The impact of garlic mustard on sandy forest soils

Garlic mustard (Alliaria petiolata), an exotic biennial herb invasive to North American forests, has potential to affect resource availability in invaded soils. Garlic mustard produces a suite of toxic chemicals that impact diversity both above and belowground and thus likely alter ecosystem processes. To examine the effects of garlic mustard on soil biota and ecosystem processes, we sampled soil from invaded and uninvaded stands at a pine plantation on sandy soils in central Illinois. Several of the pine stands were also planted with the N2-fixing tree black locust (Robinia pseudoacacia). Results indicate that pine soils underlying garlic mustard have higher pH, and exhibit higher rates of N mineralization, relative nitrification and soil respiration compared to soils without garlic mustard. Nitrogen turnover on pine invaded stands is more similar to pine soils with the black locust trees present than to uninvaded pine stands without black locust. Garlic mustard cover was much greater in stands with black locust trees planted, suggesting that garlic mustard may be attracted to high N sites. Catabolic response profiles, which provide a measure of soil microbial function, indicated a shift in substrate use for one of the substrates tested in the presence of garlic mustard. While many of the soil characteristics measured did not differ between invaded and uninvaded stands, the differences with regard to N turnover were striking and will likely have long term effects on soil nutrient status, with potential to feedback to forest health.

► We examined the effects of garlic mustard invasion on sandy forest soils. ► Garlic mustard soils had higher pH and lower bulk density than uninvaded soils. ► C and N mineralization and relative nitrification were higher in garlic mustard soils. ► Microbial activity on garlic mustard soils is stimulated by non-generalist substrates. ► Garlic mustard may impact ecosystems long-term through impacts to soil nutrients.