Rossiter-Rachor N A, Setterfield S A, Douglas M M, Hutley L B, Cook G D, Schmidt S
School of Environmental and Life Sciences, Charles Darwin University, Darwin, Northern Territory, Australia.
Ecol Appl. 2009 Sep;19(6):1546-60. doi: 10.1890/08-0265.1.
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.
非洲牧草三芒草（Andropogon gayanus）的入侵正在极大地改变澳大利亚热带贫营养稀树草原的林下植被结构。我们比较了三芒草和本地草的氮（N）关系及物候，以研究入侵对氮循环的影响，并确定三芒草具有入侵性的可能原因。与本地草林下植被相比，三芒草的地上植物生物量和根生物量分别高达10倍和4倍，地上和地下氮库分别高达7倍和2.5倍。三芒草与本地草在植物生物量和氮库上的这些显著差异与氮循环的改变有关。大多数生长发生在雨季，与本地草相比，三芒草占优势时，土壤总氮库显著降低，硝化率降低，土壤硝酸盐有效性降低多达3倍，土壤铵有效性提高多达3倍。用三种草的离体根在异地研究了不同氮源的吸收动力学。三芒草的离体根对铵的吸收速率比本地草的根高6倍以上，而本地草三齿兔尾草（Eriachne triseta）对硝酸盐的吸收速率比三芒草高3倍。我们推测，三芒草会刺激氨化作用但抑制硝化作用，正如在其非洲原生地所显示的那样，并且这种土壤氮循环的改变与该物种对铵作为氮源的偏好有关。这种机制可能导致土壤氮关系的改变，并可能增强三芒草在澳大利亚稀树草原的竞争优势和持久性。
