内蒙古典型草原生态系统中N素添加对羊草和黄囊苔草N:P化学计量学特征的影响

过度放牧是中国典型草原生态系统日趋退化的主要原因.由于养分输入-输出的不均衡,将导致土壤养分库的耗竭,而确定退化草原受何种养分的制约是对其进行恢复的重要途径之一.应用N:P化学计量学的原理和方法,有望代替传统的野外养分添加实验,来研究不同草地受养分限制的状况.本文采用这两种方法在物种水平上研究限制性养分.此外,陆地植物器官中的N:P比相对恒定是植物在地球上生存的重要适应机制,养分添加为验证这一假说提供了一种有效手段.为此,我们采用野外N素添加的方法,研究了内蒙古典型草原两种演替系列样地中(围封22年的样地A和围封2年的样地B)羊草(Leymus chinensis(Ttin.)Tzvel.)和黄囊苔草(Carex korshinskyi Kom)生物量和N:P化学计量学特征的变异.N素添加梯度分别为0、5、15、30、50、80 gNH4NO3·m-2·a-1.研究结果表明,在施肥第一年,两个物种的地上生物量和P含量均不受N素添加的影响;相关分析结果表明,在施肥第二年两种植物的N:P比不受氮素添加的影响;施肥可以显著提高羊草和黄囊苔草地上器官的含N量,P含量只是在第二年有显著增大的趋势;2001年,两块样地中羊草和黄囊苔草的氮磷含量在不同施肥处理下均呈极显著地正相关.这表明,样地A中黄囊苔草缺乏P,样地B中羊草缺乏N,施肥两年后,两个物种器官中的N、P含量具有显著的协同关系,从物种水平上验证了我们提出的假说.同时,两年的实验结果还表明,生态系统中不同物种对添加N素的响应不同,笼统地界定一个生态系统受某种元素的制约是不恰当的.

Differential Responses of N:P Stoichiometry of Leymus chinensis and Carex korshinskyi to N Additions in a Steppe Ecosystem in Nei Mongol

The typical steppe ecosystems in China are now being increasingly degraded due mainly to overgrazing. To determine the limiting nutrients is of significance in order to find out ways of successfully restoring the degraded steppe. In addition to field fertilization experiments, N:P stoichiometry is an alternative, but argumentative tool to study nutrient limitation. In this study, we used these two approaches to identify the most limiting nutrient element at the species level. Furthermore, nutrient addition experiment provides an effective means to test our hypothesis that N:P stoichiometry will remain constant because relatively narrow range of N:P ratio in tissues of the terrestrial plants is an important adaptive mechanism for plants to survive on earth. For these purposes, we designed a field experiment to examine the responses of biomass and N:P stoichiometry of the two dominant species - Leymus chinensis (Trin.) Tzvel. and Carex korshinskyi Kom. - to N fertilization at rates of 0, 5, 15, 30, 50 and 80 g NH₄NO₃.m^-2.a^-1 in two adjacent sites, one being excluded animal grazing for 22 years (site A), and another being free of grazing for only two years (site B) before the experiment was carried out. No effects of N fertilization were detected in the first year as reflected by the aboveground biomass and P concentrations of the two species. The regression analysis showed that N:P ratios of two species of both sites remained constant in the second year. N fertilization significantly increased the N concentrations of two species in both years, while only significantly increased the P contents of the two species in the second year. N and P contents of the two species were significantly correlated in all cases in 2001. Our results suggest that the L. chinensis was in short of N in site B while the growth of C. korshinskyiwas limited by P in site A, and there is a significant synergistic relationship between tissue N and P concentrations in 2001. Our hypothesis was valid on the species level since N:P ratio of the two species remained constant with increasing N application rates after two years of fertilization. We argue that it may be inappropriate to define an ecosystem which is limited by certain nutrient elements since the responses of coexisting species present in a community to nutrient additions can vary tremendously.