Effects of the invasive grass Andropogon gayanus Kunth (gamba grass) on soil nitrogen dynamics and the soil microbial community

The African grass Andropogon gayanus Kunth (gamba grass) is a serious threat to savanna ecosystems across northern Australia. The success of this high biomass grass, growing on low nitrogen (N) status savanna soils, may, in part, be due to the documented ability of gamba grass to alter the soil microbial community via allelopathy. Suppressing the activity of ammonium oxidising community reduces nitrification and conserves N in the ecosystem. This mechanism could play a key role in the invasion success and competitive superiority of gamba grass in Australian savannas. We examined the soil ammonia oxidiser (archaea and bacteria), and N-fixing communities following invasion, as well as soil N relations. Soil inorganic N availability (NO3- and NH4+) under native grass and gamba grass stands were not significantly different. This was reflected in the microbial communities, with no difference in amoA gene copy number between the native and gamba grass soils. While gene abundances were similar, the identities of some amoA phylotypes between native grass and gamba grass soils were different. The N-fixing bacterial community, as measured by nifH amplicons sequencing, provided new information about N-fixing bacteria in these soils, however there were no measurable differences between native and gamba grass soils. The dominant N fixers were Streptomyces, Actinomyces, Candidatus, Methylomirabilis and Pseudomonas.