African grass invasion alters nitrogen cycling in an Australian tropical savanna: The importance of whole-system N budgets

Background/Question/Methods Despite a proliferation of studies examining the impact of alien plant invaders on selected nitrogen (N) pools and/or fluxes, there remains a paucity of studies quantifying the impact on whole-system N budgets. N budgets may enable a clearer understanding of the longer-term impacts of invasion, as well as providing insights into the underlying mechanisms, and potential management strategies. We examined the impact of one of Australia’s worst invaders, Andropogon gayanus Kunth. (gamba grass) on savanna ecosystem N budgets. A. gayanus is a tall (~4 m), high biomass grass that is invading oligotrophic savannas in tropical northern Australia. We constructed a simple N balance for native grass and invaded savannas to examine the effect of invasion and fire on the ecosystem soil balance over time. The N balance integrated N inputs (wet deposition, N release from decomposition) and N outputs (fire, grass N uptake) as a function of three fire regimes (annual burning; fire 2 in 3 years, and 1 in 5 years). Soil N balance was estimated over a 10-year period by summing net N inputs and outputs as described above for burnt and unburnt years for each fire regime.

Results/Conclusions Savanna invaded by A. gayanus had up to 7-times higher nitrogen (N) pools, 4.5 times higher root biomass, and 2.8 times higher root N pool, than native grass understorey. The pronounced differences in biomass and N pools of A. gayanus and native grasses were associated with changes in N inputs into the soil N pool. The N budget estimates suggest that annual N inputs (wet deposition, N release from decomposition) in invaded stands were insufficient to compensate for N uptake by A. gayanus and fire mediated N losses. We conclude that without additional inputs via N fixation, infrequently (1 in 5 years) or annually burnt A. gayanus invaded sites may lose 3.1 and 8.2% of the upper soil pool over a decade, respectively, while native savanna incurs more modest losses (0.03 and 0.3%, respectively). With half of Australia’s savanna lands burning annually, A. gayanus invasion is likely to diminish the N capital of savanna ecosystems.