Fig. 4.

The metagenomic abundance ratios of methanogens to bacteria across the experimental treatments (CK, 0.5% NPs, 5% NPs) and incubation times (30 days, 160 days) at the phylum and family levels (a) and their correlations with methane production in black and red soils (b). Error bars in the column plots denote standard deviation (n = 3). Letters represent significant differences between treatments (a, b, c for 30 days; A, B, C for 160 days). Significance levels were as follows: *P < 0.05, **P < 0.01, and ***P < 0.001