Soil biota in an ungrazed grassland: response to annual grass (Bromus tectorum) invasion

Belnap, Jayne; Phillips, Susan

Bromus tectorum is an exotic annual grass that currently dominates many western U.S. semi-arid ecosystems. The effects of this grass on ecosystems in general and soil biota specifically are unknown. This study compared the soil food-web structure of two native grassland associations (Stipa and Hilaria) with and without the presence of Bromus tectorum. The study was conducted in Canyonlands National Park in southeastern Utah. Data was collected from three grassland sites: Virginia Park, Chesler Park and Squaw Flat. Sites were sampled in spring 1996 for vascular and non-vascular vegetation, soil chemistry, bacteria, soil fungi, protozoa and invertebrates. Live and dead plant-infecting fungi were sampled in spring and fall 1997. Soil biota in Hilaria-Bromus generally responded to the Bromus invasion in an opposite manner than in Stipa-Bromus, e.g. if a given component of the food web increased in one community, it generally decreased in the other. When compared to non-invaded sites, soil and soil food-web characteristics of the newly invaded sites included, 1) lower species richness and lower absolute numbers of fungi and invertebrates, 2) greater abundance of active bacteria, 3) similar species of bacteria and fungi as those found in soils invaded over 50 years ago, 4) higher levels of silt, thus greater fertility and soil water-holding capacity, 5) a more continuous cover of living and dead plant material thus facilitating the germination of the large-seeded Bromus. The results illustrate that 1) soil food-web structure can vary widely within what would generally be considered one vegetation type (semi-arid grassland) and, 2) addition of a common resource can evoke disparate responses from individual food-web compartments depending on their original structure.

Soil biota in an ungrazed grassland: response to annual grass (Bromus tectorum) invasion

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