What is it about?

Herbicides and other pesticides are applied to increase crop yield and raw material quality. However, once in the soil, they may affect non-targeted organisms by reducing their population, changing community structure or metabolic activity. Soil microorganisms has key role on the degrade herbicides, affecting their persistence in the environment. The degradation of organic compounds can be by catabolism (metabolism), in which the microorganism obtains energy or carbon from organic compounds for its growth, or by co-metabolism, in which the microorganism does not obtain energy or benefit from degradation. While transformation processes by co-metabolism are usually mediated by appropriate non-specific enzymes that are able to transform a variety of compounds. As co-metabolism does not play a role as a source of energy or nutrients for microorganisms, the populations do not grow in response to the degradation process. An extra source of carbon is necessary to sustain microorganism growth. Several formulated mixtures of herbicides, fungicides and insecticides are available for crop protection in Brazil. In fact, herbicides, applied in mixtures or in sequential applications, are common practice in agricultural areas from Brazil. Pesticides may accumulate and affect the structure of soil microorganism populations in the soil. Consequently, the rate to which organic compounds are degraded by microorganisms in the soil may change. Thus we aimed to evaluate the effect of the application of three herbicides alone and in mixture on the soil microbial activity and bacterial community structure.

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Why is it important?

Our results showed a strong effect of herbicide mixture on soil microbial respiration. The application of diuron alone promoted increase in detachment rate of 14C-CO2 after 42 days of incubation both clay and sandy soils. However, the strongest effect of the treatments was observed in the sandy soil, where there was lower emission of 14C-CO2 in the initial incubation periods (7, 28 days), suggesting that there was a deleterious effect on microbial activity. The soil type affected the microbial diversity, and in the clay soil the highest diversity indices were from the communities treated with hexazinone. While in sandy soil, the highest rates were for diuron and sulfometuron-methyl treated samples. The microbial community was structured in a complex way, with strong control separation in both soils and application alone or in mixture of herbicides.


Substrate-induced respiration evidenced the effect of herbicide application alone and in mixtures on the microbial activity in the two soils with different textures. The application of diuron, hexazinone and sulfometuron-methyl in mixture increased soil respiration in sandy soil. The percentage of 14CO2 released did not differ between diuron and sulfometuron-methyl, indicating that the herbicide mixture did not boost soil microorganism activity. Although glucose is a substrate of easy degradation by soil microorganisms, our research showed that in the presence of pesticides, the respiration rate can be changed, especially in soil with sandy texture or lower organic matter content. In the final incubation period, the 14CO2 emission rate increased in almost all treatments in the sandy soil and diuron treatment in the clayey soil, indicating the adaptation of the microbial community to the herbicides. This can be ratified by the strong change in the structure and diversity index of the bacterial community of both soils (clayey and sandy) after application of the herbicides alone and in mixture. Furthermore, diversity indices were more sensitive parameters than substrateinduced respiration to detect the impact of herbicide application alone or in mixtures on both soils.

Adijailton Jose de Souza
Luiz de Queiroz College of Agriculture / University of São Paulo

Read the Original

This page is a summary of: Respiration induced by substrate and bacteria diversity after application of diuron, hexazinone, and sulfometuron-methyl alone and in mixture, Journal of Environmental Science and Health Part B, June 2019, Taylor & Francis, DOI: 10.1080/03601234.2019.1620043.
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