施氮对空心莲子草(Alternanthera philoxeroides)和莲子草(Alternanthera sessilis)种间关系的影响

自然界的氮素释放总是呈现出空间和时间上的异质性,但关于异质性氮释放对于入侵植物和本地植物种间关系影响的研究相对较少。将入侵植物空心莲子草(Alternanthera philoxeroides)和同属本地植物莲子草(Alternanthera sessilis)分别进行单种种植(12株,无种间竞争)和混种种植(每种6株,有种间竞争),模拟大气氮湿沉降设置由两种不同施氮总量(15g N m~(-2)a~(-1)和30g N m~(-2)a~(-1))和两种不同施氮频率(每5天1次和每15天1次)交叉组成的4种施氮处理,并以不施氮为对照。施氮总量的增加显著促进了两种植物的生长,但对两种植物的种间竞争关系没有显著影响。施氮频率对两种植物的生长以及种间竞争关系都没有显著影响。两种植物在面对竞争时表现出不同的生物量分配策略,空心莲子草将更多的生物量分配到茎,而莲子草将更多的生物量分配到根。在全球变化的背景下,大气氮湿沉降可能会改变两种植物的种群结构和动态,但可能对这两种植物的种间关系影响较小。

Effects of nitrogen addition on interspecific competition between Alternanthera philoxeroides and Alternanthera sessilis

Nitrogen (N) is one of the most important soil nutrients for plants, and the amount and frequency of N release in soils is heterogeneous. Many studies have focused on the effects of heterogeneous N addition on a single plant species, but little is known about the effects of N amount and short-term N pulse on interspecific competition between invasive and native plant species. A replacement series experiment was conducted under greenhouse conditions to examine the effects of N amount and frequency release on the growth and interspecific competition between an invasive wetland clonal plant, Alternanthera philoxeroides (alligator weed), which is native in South America but highly invasive in China, and its native congener Alternanthera sessilis (sessile joyweed). Plant materials of A. philoxeroides and A. sessilis were collected from five locations in Xixi Wetland Park in Hangzhou, Zhejiang Province, China. Stem fragments of similar length (20 cm), with a stem tip for each species, were grown in monoculture (12 ramets in one container, no interspecific competition) and in mixture (six invasive plants with six native plants, with interspecific competition) in five different N treatments: control (no N added), low amount and high frequency (a total of 15 g N m^-2 a^-1 added every 5 days), low amount and low frequency (a total of 15 g N m^-2 a^-1 added every 15 days), high amount and high frequency (a total of 30 g N m^-2 a^-1 added every 5 days), and high amount and low frequency (a total of 30 g N m^-2 a^-1 added every 15 days). The results showed that N addition significantly increased the growth of both A. philoxeroides and A. sessilis. No significant difference was observed between the control and treatments for the relative yield (yield of each species in mixture divided by yield in its monoculture) for both species, indicating that N addition did not significantly affect the interspecific competition between the two species. The probable reason is that both species are from the same functional group and may have comparable capacities to compete and utilize the soil nutrients. The N pulse did not significantly modify growth or interspecific competition for either species. The sufficient soil water content and availability of nutrients other than N may explain why there were no effects of a N pulse on competition between the two species. Alternatively, high tolerance of both species to low soil nutrients and water content may lead to weaker responsiveness to a N pulse. With interspecific competition, the invasive species, A. philoxeroides, invested more biomass to stems, i.e., the stem biomass increased by 60%, which significantly decreased the root to shoot ratio. The native species, A. sessilis, allocated more biomass to roots, i.e., the root biomass increased by 250%, which significantly increased the root to shoot ratio. The results indicate that increasing atmospheric N deposition in the context of climate change may change population structure and dynamics of both species, but may not affect the interspecific competition of these plants.