Low-density polyethylene microplastics and biochar interactively affect greenhouse gas emissions and microbial community structure and function in paddy soil

Ecological effect of microplastics on soil microbe-driven carbon circulation and greenhouse gas emission: A review

Soil organic carbon (SOC) pool, the largest part of terrestrial ecosystem, controls global terrestrial carbon balance and consequently presented carbon cycle-climate feedback in climate projections. Microplastics, (MPs, <5 mm) as common pollutants in soil ecosystems, have an obvious impact on soil-borne carbon circulation by affecting soil microbial processes, which play a central role in regulating SOC conversion. In this review, we initially presented the sources, properties and ecological risks of MPs in soil ecosystem, and then the differentiated effects of MPs on the component of SOC, including dissolved organic carbon, soil microbial biomass carbon and easily oxidized organic carbon varying with the types and concentrations of MPs, the soil types, etc. As research turns into a broader perspective, greenhouse gas emissions dominated by the mineralization of SOC coming into view since it can be significantly affected by MPs and is closely associated with soil microbial respiration. The pathways of MPs impacting soil microbes-driven carbon conversion include changing microbial community structure and composition, the functional enzyme's activity and the abundance and expression of functional genes. However, numerous uncertainties still exist regarding the microbial mechanisms in the deeper biochemical process. More comprehensive studies are necessary to explore the affected footprint and provide guidance for finding the evaluation criterion of MPs affecting climate change.

Global warming potential assessment under reclaimed water and livestock wastewater irrigation coupled with co-application of inhibitors and biochar

The application of nitrification inhibitors (nitrapyrin) and urease inhibitors (N-(N-butyl) thiophosphoric triamide) under conventional water resources has been considered as an effective means to improve nitrogen utilization efficiency and mitigate soil greenhouse gas emissions. However, it is not known whether the inhibitors still have an inhibitory effect under unconventional water resources (reclaimed water and livestock wastewater) irrigation and whether their use in combination with biochar improves the mitigation effect. Therefore, unconventional water resources were used for irrigation, with groundwater (GW) control. Nitrapyrin and N-(N-butyl) thiophosphoric triamide were used alone or in combination with biochar in a pot experiment, and CO2, N2O, and CH4 emissions were measured. The results showed that irrigation of unconventional water resources exacerbated global warming potential (GWP). All exogenous substance treatments increased CO2 and CH4 emissions and suppressed N2O emissions, independent of the type of water, compared to no substances (NS). The inhibitors were ineffective in reducing the GWP whether or not in combination with biochar, and the combined application of inhibitors with biochar further increased the GWP. This study suggests that using inhibitors and biochar in combination to regulate the greenhouse effect under unconventional water resources irrigation should be done with caution.

Discrepant responses of bacterial community and enzyme activities to conventional and biodegradable microplastics in paddy soil

The prevalence of microplastics in soil ecosystems has raised concerns about their potential effects on soil properties. As promising alternatives to conventional plastics, biodegradable plastics have been increasingly applied in agricultural activities, which may release microplastics into the soil due to incomplete degradation. Compared to conventional microplastics, biodegradable microplastics in soil may induce different impacts on soil microbial properties, which have yet to be well studied. Through a 41-day microcosm experiment, we evaluated the differential effects of conventional (polypropylene, PP) and biodegradable (polylactic acid, PLA) microplastics on the geochemical properties, enzyme activities, and microbial community structure in paddy soil. Adding PLA or PP microplastics into soil significantly increased pH values and altered the contents of carbon and nitrogen nutrients. Exposure to microplastics significantly increased the activity of fluorescein diacetate hydrolase, but had varying effects on the activities of urease, sucrase, and alkaline phosphatase depending on microplastic types and doses. The addition of microplastics also influenced the structure of soil bacterial community, with Proteobacteria, Actinobacteriota, and Acidobacteriota being the dominant phyla. Significant differences in the genera of Pseudarthrobacter, Acidothermus, Bacillus, Aquisphaera, and Massilia were observed between treatments. Results of structural equation modeling (SEM) demonstrated that changes in soil carbon and nitrogen nutrients and pH values positively affected the bacterial community, while soil bacterial community as a whole exerted a negative impact on enzyme activities. FAPRPTAX analysis showed that the addition of microplastics altered the relative abundances of functional genes related to the metabolism of cellulose decomposition and ureolysis in paddy soil. Findings of this study clearly suggest that microplastic impacts on soil geochemical and microbial properties should be an integral part of future risk assessment and that to evaluate microplastic impacts, both the concentration and polymer type must be taken into account.