| Popis: |
The tropical savannas of northern Australia cover approximately one-fifth of the continent. These savannas are under threat by the invasion of the alien African grass Andropogon gayanus Kunth., which replaces the diverse native savanna grass communities with tall, dense, almost monospecific swards. This thesis examines the effects of A. gayanus invasion on nitrogen (N) dynamics in Australia’s tropical savannas. A range of N pools and fluxes were quantified in native grass savanna and were compared to those occurring in A. gayanus invaded savanna. Andropogon gayanus invasion significantly increased above- and belowground grass N pools. Compared to native grasses, A. gayanus had (i) up to 11-times higher phytomass, (ii) 7-times higher phytomass N pool, (iii) 4-times higher root biomass and (iv) 2 ½-times higher root N pool. The pronounced increase in grass N pools in A. gayanus dominated understorey were associated with changes in soil N relations. Dominance by A. gayanus resulted in significantly lower nitrification rates, 3-times lower soil nitrate availability, and up to 3-times higher ammonium availability than soil associated with native grasses. These pronounced changes in soil N availability are likely to be linked to the contrasting N ecophysiology of A. gayanus and native grasses. I suggest that the large increases in ammonium availability, but concurrent large decreases in soil nitrate availability, may be due to A. gayanus inhibiting nitrification, as it does in its native range in Africa. Compared to native grasses, A. gayanus had much higher uptake of all N sources (ammonium, nitrate or glycine), but had a strong preference for ammonium as an N source. The large necromass (grass litter) produced by A. gayanus has two likely fates in this savanna: (i) decomposition and return of N to the soil N pool, or (ii) consumption via fire and N loss to the atmospheric N pool. A study of litter vi decomposition in the two systems showed that the amount of litter decomposed in A. gayanus savanna was more than 3-times higher, and this led to up to up to 10-times higher return of N to the soil, compared to native grasses. Investigations into the firemediated N losses in the two systems showed that N losses in A. gayanus savanna were ~2-times higher than that in native grass savanna. This study also showed that these losses could be as high as 62 kg N ha-1 in cases were the phytomass of A. gayanus accumulated over several years. Andropogon gayanus invasion has changed N cycling in these savannas by altering both the N pools and fluxes, resulting in an altered N budget in invaded savanna. Once established A. gayanus speeds up N cycling in invaded savanna, and retains N in the ecosystem, possibly due to the process of nitrification inhibition. This has lead to an accumulation of ammonium in A. gayanus invaded plots, and may lead to reduced nitrate losses via leaching. These changes in N cycling are likely to increase the competitive superiority of A. gayanus and further contribute to persistence of this alien species within Australian savanna ecosystems. Despite the apparent N retention and conservation in A. gayanus invaded savanna, I propose that large, frequent, fire-mediated N losses in A. gayanus invaded savanna will lead to a permanent reduction in total soil N levels in invaded savannas in the long-term. The alteration of the N cycle due to A. gayanus invasion is further evidence of the dramatic ecosystem impacts of this alien grass invader in Australian savannas. |
