Lespedeza cuneata invasion history, rather than invader density, alters soil microbial community composition
Invasive plants that alter soil microbial communities can alter ecosystem function and generate plant-soil feedback leading to further plant community change. Lespedeza cuneata is an invasive legume American prairies and old fields, and dense stands of L. cuneata have distinctive soil microbial communities in comparison to uninvaded and lightly invaded habitats. These soil microbial changes may reflect density-dependent effects of L. cuneata, or they may be indicative of gradual microbial shifts in microbial community composition in sites with a history of L. cuneata invasion (and thus, a higher density of the invader). To evaluate whether invasion age or invasion density is a better predictor of soil microbial community composition, we used DNA fingerprinting (automated ribosomal intergenic spacer analysis) of soil bacterial and fungal communities in a restored prairie with a history of L. cuneata invasion. We used multivariate analyses to compare contemporary microbial community composition with contemporary L. cuneata biomass and with historical L. cuneata biomass from one to five years in the past. We additionally characterized microbial communities in the litter layer, in the upper 5 cm of soil, and at a depth of 5-15 cm in order to evaluate whether L. cuneata affects microbial communities through it litter or its roots.
L. cuneata invasion altered both bacterial and fungal communities. Variation in microbial community composition in 2011 was not significantly related to contemporary L. cuneata biomass, but rather it correlated with L. cuneata biomass in 2006, five years prior. Thus, areas with the longest history of L. cuneata presence also had the most distinctive microbial communities. Bacterial community composition responded to historical L. cuneata biomass in all litter and soil fractions, but fungal communities responded most strongly in the litter layer and upper 5 cm of the soil. We conclude that L. cuneata does not change microbial communities in a density-dependent fashion, but rather its effects on the soil community take time to accumulate. Fungi primarily respond to changes in litter composition and its resultant influence on the topsoil, while bacterial also respond to changes at depths associated with L. cuneata roots. These changes may establish soil legacies of L. cuneata invasion.