Invasive Andropogon gayanus (gamba grass) is an ecosystem transformer of nitrogen relations in Australian savanna

Abstract

Invasion by the African grass Andropogon gayanus is drastically altering the understory structure of oligotrophic savannas in tropical Australia. We compared nitrogen (N) relations and phenology of A. gayanus and native grasses to examine the impact of invasion on N cycling and to determine possible reasons for invasiveness of A. gayanus. Andropogon gayanus produced 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. gayanus and native grasses were associated with an altered N cycle. Most growth occurs in the wet season when, compared with native grasses, dominance of A. gayanus was 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. gayanus had an over six times higher-uptake rate of ammonium than roots of native grasses, while native grass Eriachne triseta had a three times higher uptake rate of nitrate than A. gayanus. We hypothesize that A. gayanus stimulates 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. gayanus in Australian savannas.

Alien plant invasions in tropical and sub-tropical savannas: patterns, processes and prospects

“…Introduced as a pasture grass in about the 1930s, A. gayanus has spread across the northern areas of Australia (Flores et al 2005). It inhibits soil nitrification and thereby depletes total soil nitrogen from the already nitrogen-poor soils and promotes firemediated nitrogen loss (Rossiter-Rachor et al 2009). Combined with the altered fire regime, it forms selfperpetuating positive feedback loops (Rossiter-Rachor et al 2009).…”

“…It inhibits soil nitrification and thereby depletes total soil nitrogen from the already nitrogen-poor soils and promotes firemediated nitrogen loss (Rossiter-Rachor et al 2009). Combined with the altered fire regime, it forms selfperpetuating positive feedback loops (Rossiter-Rachor et al 2009). We return to the role of fire as a mechanism of invasion in a later section.…”

Why non-native grasses pose a critical emerging threat to biodiversity conservation, habitat connectivity and agricultural production in multifunctional rural landscapes

“…This is certainly true of A. gayanus, which has an extended growing season compared with native grasses (Rossiter-Rachor et al 2009) and P. aquatica, which increases pasture water use, reduces deep drainage (Johnston et al 2003), but increases soil water deficits in dry times. The photosynthetic pathways of E. curvula, N. neesiana and N. trichotoma (Table 1) often differ significantly from the dominant grasses that they displace (e.g., Hattersley 1983), and so these species are also likely to alter soil water regimes in infested areas.…”

Unassisted invasions: understanding and responding to Australia’s high-impact environmental grass weeds

“…The fire-generated transition from savanna woodland to gamba grassland can occur within a decade (Brooks et al 2010). This transition can be assisted by increased nitrogen losses during fires (Rossiter-Rachor et al 2009) and other invasion-related changes in soil nutrients and water required by native species (Rossiter-Rachor et al 2009), resulting in the crossing of an 'abiotic threshold' beyond which ecosystem function is very different (Brooks et al 2010). …”