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Effect of arsenic-contaminated irrigation water exposeure combined with conventional and biodegradable microplastics on arsenic fractionation in the paddy soil
Summary
When farmers irrigate rice fields with arsenic-contaminated water, plastic pollution in the soil makes things worse—and surprisingly, "biodegradable" plastic (PLA) may be more harmful than regular plastic in this case. High amounts of biodegradable plastic actually released more toxic arsenic into the soil and water than conventional plastic, and also reduced the diversity of helpful soil microbes. This matters because arsenic can build up in rice and enter our food supply, so switching to "eco-friendly" plastics isn't automatically safer—it depends on the specific pollution context, and this needs more attention
Microplastics (MPs) and arsenic (As)-contaminated irrigation water frequently coexist in agricultural soils, yet their relative contributions to As migration in the water-soil system and impacts on soil microbes remain poorly quantified-limiting accurate risk assessment of this emerging composite pollution. This study conducted a laboratory incubation experiment to investigate the effects of As-contaminated irrigation water and MPs (conventional PE vs biodegradable PLA, at 1% and 3% w/w) on pore water As concentration, soil As fractions, and soil microbial community structure. The results revealed that As-contaminated water irrigation only induced a marginal increase in pore water As concentration (T06:12.5 μg/L to T02:18.7 μg/L), whereas MPs type (T08:113.41 μg/L to T06:12.5 μg/L) was the dominant regulators of pore water As levels. Specifically, high-concentration (3%) MPs elevated pore water As by 2.8-4.5 folds compared to low-concentration (1%) MPs, with PLA inducing 1.9-2.3 folds higher As release than PE. For soil As fractions, As-contaminated water irrigation increased total soil As content, with a notable enrichment in exchangeable As; notably, PLA further promoted exchangeable As accumulation, which contrasts with previous studies focusing on PE-induced As immobilization. The irrigation water quality determines the total As input load, while MPs type primarily controls As speciation and mobility. For soil microbial communities, MPs impacts exhibited a clear concentration threshold: low-concentration (1%) MPs caused negligible shifts in community composition, whereas high-concentration (3%) MPs significantly reduced microbial alpha-diversity. These findings highlight that, given the continuous input of total As load from As-contaminated irrigation water, high-concentration PLA MPs play a significant role in regulating the speciation and mobility of soil As pollution. Therefore, they should be given particular attention in the risk management of agricultural soil As pollution, providing critical parameters for revising composite pollution risk assessment frameworks.