Nitrogen Fertilization Alleviates Microplastic Effects on Soil Protist Communities and Rape (Brassica napus L.) Growth

Agricultural plastic mulch enhances crop yields but leads to persistent microplastic contamination in soils. Concurrently, nitrogen (N) fertilization and atmospheric deposition profoundly reshape microbial ecosystems. This study examined the individual and interactive effects of polyethylene microplastics (PE, 1% <i>w</i>/<i>w</i>) and nitrogen addition (N, 180 kg ha^-1 yr^-1) on soil protist communities and rape (<i>Brassica napus</i> L.) productivity. High-throughput sequencing and soil-plant trait analyses revealed that PE alone reduced the soil water retention and the rape biomass while elevating the soil total carbon content, C/N ratios, and NH₄⁺-N/NO₃^--N levels. Conversely, N addition significantly boosted the rape biomass and the chlorophyll content, likely through enhanced nutrient availability. Strikingly, the combined PE_N treatment exhibited antagonistic interactions; protist diversity and functional group composition stabilized to resemble the control conditions, and the rape biomass under the PE_N treatment showed no difference from the CK (with basal fertilizer only), despite significant reductions under the PE treatment alone. Soil nutrient dynamics (e.g., the SWC and the C/N ratio) and the protist community structure collectively explained 96% of the biomass variation. These findings highlight the potential of nitrogen fertilization to mitigate microplastic-induced soil degradation, offering a pragmatic strategy to stabilize crop productivity in contaminated agricultural systems. This study underscores the importance of balancing nutrient management with pollution control to sustain soil health under global microplastic and nitrogen deposition pressures.