Abstract
The objective of our study was to test whether limited microbial degradation at low pesticide concentrations could explain the discrepancy between overall degradability demonstrated in laboratory tests and their actual persistence in the environment. Studies on pesticide degradation are often performed using unrealistically high application rates seldom found in natural environments. Nevertheless, biodegradation rates determined for higher pesticide doses cannot necessarily be extrapolated to lower concentrations. In this context, we wanted to (i) compare the kinetics of pesticide degradation at different concentrations in arable land and (ii) clarify whether there is a concentration threshold below which the expression of the functional genes involved in the degradation pathway is inhibited without further pesticide degradation taking place. We set up an incubation experiment for four weeks using 14C-ring labeled 2-methyl-4-chlorophenoxyacetic acid (MCPA) as a model compound in concentrations from 30 to 20,000 μg kg–1 soil. To quantify the abundance of putative microorganisms involved in MCPA degradation and their degradation activity, tfdA gene copy numbers (DNA) and transcripts (mRNA) were determined by quantitative real-time PCR. Mineralization dynamics of MCPA derived-C were analyzed by monitoring 14CO2 production and 14C assimilation by soil microorganisms. We identified two different concentration thresholds for growth and activity with respect to MCPA degradation using tfdA gene and mRNA transcript abundance as growth and activity indices, respectively. The tfdA gene expression started to increase between 1,000 and 5,000 μg MCPA kg–1 dry soil, but an actual increase in tfdA sequences could only be determined at a concentration of 20,000 μg. Accordingly, we observed a clear shift from catabolic to anabolic utilization of MCPA-derived C in the concentration range of 1,000 to 5,000 μg kg–1. Concentrations ≥1,000 μg kg–1 were mainly associated with delayed mineralization, while concentrations ≤1,000 μg kg–1 showed rapid absolute dissipation. The persistence of pesticides at low concentrations cannot, therefore, be explained by the absence of functional gene expression. Nevertheless, significant differences in the degradation kinetics of MCPA between low and high pesticide concentrations illustrate the need for studies investigating pesticide degradation at environmentally relevant concentrations.
Highlights
Pesticide application is the dominant pest control method utilized by farmers in Germany, with an average application rate of 2.8 kg ha−1 yr−1 on arable land (Baumgarten et al, 2008)
The persistence of pesticides at environmentally low concentrations has been rarely investigated, their degradation kinetics may substantially differ from degradation at high concentrations (Fomsgaard, 1997)
This study focused on the biodegradation of gradually increasing pesticide concentrations using a combination of 14C isotope analysis to model degradation kinetics, analysis of functional genes, and transcript abundance
Summary
Pesticide application is the dominant pest control method utilized by farmers in Germany, with an average application rate of 2.8 kg ha−1 yr−1 on arable land (Baumgarten et al, 2008). A considerable fraction ends up in the soil, where filtering functions, such as immobilization by clay minerals and organic matter (Sun et al, 2010), chemical and microbial degradation provide important ecosystem services for groundwater and surface water protection (Keesstra et al, 2012). Despite pesticide biodegradation, which is considered as the most important degradation pathway (Nowak et al, 2011), remnants of multiple pesticides (including their metabolites), persist at low but detectable and environmentally relevant concentrations in soils. While 25% of soils contained specific pesticide residues, 58% contained a mixture of several pesticides in medium and maximum concentrations of 0.02 to 0.04 mg kg−1 and 0.31 to 0.41 mg kg−1, respectively (Silva et al, 2019)
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