Exotic plants invading new habitats frequently initiate broad changes in ecosystem functioning and have characteristics of ecosystem engineers. Sorghum halepense (Johnson grass) is one such invasive grass. Growing in nitrogen (N)-poor prairie soils, S. halepense creates near monocultures in once phylogenetically diverse-communities. This successful invader has many properties which suggest that it has a high N-demand and may impact the N-cycle of areas it invades. Most obvious among these properties is the presence of the cyanogenic constitutive defense chemical, dhurrin, contained within the leaves. In addition to containing dhurrin, S. halepense also has considerable above-ground biomass; it often grows as a densely-packed monoculture. In this work we investigated the role of S. halepense as an ecosystem engineer through its influence on soil biogeochemical cycling. We measured soil nutrient pools in areas invaded by S. halepense and compared them to soil nutrient pools in native prairie. In addition, we explored the hypothesis that this invasive grass harbors N-fixing bacteria.
Results/Conclusions
Invaded soils typically contained 2-4 times greater concentrations of alkaline metals, essential micronutrients, and essential plant nutrients than native prairie soils. The notable exception was Ca+2, which was always significantly lower in invaded soils. The N-content of S. halepense above-ground biomass was 6.4 mg g-1 or 320 mg N plant-1 and suggested the presence of a supplemental N source supporting plant growth. Altered soil biogeochemistry in invaded areas coupled with high above-ground biomass in N-poor soils suggested enhanced N-fixing activity in soils associated with S. halepense. Assessment of nitrogenase activity indicated that N-fixation was occurring in, and largely restricted to, S. halepense roots and rhizomes. The invasive grass was found to fix N through an association with endophytic bacteria inhabiting rhizomatous tissue. In addition to N-fixing bacteria, other endophytic growth-promoting bacteria, largely those capable of mobilizing phosphorus through acidification, were isolated from roots and rhizomes. Our results indicate that endophytic plant-growth promoting bacteria enhance the ability of S. halepense to invade and persist by altering fundamental ecosystem properties and functions as an ecosystem engineer.
