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 (O 2 ) 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 . O 2 diffusivity differences were determined in microcosm using an oxygen-free chamber. The O 2 concentration in the soil was also measured in optimal conditions for respiration. O 2 diffusivity and concentration were measured using optode imaging. Respiration was estimated from cumulative CO 2 and changes in the size of the water-extractable carbon pool. Adding MPs decreased O 2 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 O 2 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 O 2 dynamics. However, the MPs' impact on O 2 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 functions.

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