The size-variation of microplastics alters plant nitrogen uptake by regulating soil gross nitrogen transformation rates

Abstract

The accumulation of microplastics (MPs) has been shown to affect ecosystem nitrogen (N) cycling. However, direct quantitative studies investigating how variations in MPs type and size affect soil N processes and plant N uptake are still lacking. Thus, a¹⁵N tracing was carried out to investigate the effects of variations in size (i.e., 1200–1400, 600–700, 120–150, 25–38 μm) of conventional polyethylene (PE-MPs) and biodegradable polylactic acid MPs (PLA-MPs) on the interactions between soil gross N transformation and plant N uptake rates. Results showed that variations in MPs size affected plant N uptake through regulating soil gross N transformation rates, with significant differences between PE-MPs and PLA-MPs. For PE-MPs, plant N uptake was significantly higher under the medium size (i.e., 600–700 μm) than other sizes. This was mainly due to the significantly higher nitrification rate (13 mg N kg⁻¹ day⁻¹) at 600–700 μm compared to other sizes, in turn, increasing NO₃⁻ supply to plants. Furthermore, decreased NH₄⁺ immobilization (I_NH4) and increased dissimilatory NO₃⁻ reduction to NH₄⁺ in 600–700 μm PE-MPs resulted in increasing NH₄⁺ residence time, which ultimately could increase the available NH₄⁺ to plants. For PLA-MPs, plant N uptake was significantly reduced by small size (i.e., 25–38 μm). Despite N mineralization rate (M) in 25–38 μm was 6–11 times higher than other sizes, I_NH4 was much higher than M, resulting in a stimulation of NH₄⁺ residence time, in turn, inhibiting plant NH₄⁺ uptake. Lower autotrophic nitrification rate due to decreased gene abundances of nitrification, as well as much higher NO₃⁻ immobilization rate suppressed plant NO₃⁻ uptake in 25–38 μm PLA-MPs. Our findings highlight that the extent to which MPs influence N processes depends on their type and size. The small-size degradable MPs can pose a major threat to plants, so the managements of degradable plastics requires special attentions.