that acid addition substantially altered the soil microbial community (Fig. 3a–c). Acid addition significantly decreased total FAs and bacterial FAs but increased fungal FAs and fungi/bacteria ratio (Figs. 3a–c and Fig. S3). Acid addition significantly decreased the activity of four extracellular enzymes (xylosidase, cellobiohydrolase, acid phosphatases, and leucine aminopeptidase), increased the activity of β-N-acetylglucosaminidase, and did not change the activity of a-glucosidase or β-1,4-glucosidase (Fig. S4). Acid addition also decreased the PC1 of the seven extracellular enzyme activities (EEA) (Fig. 3d). According to regression analyses, BB/AB, PR, and PR/PB increased and AB and PB decreased with increasing acid addition rate (Fig. 3e–h)