Sorption and mineralisation of S-metolachlor in soils from fields cultivated with different conservation tillage systems
Highlights
► SMOC sorption and mineralisation are variable among conservation tillage samples. ► Sowing cover crops in conservation tillage systems increases SMOC sorption. ► SMOC mineralisation is correlated with the ratio ‘microbial biomass/sorption coefficient’.
Introduction
Conservation tillage is promoted to enhance the sustainability of agricultural systems and reduce the environmental impact of agricultural activities. Reduced intensity of tillage leads to significant and complex changes (usually interrelated) in soil physical, chemical and biological properties. Often these changes strongly modify the fate of the applied pesticides (Alletto et al., 2010). Although many studies have been made, mainly on the north American continent, the environmental fate of pesticides under conservation tillage presents many contradictions and remains poorly understood (Alletto et al., 2010, Aubertot et al., 2005). A better understanding of pesticide retention and degradation processes that control their movements in soils is needed to assess their persistence and their risks of transfer in the environment under conservation techniques.
One of the main changes resulting from the implementation of conservation tillage compared with conventional methods is the accumulation of organic residues on the soil surface (mulch) and the increase in soil organic matter in the top few centimetres of the soil profile (Balesdent et al., 1990, Tebrügge and During, 1999). Generally, organic carbon content is high in surface soil due to the presence and decomposition of the mulch, and gradually decreases with depth (Lal et al., 1994, Pinheiro et al., 2004, Six et al., 1999). These changes in organic matter location have a strong influence on soil properties, which greatly modify pesticide fate (Alletto et al., 2010). For most pesticides, organic carbon content and pesticide adsorption are positively correlated, resulting in a higher adsorption in the surface soil under conservation tillage than under conventional tillage (Locke, 1992, Reddy and Locke, 1998, Zablotowicz et al., 2000).
Soil pH may also be modified by tillage techniques but in different ways according to the study. Conservation tillage can lead to an increase in pH (Reddy and Locke, 1998), no change (Comia et al., 1994, Rasmussen, 1991) or most often, to a decrease in pH, especially in surface soil due to the accumulation of organic matter and fertilisers (Arshad et al., 1999, Doran, 1980, Levanon et al., 1994). Soil pH can also play a role in pesticide sorption; thus in general sorption increases when soil pH decreases because the charges for the polar or ionic pesticides are modified and/or the variable charges of the soil constituents decrease, favouring the adsorption of non-polar pesticides (Barriuso and Calvet, 1992, Barriuso et al., 1992, Grey et al., 1997).
Mineralisation is regarded as the final stage in pesticide degradation, leading to its complete removal from the soil. Soil microbial activity can be correlated with mineralisation of the molecules (Lavorenti et al., 2003), but not always (Reddy and Locke, 1998). The increase in soil microbial activity under conservation techniques does not always imply that specific microbial populations involved in the degradation of a compound are more abundant (Gaston and Locke, 2000). In some cases, crop residues on the soil surface under conservation tillage seemed to disrupt degradation (Locke and Harper, 1991, Sorenson et al., 1991), and compared with conventional tillage, lag phases may occur in the activation of mineralisation (Seifert et al., 2001). Competition between retention and degradation is also frequently mentioned to explain lower pesticide mineralisation under conservation tillage (Zablotowicz et al., 2000). In addition, a greater availability of carbon under conservation tillage compared with conventional tillage favours the development of non-specialised microorganisms in competition with microorganisms specialised in pesticide degradation. This could defer the use of pesticides as a source of carbon and thus their degradation in soil (Locke and Harper, 1991).
S-metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(methoxy-1-methylethyl) acetamide] (SMOC) is a selective chloroacetamide herbicide widely used for pre- and post-emergence weed control in a variety of crops including maize, soybean, sunflower and sorghum (Ahrens, 1994). SMOC is more persistent in soils than other acetanilide herbicides (Funari et al., 1998), moderately sorbed on soil components (adsorption coefficient Kd ≈ 2.8 L kg−1) (Bowman, 1989, Weber et al., 2004) and relatively highly soluble in water (Sw = 530 mg L−1 at 20 °C). Due to these properties, SMOC has the potential to migrate into ground and surface waters and is frequently detected by environmental monitoring (Institut Français de l’Environnement, 2007, Laabs et al., 2000, Ritter et al., 2000, Vecchia et al., 2009).
Our purpose was to investigate the variability of S-metolachlor sorption and mineralisation under several soils and agricultural systems using conservation techniques. More precisely, the hypotheses tested in this study were that (a) the effect of soil organic matter on SMOC sorption is due to tillage management (= location and quantity of residues) and crop rotation and fallow period management (= nature of residue); (b) SMOC mineralisation could be increased on fields with previous SMOC application and (c) a high sorption of SMOC could limit its biological degradation. A relatively large number of situations (soil types, crop and tillage practices) were chosen in order to get statistically significant trends.
Section snippets
Tillage systems and soil sample characteristics
Soil samples were collected from 51 agricultural fields located in the central basin of the Midi-Pyrenees, south-west France, from April to June 2010. According to the World Reference Base for Soil Resources (ISSS-ISRIC-FAO, 1998), soils were mainly classified as Luvisols, Cambisols and Calcisols. All these fields had been cultivated with conservation techniques for 2–23 years, but with differences in tillage operations according to the crop. Winter crops were always sown with no tillage, while
Description of the samples and variability of soil properties
On the 51 agricultural fields, 7 were cultivated with DT, 18 with RT and 26 with NT practices. Most of the fields (31) were managed with a long rotation (>4 years). The main crop residues found on the soil surface in 2010 (from crops grown in 2009) were maize (9 fields), sunflower (9 fields), wheat (soft and durum, 9 fields), sorghum (7 fields) and rape (7 fields). These residues from the previous crops were mixed with residues of the cover crop sown during the fallow period on 18 fields. The
Conclusion
Adsorption and mineralisation of the herbicide S-metolachlor have been studied in soil samples collected at 0–5 and 5–10 cm depth in 51 agricultural fields cultivated with conservation tillage. This study showed that SMOC sorption was positively correlated with soil organic carbon content, leading to a higher sorption in the topsoil samples (0–5 cm) than in the underlying layer (5–10 cm). A novelty of this experiment was the finding that sowing cover crops during the fallow period in conservation
Acknowledgements
The authors would like to thank Syngenta for providing the 14C and technical grade S-metolachlor. This work was financially supported by the CASDAR project 8102 (coordination by the ‘Chambre Régionale d’Agriculture de Midi-Pyrénées’).
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