Herbicides in the soil environment: linkage between bioavailability and microbial ecology.

ZABALOY CELINA; ZANINI GRACIELA; BIANCHINOTTI MARÍA VIRGINIA; GOMEZ MARISA; GARLAND JAY

Herbicides, Theory and Applications

INTECH

Lugar: Vienna; Año: 2011; p. 161 - 192

Modern agriculture relies heavily on herbicides for the control of weeds in crops and pasturesto maximize yields and economical benefits to sustain an increasing world population. The introduction of herbicide-resistant traits in several crops, such as glyphosate-resistant soybean,maize and canola, has further increased herbicide consumption worldwide (Cerdeira & Duke,2006). Only in the United States, herbicide consumption was roughly of 200 million kilogramin 2001, and about 20 % of it was glyphosate. Glyphosate is, undoubtedly, the most used herbicide worldwide (Woodburn, 2000). In Argentina, where GR soybean accounts for almost90% of planted soybean, it was estimated that 160 million liters of glyphosate were used inthis crop in 2004 (Altieri & Pengue, 2006; Pengue, 2004).The environmental fate of herbicides is a matter of recent concern given that only a small fractionof the chemicals reaches the target organisms (Pimentel, 1995). This concern is mainlydue to the impact that herbicides may have in soil and water pollution and thus on human,animal and plant health. J. (1993) wrote in 1993 ”It is useful to keep in mind, the concept thata pollutant is a substance in the wrong place, at the wrong time, or in the wrong amount”.This means that although herbicides are very important to agriculture, under certain circumstancesthey may act as pollutants that can lead to the deterioration of soils, groundwatersand surface waters. While most herbicides are not intentionally applied onto soil, they canenter the soil environment from direct interception of spray by the soil surface during earlyseason or post-harvest applications, from runoff or leaching of the herbicide from vegetationand/or from dead plant material. The herbicide concentration may vary from a few microgramsto miligrams per kilogram soil, as most of the applied chemical is retained within thetop 5 cm of soil. Adsorption to soil is of critical importance for the regulation of herbicidepersistence and mobility throughout the environment because sorption processes control theamount of herbicide present in the soil solution. These processes are dependent on severalfactors related to soil characteristics such as mineral composition, organic matter content, soilsolution chemistry and to chemical characteristics of the herbicide. Soil-bound herbicide orresidues are temporarily inactivated, which prevents harmful effects on soil biota but alsomakes them less bio-available for microbial degradation because most microbial species arenot able to utilize herbicides in the sorbed state. Soil biochemical and biological processeshave an utmost importance for ecosystems functioning, as microbes have key roles in organicmatter transformations, nutrient cycling and degradation of organic pollutants, including pesticides(Beck et al., 2005). Biological degradation mediated by microbial enzymes is the main route for pesticides detoxification in soils (van Eerd et al., 2003). Most isolated herbicidedegradingmicroorganisms belong to bacterial species, but fungi are also well-known for theircapacity to degrade complex substrates, and may be more important than present isolationapproaches have suggested (Smith & Collins, 2007). Differential toxicity of herbicides to soilmicroorganisms may alter community structure, including potential increases in plant or animalpathogens. Herbicides may also cause changes in microbial community function andconcomitant impacts on soil health and ecosystem processes. Even though functions mayappear unaltered, due to species redundancy in soil, the extinction of resistant species maycompromise the continuity of such processes.This chapter will present aspects of the behavior of herbicides in soils, focusing on soil retentionand microbial degradation as main factors controlling persistence. The potential impactof herbicides on non-target soil microbes, on their processes and interactions, will bealso discussed. The enormous variety of herbicides commercially available today makesit impossible to review all of them. Thus, this work will focus on some of the herbicidesmost used in the (semiarid) Pampa region of Argentina and worldwide (i.e., glyphosate, 2,4-dichlorophenoxyacetic acid, atrazine, metsulfuron-methyl), based on our own research data.