Mixed litter and incubation sites drive non-additive responses in seed germination and seedling growth of lettuce

Litter decomposition is a crucial biochemical process in terrestrial ecosystems. Litter mixture consisting of litter from multiple plant species may have non-additive decomposition rates relative to litter of single-species. The litter-mixing effect (LME) on the decomposition process in plants has been explored widely. Allelopathic effects from leaf litter of invasive plants may suppress growth in neighbouring species. Yet, few studies have examined the association between the effects of allelopathy on plant invasions and changes in litter decomposition. To test the relationship between allelopathy and LME during litter decomposition, we conducted both field and greenhouse experiments. The allelopathic effect of invasive Solidago canadensis and native Pterocypsela laciniata was tested on lettuce (Lactuca sativa) using litter extracts. We found that overall invasive litter had a strong allelopathic effect before litter decomposition compared with native litter. Litter extracts in the mixed and single treatments showed both additive and non-additive effects on seed germination and seedling growth in lettuce. Invasive and native litter significantly increased total inorganic nitrogen content (NO3--N and NH4+-N), and decreased soil pH value, total phenolics, and total flavonoids after litter decomposition under field conditions. Additionally, the effect of litter types on germination and growth varied between the field and the greenhouse. These findings demonstrate a general relationship between plant performance indices and non-additive responses to allelopathy and suggest that the incubation site may be an important factor in successful invasions.