Microplastics in Agricultural Soils: The Role of Soil Texture in Modulating Oxygen Diffusivity and Soil Respiration

Abstract

The presence of microplastics (MPs) in soils impacts nutrient cycling and soil respiration. However, the mechanisms underpinning the direction and magnitude of these effects on soil are uncertain. We hypothesized that the presence of MPs affects pore connectivity, leading to changes in oxygen (O₂) diffusivity and soil respiration. Furthermore, we anticipated that the magnitude of the effects would be dependent on both soil texture and MPs morphology. 1 % (w/w) PET MPs fibers (500 μm length) and fragments (125 – 250 μm) were spiked into rhizotrons filled with either clay or sandy loam soils. O₂ diffusivity differences were determined in microcosm using an oxygen-free chamber. The O₂ concentration in the soil was also measured in optimal conditions for respiration. O₂ diffusivity and concentration were measured using optode imaging. Respiration was estimated from cumulative CO₂ and changes in the size of the water-extractable carbon pool. Adding MPs decreased O₂ concentration in the sandy loam soil (167.4 ± 6.1 mg L^-1 air), with a greater reduction observed for fragments (15%) compared to fibers (12%). Soil respiration decreased by 40% in both fragment and fiber treatments in alignment with the reduction in oxygen concentration. Conversely, in the clay soil, the addition of fibers and fragments resulted in a 13 and 7% increase in O₂ concentration compared to the control (177.9 ± 3.8 mg L^-1 air). Both changes in oxygen concentration and diffusivity, show a similar response to MPs for the two soils. These findings indicate that the effects of MPs on soil respiration are likely driven by changes in O₂ dynamics. However, the MPs' impact on O₂ dynamics depends on soil particle size distribution. Future research should consider MP size, morphology, and soil particle distribution interactions to assess MPs' impacts on soil function.