Dose-dependent responses of N2O emissions to biodegradable microplastics and warming in contrasting subtropical soils of Southwest China

Abstract

Microplastic pollution and climate change are increasingly recognized as key drivers of biogeochemical processes in soil ecosystems, however, their interactive effects on greenhouse gas (GHG) emissions remain poorly understood. Here, we conducted a 168-h laboratory incubation. We used two contrasting subtropical soils from the karst region of Southwest China: a natural Masson pine (Pinus massoniana) forest soil and an adjacent farmland soil converted from the same forest and continuously cultivated with chili (Capsicum annuum) for two decades. Treatments included biodegradable polylactic acid (PLA) microplastics at 0 %, 1 %, and 10 % (w/w) and two temperatures (25 and 30 °C). Incubations were run under oxic (20 % O₂) and anoxic (0 % O₂) conditions. Soil analyses and quantitative PCR conducted prior to incubation revealed that farmland soils had higher NO₃⁻ (+229 %) and NH₄⁺ (+45 %) concentration, enzymatic activity (β-1,4-N-acetyl-glucosaminidase: +16 %, β-1,4-glucosidase: +15 %), microbial Simpson diversity (+8 %), and lower DOC concentration (−64 %) compared to forest soils. Additionally, farmland soil had significantly higher abundances of denitrification-related functional genes (+2 %–7 %) and lower ammonia-oxidizing bacteria (amoA-AOB, −2 %). These changes potential strongly promoted cumulative CO₂ (+18 %) and N₂O (+384 %) emissions in farmland soils (CO₂: 133 ± 16 g C kg⁻¹, N₂O: 94 ± 19 mg N kg⁻¹) compared to natural forest. CO₂ emissions showed additive effects, with maximum CO₂ emissions under 10 % w/w PLA at 30 °C. Interestingly, a nonlinear response in N₂O emission was observed: emissions peaked at 1 % PLA (53 ± 11 mg N kg⁻¹) but declined at 10 % (16 ± 4 mg N kg⁻¹), suggesting a concentration threshold potentially due to microbial priming at low PLA and substrate limitation at higher doses. Our results state that forest-to-farmland conversion substantially increases CO₂ and N₂O emissions due to elevated inorganic nitrogen and denitrifier gene abundance. Moreover, we reveal a threshold effect of microplastics on N₂O fluxes and a synergistic effect of warming and microplastics on CO₂, but not N₂O, emission. These results highlight the need to consider land use history, pollutant concentration, and gas-specific microbial pathways when predicting soil GHG responses under future climate scenarios. However, these insights are based on short-term incubation, and further long-term studies are required to validate whether these patterns persist or shift over extended exposure.