Exotic grass invasion alters nitrogen dynamics in the tropical savanna of northern Australia.
Rossiter, Natalie*,1, 2, Setterfield, Samantha1, Douglas, Michael1, Hutley, Lindsay1, 1 Charles Darwin University & Tropical Savannas CRC, Darwin, Northern Territory, Australia2 CSIRO Tropical Ecosystems Research Centre, Darwin, Northern Territory, Australia
ABSTRACT- Invasive exotic grasses may alter ecosystem processes by changing the resource pools of an ecosystem, or the rates of resource consumption and supply. Andropogon gayanus Kunth. (Gamba grass), a tall perennial African grass has invaded ecosystems in the Top End of northern Australia. A. gayanus invasion significantly increases biomass production by up to 10 times. This invasion could have significant ecological consequences for the nitrogen limited savanna ecosystems. We hypothesized that A. gayanus may reduce the nitrogen availability of invaded sites due to higher rates of nitrogen uptake. We therefore compared N availability and N uptake of invaded sites with those of native grass savannas. Nitrogen availability was measured in situ using mixed ion-exchange resin bags, at five sites at Wildman Reserve, approximately 250 km south east of Darwin. N uptake was estimated by harvesting above ground grass biomass, determining the standing pool of N and using this a surrogate for N uptake. We found sites invaded by A. gayanus had reduced available soil nitrate levels by up to 70%, compared with native grass sites. This may be explained in part by rates of A. gayanus N uptake which were 50-70 % higher than native grasses. These results suggests that A. gayanus has the potential to effect savanna nitrogen dynamics and native plant communities.
Key words: Andropogon gayanus, biological invasion, invasive species, nitrogen
All materials copyright The Ecological Society of America (ESA), and may not be used without written permission.