Frequent Fire Reduces the Magnitude of Positive Interactions Between an Invasive Grass and Soil Microbes in Temperate Forests

Fire activity is increasing in many regions. Although increased fire activity is expected to promote plant invasion, over longer time periods, frequent fire can shift the nutrient status of ecosystems, which may alter interactions between invasive plants and soil microbial decomposers. Here, we applied a fire treatment to plots embedded in deciduous forests under regimes of either fire exclusion or frequent fire and invaded by the most widespread invasive grass in the eastern USA (Microstegium vimineum) to determine how frequent fire affects plant–soil interactions while controlling for time after fire. We predicted that frequent fire would increase microbial nitrogen (N) limitation, leading to lower Microstegium productivity and reduced soil carbon (C) and N loss. We found that Microstegium leaf, root, and microbial biomass C/N ratios were significantly wider under frequent fire than fire exclusion. Consistent with these patterns, dissolved organic N concentration was 22% lower and the activity of an exoenzyme targeting N acquisition was 59% higher under frequent fire, indicating that frequent fire increased microbial N limitation. Higher surface soil C and N and fire-induced increases in particulate soil organic matter N suggest that frequent fire in these grass-invaded forests enhanced microbial N limitation through the accumulation of microbially resistant pyrogenic N. A structural equation model confirmed that these changes were interrelated and associated with 87% lower Microstegium aboveground biomass and 85% higher fine root biomass. These findings provide new insights into how frequent fire impacts plant–soil microbial interactions and thus may feedback to plant invasion over longer time periods.