Interactive Effects of Microplastic Pollution and Global Warming on Soil Carbon and Nitrogen Dynamics in Subtropical Forests

Microplastics (MPs) have become ubiquitous in terrestrial ecosystems, while the responses of biogeochemical cycles in forest soils to MPs remain poorly understood. This study aimed to explore the potential disturbances of MPs on soil carbon and nitrogen cycling and their associations with soil chemical properties under different MP input treatments in a global warming trend.

Surface soil samples (0-10 cm) were collected from a subtropical forest in Hong Kong and incubated with four MP types (PE, PP, PLA, and PBAT), three concentrations (0% as controls, 1%, and 5%), and two temperatures (20℃ and 30℃) for three months. Key chemical properties (e.g., total carbon (TC), total nitrogen (TN), soil organic carbon (SOC), dissolved total carbon (DTC), nitrate nitrogen (NO₃-N), ammonium nitrogen (NH₄-N), etc.) and cumulative greenhouse gas emissions (CO₂, CH₄) were measured. After that, three-way ANOVA was used to analyse the main and interactive effects of MP types, concentrations, and temperatures on soil properties, while Spearman’s correlation analysis was applied to explore the associations between soil properties. Also, redundancy analysis (RDA) was used to understand the synergistic relationships of soil property changes defined by key driving factors.

Preliminary RDA analysis revealed that temperature and concentration jointly explained approximately one-third of the total variation in soil chemical properties, with temperature being the dominant driver. However, MP types alone did not significantly structure the overall property matrix, three-way ANOVA revealed significant interactive effects. It indicated that MP types could interact with either temperature or concentration to significantly affect key processes such as NO₃-N content and cumulative CO₂ emission. Spearman’ s correlation analysis also illustrated that these interactions were triggered by a temperature-dependent shift in carbon-nitrogen coupling. At 20 ℃, cumulative CO₂ emission was strongly negatively correlated with NO₃-N, whereas at 30℃ it became positively linked with NH₄-N and DTC, suggesting a shift toward a more rapid, tightly coupled mineralization pathway under warmer conditions. Notably, the temperature sensitivity (Q10) of soil respiration was altered by MP addition, certain polymers (e.g., PBAT) exhibited higher Q10 values than the control, indicating an amplified respiratory response to warming.

In conclusion, the combination of different statistical analysis methods suggests that MPs may disturb the key carbon and nitrogen cycling of subtropical forest soil not merely by changing the content of soil properties, but also by modifying the system’ s temperature sensitivity and by differentially occurring certain metabolic pathways at specific temperatures. This work aligns with the SSS7 focus on anthropogenic influences on soil systems and supports further research on MP-microbe-climate feedbacks.