Influence of biochar and microplastics on microbial necromass accumulation and CO2 and N2O emission in a Calcaric Fluvisol

Biochar application represents a promising soil management strategy for mitigating CO₂ and N₂O emissions, yet microplastics, as concurrent soil pollutants, may interact with biochar. In a 91-day microcosm experiment (25 ºC, 60 % of WHC), we investigated soil C and N dynamics following the addition of polyethylene microplastics (1 and 5% (w/w)) to a Calcaric Fluvisol already amended with biochar for one month. Biochar alone reduced CO₂ and N₂O emissions by 11% and 3%, while polyethylene reduced CO₂ and N₂O emission by 11‒26% and 4‒14%, respectively. The suppression of CO₂ emissions by biochar and polyethylene was due to reduced soil aggregation and dissolved organic matter (DOM) content, as well as increased DOM aromaticity, all of which collectively led to diminished bacterial biomass and β-N-acetyl-glucosaminidase activity. Biochar decreased N₂O emissions by suppressing <i>nirS</i> and <i>nirK</i> genes while increasing <i>nifH</i> gene; polyethylene decreased N₂O emission primarily by lowering <i>nirK</i> gene. Biochar alone resulted in a 34.6% decrease in microbial necromass carbon (MNC), primarily due to the suppression of bacterial abundance, thus leading to reduced efficiency in bacterial MNC production. On the other hand, polyethylene microplastics had a modest impact, slightly decreasing total MNC by 7.8‒11.0% in control soil, mainly due to dilution effects. However, in biochar-treated soil, polyethylene exerted a profound influence, markedly increasing MNC by 33.1‒60.8%. This enhancement can be attributed to the disruption of soil aggregation, which provided better protection for MNC, and a reduction in β-N-acetyl-glucosaminidase activity, responsible for MNC decomposition. In summary, our findings reveal the interactive effects of biochar and polyethylene microplastics on MNC accumulation, as well as CO₂ and N₂O emissions. These insights shed light on the resilience and carbon sequestration potential of biochar-treated soil ecosystems in the face of environmental disturbances.