Long-term mulching of biodegradable plastic film decreased fungal necromass C with potential consequences for soil C storage

Discrepant soil microbial community and C cycling function responses to conventional and biodegradable microplastics

Biodegradable microplastics (MPs) are promising alternatives to conventional MPs and are of high global concern. However, their discrepant effects on soil microorganisms and functions are poorly understood. In this study, polyethylene (PE) and polylactic acid (PLA) MPs were selected to investigate the different effects on soil microbiome and C-cycling genes using high-throughput sequencing and real-time quantitative PCR, as well as the morphology and functional group changes of MPs, using scanning electron microscopy and Fourier transform infrared spectroscopy, and the driving factors were identified. The results showed that distinct taxa with potential for MP degradation and nitrogen cycling were enriched in soils with PLA and PE, respectively. PLA, smaller size (150-180 µm), and 5% (w/w) of MPs enhanced the network complexity compared with PE, larger size (250-300 µm), and 1% (w/w) of MPs, respectively. PLA increased β-glucosidase by up to 2.53 times, while PE (150-180 µm) reduced by 38.26-44.01% and PE (250-300 µm) increased by 19.00-22.51% at 30 days. Amylase was increased by up to 5.83 times by PLA (150-180 µm) but reduced by 40.26-62.96% by PLA (250-300 µm) and 16.11-43.92% by PE. The genes cbbL, cbhI, abfA, and Lac were enhanced by 37.16%- 1.99 times, 46.35%- 26.46 times, 8.41%- 69.04%, and 90.81%- 5.85 times by PLA except for PLA1B/5B at 30 days. These effects were associated with soil pH, NO3--N, and MP biodegradability. These findings systematically provide an understanding of the impact of biodegradable MPs on the potential for global climate change.

Long-term biogas slurry application increases microbial necromass but not plant lignin contribution to soil organic carbon in paddy soils as regulated by fungal community

Biogas slurry (BS) is widely considered as a source of organic matter and nutrients for improving soil organic carbon (SOC) sequestration and crop production in agroecosystems. Microbial necromass C (MNC) is considered one of the major precursors of SOC sequestration, which is regulated by soil microbial anabolism and catabolism. However, the microbial mechanisms through which BS application increases SOC accumulation in paddy soils have not yet been elucidated. A 12-year field experiment with four treatments (CK, no fertilizers; CF, chemical fertilizer application; BS1 and BS2, biogas slurry application at two nitrogen rates from BS) was conducted in rice paddy fields. The results showed that long-term BS application had no effect on lignin phenols proportion in SOC relative to CF. In contrast, BS application elevated the MNC contribution to SOC by 15.5-20.5 % compared with the CF treatment. The proportion of fungal necromass C (FNC) to SOC increased by 16.0 % under BS1 and by 25.8 % under BS2 compared with the CF treatment, while no significant difference in bacterial necromass C (BNC) contribution to SOC was observed between the BS and CF treatments. The MNC was more closely correlated with fungal community structures than with bacterial community structures. We further found that fungal genera, Mortierella and Ciliophora, mainly regulated the MNC, FNC and BNC accumulation. Collectively, our results highlighted that fungi play a vital role in SOC storage in paddy soils by regulating MNC formation and accumulation under long-term BS application.