|Publication Type||Journal Article|
|Year of Publication||2009|
|Authors||Rossiter, NA, Setterfield, SA, Douglas, MM, Assoc Prof Hutley, LB, Cook, GD, Schmidt, S|
|Pagination||1546 - 1560|
|Keywords||ammonium, exotic grasses, invasive alien species, nitrate, nitrification inhibition, nitrogen cycling, nitrogen uptake|
Invasion by the African grass Andropogon gayanusis drastically altering the understory structure of oligotrophic savannas in tropical Australia. We compared nitrogen (N) relations and phenology of A. gayanusand native grasses to examine the impact of invasion on N cycling and to determine possible reasons for invasiveness of A. gayanus. Andropogon gayanusproduced up to 10 and four times more shoot phytomass and root biomass, with up to seven and 2.5 times greater shoot and root N pools than native grass understory. These pronounced differences in phytomass and N pools between A. gayanusand native grasses were associated with an altered N cycle. Most growth occurs in the wet season when, compared with native grasses, dominance of A. gayanuswas associated with significantly lower total soil N pools, lower nitrification rates, up to three times lower soil nitrate availability, and up to three times higher soil ammonium availability. Uptake kinetics for different N sources were studied with excised roots of three grass species ex situ. Excised roots of A. gayanushad an over six times higher uptake rate of ammonium than roots of native grasses, while native grass Eriachne trisetahad a three times higher uptake rate of nitrate than A. gayanus. We hypothesize that A. gayanusstimulates ammonification but inhibits nitrification, as was shown to occur in its native range in Africa, and that this modification of the soil N cycle is linked to the species' preference for ammonium as an N source. This mechanism could result in altered soil N relations and could enhance the competitive superiority and persistence of A. gayanusin Australian savannas.