Disentangling microplastics effects on soil structure, microbial activity and greenhouse gas emissions

There is an expected increase of microplastics (MPs) concentrations in terrestrial ecosystems in the next decades from a variety of sources. Understanding the responses of soil ecosystems to the presence of MPs becomes increasingly important as multiple stressors can act together to negatively impact this environmental compartment, especially in the context of global warming. It has already been shown that MPs (particularly fibers) can influence several parameters of soil structure and function, including aggregate formation, water holding capacity and microbial activity. Furthermore, recent studies suggest that the presence of MPs in soils affects the emissions of the greenhouse gases (GHG) carbon dioxide (CO₂) and nitrous oxide (N₂O). The mechanisms underpinning the direction and magnitude of MPs effects on GHG emissions from soils are uncertain, mainly due to the lack of knowledge of how the presence of MPs drives changes in soil structure and the subsequent link between soil structure and microbial activity. Here, we hypothesized that the presence of MPs affects soil structure by increasing porosity, leading to higher O₂ availability and consequently higher decomposition of soil organic matter (SOM) and lower denitrification activity. In this study, we spiked MPs of different polymers (PET, PLA), morphologies (fragments, fibers) and sizes to a custom built rhizotron (7 x 4 x 1 cm) filled will a clay soil with a MP treatment of 5 w/w%. The soil was initially sterilized, but we added microbial inoculum collected from the same soil and glucose (as a substrate source) in known concentrations to assess soil respiration over time. We determined the spatial distribution of microbial respiration by mapping O₂ concentrations using optode imaging, with a resolution of 1 image every 10 minutes over the course of 48 hours. Soil pore size, pore distribution and the pore connectivity network were determined by using X-ray micro-tomography (µCT). GHG emissions were measured by placing replicate set-ups in a Tedlar bag and collecting CO₂ and N₂O from the headspace in exetainers to be analyzed by gas chromatography. This approach allowed us to collect real-time O₂ distribution and compare this to X-ray micro-tomography (µCT) data from the same soil matrix to assess MPs impacts to the soil structure and link it to GHG emissions compared to the control (i.e. no MPs addition). Collectively, in this presentation we will discuss the impacts of MPs addition to soil and on the linkages between soil structure, microbial activity and GHG emissions. This study can serve as a baseline for understanding the important impacts of MPs to soil functioning, which is particularly relevant as plastics are increasingly used directly in agriculture and can have direct releases to the terrestrial ecosystem.