The invasive Sorghum halepense harbors endophytic N2-fixing bacteria and alters soil biogeochemistry

Exotic plants invading new habitats frequently initiate broad changes in ecosystem functioning. Sorghum halepense is an invasive grass capable of growing in nitrogen (N)-poor tallgrass prairie soils that creates near monocultures in once phylogenetically diverse-communities. The biogeochemistry of soils invaded by S. halepense was compared to that of uninvaded native prairie soils. Invaded soils contained two to four times greater concentrations of alkaline metals, 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) (320 mg N plant(-1)) and suggested a supplemental N source supporting plant growth. Altered soil biogeochemistry in invaded areas coupled with high above-ground biomass in N-poor soils suggested N-2-fixing activity associated with S. halepense. Nitrogenase activity of plant tissues indicated that N-2-fixation was occurring in, and largely restricted to, S. halepense rhizomes and roots. A culture approach was used to isolate these N-2-fixing bacteria from plant tissues, and 16S rRNA gene sequencing was used to identify these bacterial isolates. Nitrogenase activity of bacterial isolates indicated several were capable of N-2-fixation. In addition to N-2-fixation, other roles involved in promoting plant growth, namely mobilizing phosphorus and iron chelation, are known for closest matching relatives of the bacterial isolates identified in this work. Our results indicate that these plant growth-promoting bacteria may enhance the ability of S. halepense to invade and persist by altering fundamental ecosystem properties via significant changes in soil biogeochemistry.