Effects of a fungicide on oligochaetes and soil organic matter dynamics under controlled conditions
Résumé
Pesticides are used in agriculture to protect crops and ensure satisfying yields. However, they
can affect non-target organisms and the related ecological functions (e.g. organic matter
decomposition, soil structure). Earthworms (macrofauna > 2 mm in diameter) and enchytraeids
(mesofauna < 2 mm and > 0.2 mm in diameter) are Annelida Oligochaeta involved in organic
matter transformation. These organisms are considered as ecosystem engineers because they
modify environmental conditions for other organisms through their bioturbation activity. For
example, Huang and Xia (2018) had shown that earthworm mucus promoted microbial activity
that increased organic matter mineralization by specific bacteria. If this process has been quite
well studied for earthworms, much less is known about the effects of enchytraeids on soil
organic matter dynamics, and even less after pesticide applications.
We conducted a laboratory experiment with a natural soil to test the assumption that the
application of a commercial formulation of fungicide (Swing® Gold, composed of 50 g.l-1
epoxiconazole and 133 g.l-1 dimoxystrobin) would lead to direct effects on terrestrial
Oligochaeta (mortality, weight gain) and indirect effects on organic matter mineralization (CO2
release). We also tested whether these effects were in the same magnitude for earthworms and
enchytraeids. For that, we used different concentrations of fungicide (no fungicide, one, and
three times the recommended dose RD) and four conditions related to soil fauna (no fauna,
earthworms Aporrectodea caliginosa alone, enchytraeids Enchytraeus buchholzi alone, a mix of
earthworms and enchytraeids) during 26 days. To estimate the mineralization of organic matter,
CO2 release was measured at days 1, 5, 8, 12, 15, 19, 22 and 26. The total mineralization was
calculated on the entire experiment duration. For trapping CO2, NaOH was placed in a dish and
inserted in each microcosm for 24 hours, then titrated with HCl to determine the CO2 content.
No mortality was recorded at the end of the experiment, whatever the treatment. We found
no significant effect of the pesticide on animal weight gain. Organic matter mineralization
decreased with increasing concentration of pesticide until day 19. After this period, the contrary
occurred since the release of CO2 was higher in the treatments with the fungicide at 3 times the
RD than in treatments at the RD and without fungicide. This can be explained by some bacteria
species which can use pesticides as energy sources, thus enhancing their activities, such as
mineralization. Fauna assemblage had no effect on mineralization until day 22. Then, CO2
release was higher when earthworms or enchytraeids were present (no difference between these
treatments) than in the treatment without animals. The treatment involving earthworms and
enchytraeids had a middle level of mineralization. This could be explained by a competition
between earthworms and enchytraeids for food resources.
At the end of the experiment, the highest mineralization rate was recorded in the treatment
with earthworms and without pesticide. This result confirms the positive role of earthworms in
soil organic matter mineralization. We also found an effect of the interaction between the
pesticide and fauna treatments on mineralization at days 19 and 22. With no pesticide,
enchytraeids were less efficient than earthworms but, probably withstanding higher pesticide
contamination, they displayed a higher rate of mineralization than earthworms in conditions
with pesticide at 1 and 3 times the RD.
Domaines
Sciences du Vivant [q-bio]Origine | Fichiers éditeurs autorisés sur une archive ouverte |
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