Linking species with their effects on ecosystem processes: Can an invasive species’ density and relative abundance account for variability in soil net nitrification potential?

Background/Question/Methods

Plant species, notably invasive species, have the capacity to change ecosystem processes. While it is generally assumed that species’ effects on an ecosystem process scale linearly with the species’ abundance per area or as a function of the species’ proportion in its plant community, this assumption is rarely tested. Moreover, the identity of co-occurring plant species may modify how the ecosystem process scales with the target plant species by influencing pathways with which the target species’ alters the ecosystem process. The invasive grass, Microstegium vimineum, has been associated with elevated soil nitrate concentrations and net potential nitrification rates in observational and experimental studies. To determine how (1) M. vimineum density and (2) proportion of M. vimineum in simple plant communities scale with net potential nitrification rates and (3) the effect of co-occurring plant species on these relationships, we grew 0, 1, 2, 4, 5, and 6 M. vimineum individuals in greenhouse pots in the absence and presence of either Panicum virgatum, which naturally co-occurs with M. vimineum in its invasive range, or Sorghum bicolor, which produces nitrification-inhibiting compounds.

Results/Conclusions

We found no relationship between nitrification potential and M. vimineum density per pot when grown alone, with Panicum virgatum, or with Sorghum bicolor. No relationship was detected between the relative abundance of M. vimineum when grown with Panicum virgatum. Moreover, contrary to our expectation, a weak negative relationship was detected between M. vimineum relative abundance and nitrification potential when grown with Sorghum bicolor (p=0.02). These results suggest that factors other than M. vimineum density and relative abundance when grown with Panicum virgatum or Sorghum bicolor are important in determining changes in soil net nitrification potential. Future research should investigate forces that underlie variability M. vimineum’s influence on net potential nitrification rates so that we may better understand ecosystem-level changes observed in invaded areas.