Comparative fungal responses in managed plant communities infested by spotted (Centaurea maculosa Lam.) and diffuse (C. diffusa Lam.) knapweed

Spotted knapweed (Centaurea maculosa Lam.) and diffuse knapweed (Centaurea diffusa Lam.) infested plant communities that are being studied for improved management practices to enhance native perennial grass species were evaluated for selected fungal responses during 2000-2003. Replicated plots were treated using combinations of carbohydrate addition to reduce soil N availability, reseeding with desirable mid- and late-seral plant species, and reintroducing a native late-seral soil microbial community using whole soil inoculum. The responses of extraradical filamentous fungi, mycorrhizae (infection intensity, arbuscules and vesicles) and systemic endophytic fungal (SEF) of native perennial grasses were evaluated in relation to plant community relative biomass, litter mass and soil C:N ratio in treated versus replicated untreated control plots. Soil filamentous fungal and plant community changes occurred with combined treatments (sucrose, seeding, soil inoculum) and were most evident in the diffuse knapweed-invaded sites during 2003. Based on overall correlations for the 2000-2003 experimental period, total and active filamentous fungi were inversely related to the relative biomass of invasive plants, particularly of spotted knapweed. The native perennial grasses had lower mycorrhizal infection intensity at the spotted knapweed site, in comparison with the diffuse knapweed infested site, where soil inoculation also appeared to be related to higher SEF and mycorrhizal infection occurrence. These results suggest that a mycocentric approach, involving soil management to maintain filamentous fungal hyphal lengths, while decreasing active fungal development, may create a soil environment more conducive to maintenance of desired indigenous plant species.