbicide application (14C-labelled) in physically extracted soil fractions of topsoil layers of undisturbed soil columns with and without incorporated maize straw (14C-labelled). In addition, a variety of wet-chemical and spectroscopic methods were used to characterise the structure of organic carbon within the different soil fractions. The addition of crop residues increased the amount of dissolved organic carbon, enhanced the aromaticity of the organic carbon structure and enforced the aggregation of organomineral complexes. After incorporation of crop residues, an increase in the formation of metabolic compounds of benazolin and of non-extractable residues was detected. These results indicate that the addition of crop residues leads to a decrease in mobility and bioaccessibility of benazolin and its metabolites.

Introduction

The incorporation of crop residues into farmland soil may influence the mobility and bioavailability of pesticides. Despite many studies concerning the strong correlation between soil organic matter content and the adsorption or sequestration of xenobiotics, their binding mechanisms are still not understood precisely. It is commonly reported that organic amendments enrich soils of low organic matter content and consequently promote adsorption of pesticides and reduce pesticide mobility (Printz et al., 1995). However, only a few studies indicate an increase in the adsorption of anionic herbicides when applied to amended soils (Cox et al., 2000). There is also evidence in the literature that organic matter may block adsorption sites and thus inhibit the sorption of anionic pesticides (Celis et al., 2005, Vereecken, 2005). Furthermore, the incorporation of decomposable carbon may stimulate the biodegradation of pesticides by increasing the soil microbial activity (Wanner et al., 2005). On the other hand, soils amended with fresh organic matter can protect xenobiotics from degradation through enhanced sorption and preserve them in an extractable form (Barriuso et al., 1997). Generally, it has been found that the addition of organic amendment to soil increases the proportion of non-extractable residues of pesticide (Barriuso et al., 1997). The formation of non-extractable residues, also named as ‘bound’ residues, may be due to physical sequestration (Pignatello and Xing, 1996), covalent binding (Hatcher et al., 1993) and also to incorporation into the soil microbial biomass (Stott et al., 1983). Thus, binding mechanisms can be part of various physical and chemical interactions between compounds and the soil structure. All these findings underline the important role played by soil organic matter in the mobility and bioaccessibility of pesticides.

To elucidate the interactions between organic matter and pollutants in soil, physical methods based on particle-size fractionation, initially developed to study soil organic matter dynamics (Christensen, 2001), have recently been further developed and applied (Séquaris et al., 2005). These physically based methods are expected to only slightly modify the organic carbon compounds and the nature of their association with the soil's mineral components. The use of physical fractionation methods in sorption studies of pesticides in soils has increased steadily over the past few years (Bayard et al., 1998, Taylor et al., 2004). However, only a few research groups have combined physical and chemical methods of soil organic matter fractionation in order to study pesticide-soil organic matter interactions and localisation of pesticide ‘bound’ residues (Benoit et al., 2000, Doick et al., 2005). To our knowledge, the use of 14C-labelled crop residues and pesticides in combination with a gentle particle-size fractionation and chemical extraction linked to further studies of structural changes of organic carbon pools has not yet been attempted.

In our work, the influence of maize straw on the mobility and bioaccessibility of the auxin herbicide benazolin was investigated in physically extracted soil fractions. Benazolin (4-chloro-2-oxobenzothiazolin-3-yl acetic acid) is a selective, systemic, growth-regulator, post-emergence herbicide (pKa = 3.04, 20 °C). Benazolin is the corresponding acidic and first transformation product of benazolin-ethyl (ethyl 4-chloro-2-oxobenzothiazolin-3-yl) that is degraded in soils by de-esterification with a half-life of 1–2 days (Leake, 1989). Further transformation of benazolin occurs via cleavage of the acetic acid moiety (half-life 2–4 weeks) to thiazolin (4-chloro-benzothiazolin-2-one) (Leake, 1989). After ring-opening and the formation of several metabolites, thiazolin finally mineralises to CO2. Previous studies have been devoted to the fate and transport of benazolin in lysimeter and column experiments (Burauel et al., 1995, Drewes, 2005, Jene, 1998, Leake, 1991).

A variety of wet-chemical and spectroscopic methods can be used to characterise the structure of the organic carbon within the different particle-size fractions in the course of decomposition processes. As shown by numerous studies, due to their sensitivity and non-destructive nature, fluorescence techniques are well suited for studies of the chemical and physical properties of dissolved organic carbon (DOC) (Burauel and Baßmann, 2005, Senesi et al., 1991). Their colloidal behaviour in solution and thus the stability of the clay-sized particles can be studied by using photon correlation spectroscopy (particle hydrodynamic diameter) and electrophoretic mobility (zeta potential) (Burauel and Baßmann, 2005). So far little attention has been paid to the potential of photon correlation spectroscopy (PCS) in determining particle size distributions in natural waters and soil solutions (Séquaris and Lewandowski, 2003). Zeta potential measurements have been used to characterise the role of reactive surface sites of clay minerals and iron oxide particles and their complexation by humic substances or natural organic matter and of real soil systems (Séquaris and Lewandowski, 2003, Tombácz et al., 2004).

The specific objective of this study is to determine the mobility and bioaccessibility of benazolin by coupling physical soil fractionation, chemical extraction and physico-chemical techniques (Fig. 1). This will enable us to evaluate the distribution and retention properties of organic carbon and the amount of ‘bound’ residues and mobile herbicide metabolites in unamended and amended undisturbed soil column experiments. These results will enable a better understanding of binding and remobilisation processes of chemicals in soils.

Section snippets

14C-labelled maize straw

Maize plants were grown in a phytochamber CMP 4030 (Conviron) under controlled conditions. For the labelling procedure, 14CO2 was produced in a flask outside the chamber by continuously pumping (100 ml d−1) an aqueous solution of NaH14CO3 (14.2 mg of 50 mCi mmol−1, American Radiolabeled Chemicals, Inc.) into a solution of hydrochloric acid (3 M). A constant stream of CO2 passing through the hydrochloric acid transported the 14CO2 into the chamber. The plant shoots were exposed to the 14CO2-enriched

Leaching of maize straw and benazolin compounds

The results of the column experiments for 14C-labelled maize straw and benazolin are shown in Fig. 2. Six months after incorporation of the 14C-labelled maize straw, up to 25% of the 14C-activity was located in the top 0–5 cm soil layer. Only traces of penetration below 10 cm were detected (Fig. 2A). The amount of recovered 14C-activity in the leachate did not exceed 2% of the applied 14C-activity (data not shown). Most of the applied 14C-activity was collected as 14CO2, indicating a strong

Conclusions

The use of 14C-labelled maize straw and benazolin in combination with a gentle particle-size fractionation followed by a chemical extraction opened up the possibility of characterising the distribution of organic carbon in different size fractions and evaluating the amount of ‘bound’ residues and mobile herbicide metabolites in them. Furthermore, the changing structure of soil organic carbon, and thus its possible retention property, was examined by using wet-chemical and spectroscopic methods

Acknowledgements

The authors gratefully acknowledge the assistance of A. Steffen, J. Noël and M. Lièvre.

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