Ecotoxicological Impact of the Bioherbicide Leptospermone on the Microbial Community of Two Arable Soils

Abstract

The ecotoxicological impact of leptospermone, a β-triketone bioherbicide, on the bacterial community of two arable soils was investigated. Soil microcosms were exposed to 0 × (control), 1 × or 10 × recommended dose of leptospermone. The β-triketone was moderately adsorbed to both soils (i.e.,: K fa ~ 1.2 and K oc ~ 140 mL g(-1)). Its dissipation was lower in sterilized than in unsterilized soils suggesting that it was mainly influenced by biotic factors. Within 45 days, leptospermone disappeared almost entirely from one of the two soils (i.e., DT50 < 10 days), while 25% remained in the other. The composition of the microbial community assessed by qPCR targeting 11 microbial groups was found to be significantly modified in soil microcosms exposed to leptospermone. Pyrosequencing of 16S rRNA gene amplicons showed a shift in the bacterial community structure and a significant impact of leptospermone on the diversity of the soil bacterial community. Changes in the composition, and in the α- and β-diversity of microbial community were transient in the soil able to fully dissipate the leptospermone, but were persistent in the soil where β-triketone remained. To conclude the bacterial community of the two soils was sensitive to leptospermone and its resilience was observed only when leptospermone was fully dissipated.

Keywords: bacterial community; biodegradation; bioherbicide; leptospermone; microbial ecotoxicology.

Figure 1

Dissipation kinetics of 1 × recommended rate leptospermone (D1; 5 μg g⁻¹) in sterilized (●) and unsterilized soils (■) from (A) P and (B) SJF, and 10 × the recommended rate (D10; 50 μg g⁻¹) in the soil from (C) P and (D) SJF. Standard deviations are indicated (n = 3).

Figure 2

UniFrac analysis of the effect of leptospermone applied at different concentrations (D0, D1, and D10) on the bacterial community composition of P and SJF soils at 4 and 45 days (d4, d45). The first three axes of the PCoA of the weighted UniFrac distance matrix of 16S rRNA amplicon pyrosequencing are shown. The percent of variance explained by each axis is given. For both soils, treatments are as follows: at 4 days for D0, D1, and D10 (gray, cyan, and dark violet), and at 45 days for D0, D1, and D10 (brown, blue and red).

Figure 3

Phylogenetic relationships and distribution of 16S rRNA OTUs observed in P and SJF soils at 4 days (d4). The maximum likelihood phylogeny consists of representative nucleotide sequences for OTUs. Node confidence (n = 1000 bootstrap replicates) between 80 and 100% is shown by black dot. The affiliation of the represented OTUs to the main microbial groups (at phylum or class level) is indicated by different colors on the internal ring. Relative abundances of each OTU represented by bar plots are expressed as a proportion of the maximum abundance detected in each treatment, and indicated for P and SJF soils as follows (from inside to outside of the circle): gray, cyan, and dark violet for D0, D1, and D10, respectively.

Figure 4

Relative abundance of bacterial phylum and Proteobacteria classes (expressed as % of the total number of OTUs) detected in the P and SJF soils at 4 and 45 days.