Loss of microbial diversity increases methane emissions and arsenic release in paddy soils

Microorganisms are vital to the emission of greenhouse gases and transforming pollutants in paddy soils. However, the impact of microbial diversity loss on anaerobic methane (CH₄) oxidation and arsenic (As) reduction under flooded conditions remains unclear. In this study, we inoculated microbial suspensions into natural As-contaminated paddy soils using a dilution approach (untreated, 10^-2, 10^-4, 10^-6, 10^-8 dilutions) to manipulate microbial diversity levels. The results revealed that the 10^-4 and 10^-6 dilutions resulted in the highest CH₄ emissions (97.0 μmol and 102.3 μmol) compared to untreated groups (27.6 μmol). However, anaerobic CH₄ oxidation was not observed in 10^-4 dilution groups and higher dilutions, suggesting the loss of diversity inhibited the natural reduction of CH₄. Moreover, the porewater As concentration in the dilution groups was 1.8-8.2 times greater than in the untreated groups. The loss of microbial diversity promoted the reductive dissolution of iron (Fe) minerals bearing As, leading to increased concentrations of Fe(II) and dissolved organic carbon (DOC), which further enhanced As release (Fe(II), R = 0.9, p < 0.001) (DOC, R = 0.8, p < 0.001) from soil to porewater. However, CH₄-dependent As(V) reduction was almost entirely inhibited under diversity loss. The decline in microbial diversity increased the relative abundances of methanogens (e.g., Methanobacterium and Methanomassiliicoccus), Fe(III)/As(V)-reducing bacteria (e.g., Bacillus, Clostridium_sensu_stricto_10, and Geobacter), and the related functional genes (i.e., mcrA and Geo). These findings suggest that microbial diversity is critical for specialized soil processes, highlighting the detrimental effects of biodiversity loss on CH₄ emissions and As release in As-contaminated paddies.