Litter decomposition is commonly thought to be primarily controlled by a combination of climate and litter chemistry (C:N, N:lignin, polyphenol concentrations).  However, changes in the soil microbial community may also impact decomposition rates, as suggest by litter transplant experiments.  When plant communities change in species composition, e.g., through natural succession or biological invasions, litter decomposition of a new species takes place on soil conditioned by the litter of the former species.  Such litter-induced shifts in microbial function could contribute to a potential plant-soil feedback loop, where litter chemistry influences soil microbes, which in turn impact litter decomposition. We examined the effects of soil conditioning by litter of one species on the decomposition of its own and different species’ litter in the context of exotic plant invasions.  We constructed microcosms in pots of soil collected from uninvaded deciduous forests, and conditioned them with litter from one of four species (two common co-occurring invasive species, Berberis thunbergii and Microstegium vimineum, and two common co-occurring native species, Viburnum acerifolium and Vaccinium corymbosum) for 18 months. After the conditioning period, a subsample of pots was destructively sampled to determine microbial community structure.  In half of the remaining pots, litter was replaced with the same litter type, while the other half received litter of a different species.  This litter decomposed for 7 months, then collected and mass loss was calculated.

Results/Conclusions

Pre-conditioning of the soil significantly influenced the rate of subsequent litter decomposition (p<0.01), but not in the same way for all species (significant pretreatment X species interaction, p<0.001).  Litter of the three shrub species decomposed significantly slower after preconditioning with the same litter type (p<0.001), while the fourth species (M. vimineum, a grass) decomposed significantly faster after preconditioning under its own litter (p<0.001). These effects on decomposition corresponded to changes in the phospholipid fatty acid (PLFA) profile of the soil microbial community as well. The results suggest that 1) negative and positive plant-soil feedback acts through litter decomposition and soil microbial communities, 2) there is no consistent difference between native and exotic species decomposing on the same soil, and 3) feedback processes generated by the grass, an unusual plant form in forests, may be different from those generated by woody shrubs, a common plant form in forests.  In conclusion, changes in plant communities that result from both natural succession and exotic species invasions will have legacy effects on decomposition and soil microbial ecology.