Effect of emerging contaminants on soil microbial functioning: Consequences of metallic nanoparticles on N-cycling
Numerous emerging contaminants are identified every year in soil (nanoparticles, antibiotics, pesticides…), but their consequences on soil functioning are poorly documented. Titanium dioxide nanoparticles (TiO2-NPs) are commonly used in commercial products, like sunscreens, paints or papers. Along their life cycle from production to disposal, TiO2-NPs can be released in the environment, especially the soil, through various pathways such as agricultural amendments of sewage sludge or landfills. What is the response of soil microbial communities exposed to these new components? Does the toxicity is mitigated by soil characteristics, such as texture or organic matter (OM) content? Are there some microbial processes more sensitive than others to this disturbance?
To answer these questions, six contrasted agricultural soils were exposed in microcosms for 90 days to 1 and 500 mg TiO2-NPs kg-1 dry soil, representing an environmentally relevant concentration and an accidental spiking respectively. Soil respiration and activities of the nitrogen cycle (nitrification and denitrification) were measured to assess the effect of TiO2-NP on soil microbial functioning. Microbial abundance of nitrifying and denitrifying microbial communities were also determined by qPCR targeting functional genes (amoA, nirK, nirS). We used the Illumina sequencing platform (MiSeq) to explore the effects of TiO2-NPs on the diversity of bacterial and archaeal communities involve in nitrification: the ammonia-oxidizing bacteria and archaea (AOB and AOA, amoAgenes).
In most soils, TiO2-NPs did not affect the activity of microbial communities, except in a silty-clay soil (high OM content), where soil respiration, nitrification and denitrification were significantly decreased, even with the low NPs concentration. Our results suggest that TiO2-NPs toxicity does not depend on soil texture but likely on pH and OM content.
In the silty-clay soil, the nitrification rates were strongly reduced (-40%) due to the decrease of AOA (-56%), which seem to be the key driver of nitrification in this soil. A path analysis indicated that the mortality of AOA had cascading effects on nitrification and denitrification activities. The diversity analysis suggests that the higher sensitivity of the nitrification process to TiO2-NPs could be related to the low functional diversity of AOA in soil.
Our results highlight the detrimental effect of TiO2-NPs on soil functioning, especially on the nitrification process which is durably lowered. Moreover, we show that archaeal communities have a key functional role in soil and in contrast with the general view that they can be more sensitive to chronic stress conditions than bacteria.