Short-term microplastic addition modifies the impacts of Solidago canadensis L. invasion on soil faunal communities

Plant invasions and microplastic (MP) pollution are widespread environmental challenges with increasing impacts on soil ecosystems. Although their individual effects on soil faunal diversity are well-documented, the interactive effects of these co-occurring stressors remain poorly understood. We investigated the short-term (two months) responses of soil faunal communities to conventional (low-density polyethylene, LDPE) and biodegradable (polylactic acid, PLA) MPs in habitats with and without invasion by Solidago canadensis L. We found that invasion by S. canadensis significantly increased overall abundance and richness of soil fauna, particularly microbivores. However, MP addition—either alone or in combination with plant invasion—did not significantly affect the abundance or richness of the total soil fauna or most trophic groups. Structural equation modeling revealed that MPs not only attenuated the positive effect of invasion on overall abundance by 48.4%, but also reversed its effect on richness from positive to a net decrease of 120.6%. Moreover, while S. canadensis invasion significantly altered soil faunal community composition in plots without MPs, this effect disappeared in most MP-amended treatments. These findings suggest that short-term MP pollution may alter the impact of S. canadensis invasion on soil fauna. However, the magnitude of MP effects varied across trophic groups, with microbivores exhibiting the most notable response. This study underscores the necessity of considering interactions among co-occurring environmental stressors in soil biodiversity assessments, as their combined effects can reshape outcomes for key functional groups within ecosystems.