• This study discovered the direct responses of soil fauna to acid deposition.

Excessive acid deposition causes soil acidification and changes the soil microhabitat, affecting the survival and reproduction of soil organisms. Folsomia candida (Collembola, Isotomidae) is used internationally as a model organism for assessing chemical toxicity in soil and it is feasible to use its avoidance response as an indicator of environmental changes as well. In this study, we used Folsomia candida avoidance behavior to assess the risks of acid deposition on soil ecosystems. Different pH (3.0, 3.5, 4.0, 4.5, 5.0, and 5.5) treatments were set up in petri dish experiments, and the avoidance behavior of Folsomia candida was measured after 12, 24, and 48 h of exposure to the pH conditions. The results indicated that (1) both the exposure duration and pH level influenced collembolan avoidance behavior. (2) After 12 h exposure, most of the insects showed avoidance behavior but without significant differences among the treatment conditions. (3) After 24 h exposure, significant avoidance behavior was observed at pH 3.0, 3.5, and 4.0. (4) After 48 h exposure, avoidance behavior was seen in all treatment conditions except for pH 5.5. This study described the direct responses of soil fauna to acid deposition and indicated that both pH and length of exposure interacted to influence the avoidance behavior of Folsomia candida. During the experimental period, the insects reacted negatively and show consistent avoidance behavior at pH 3.0, 3.5, and 4.0. Reversed avoidance behavior was apparent between pH 4.5 and 5.0 and not observed at pH 5.5, indicating that the latter was the preferred pH environment.