火烧对内蒙古草原中坚韧胶衣固氮活性的影响

EFFECT OF RANGE FIRE ON NITROGEN FIXATION OF COLLEMA TENAX IN A SEMIARID GRASSLAND OF INNER MONGOLIA,CHINA

坚韧胶衣(Collema tenax)是干旱和半干旱草原中常见的一种固氮地衣,是草原生态系统中生物土壤结皮(Biological soil crust)的主要组成部分,对生态系统氮循环具有重要的影响。火烧作为一种干扰因子,是草原生态系统结构和功能维持的重要因素之一。该文采用乙炔还原法(Acetylene reduction assay),研究了火烧对内蒙古草原生态系统中坚韧胶衣固氮活性的短期影响。结果表明,在个体尺度上,与对照相比,火烧区中地衣体烧损的坚韧胶衣固氮活性降低了42.3%,而无烧损的个体固氮活性则升高了28.4%。这表明火烧对坚韧胶衣的固氮功能在个体尺度上具有双重影响:1)通过烧损地衣体、恶化地表温度和水分条件,而抑制个体的固氮活性;2)通过改善光照条件,使表土养分呈现脉冲式增高,而促进未烧损个体的固氮活性。在种群尺度上,火烧与对照之间固氮活性并无显著差异,这可能是由于火烧在个体尺度上对坚韧胶衣的固氮活性的双重影响相互抵消所致。

Aims Collema tenax is a common N-fixing lichen in arid and semiarid grasslands, where range fire can be an important factor influencing ecosystem structure and function. The response of N-fixing activity of lichens to range fire has received little attention in ecological studies. Our purpose was to investigate the short-term effects of range fire on N fixation of C. tenax. Methods Three months after a controlled bum at the Inner Mongolian semiarid grassland, we measured plant cover, litter cover and N-fixing activity of C. tenax in both burned plots and control plots. N-fixing activity was measured using acetylene reduction assay. One-Way ANOVA was applied to compare N-fixing activity between treatments. Important findings At the individual scale, N-fixing activity of C. tenax decreased by 42.3% for individuals with a burned thallus, but increased by 28.4% for those with an intact one, as compared to that observed in unburned plots. These results show counteractive effects of burning on N fixation of C. tenax at individual scale, i.e., 1） negative effect through damage on lichen thallus and potential deterioration in surface water and temperature conditions and 2） positive effect as a result of increased light availability via removal of litter and plant biomass and, to a lesser extent, a fire-induced nutrient pulse on the soil surface. At the population scale, no significant effects of burning on N-fixing activity of this lichen were found, which may be attributable to the counteractive responses of N fixation of C. tenax at the individual scale.