长期氮添加对典型草原植物多样性与初级生产力的影响及途径

氮(N)沉降对陆地生态系统的结构和功能已产生了重要的影响, N也是中国北方草原植物生长和初级生产力的主要限制性元素。物种多样性和功能多样性是揭示生物多样性对生态系统功能维持机制的关键指标, 然而, 关于长期N添加下草原物种多样性与功能多样性的关系, 及其对初级生产力的影响途径及机制, 尚不十分清楚。为此, 该研究依托在内蒙古典型草原建立的长期N添加实验平台, 实验处理包括1个完全对照(不添加任何肥料)和6个N添加水平(0、1.75、5.25、10.50、17.50和28.00 g·m^-2·a^-1), 研究了长期N添加对典型草原物种多样性、功能多样性和初级生产力的影响大小及途径。结果表明: 1) N添加显著降低了典型草原的物种丰富度和Shannon-Wiener指数, 但对功能多样性(包括功能性状多样性指数和群落加权性状值)无显著的影响。2)结构方程模型分析表明, 功能多样性主要受物种丰富度的影响, 但是物种多样性减少并没有导致功能多样性降低, 其原因主要是功能群组成发生了改变, 即群落内多年生根茎禾草所占比例显著增加, 以致群落加权性状值变化不大。3) N通过影响物种丰富度和功能群组成, 间接影响群落加权性状值, 进而影响群落净初级生产力。其中, 群落加权性状值是最重要的影响因子, 可解释48%的初级生产力变化, 表明初级生产力主要是由群落内优势物种的生物量及功能性状所决定, 因此该研究的结果很好地支持了质量比假说。

Effects and pathways of long-term nitrogen addition on plant diversity and primary productivity in a typical steppe

Aims Nitrogen (N) deposition has made great impacts on the structure and function of terrestrial ecosystems in recent decades, and N is known as the main limiting element of plant growth and primary productivity in the northern grassland of China. Species diversity and functional diversity have been widely recognized as key indicators of the mechanisms of biodiversity maintenance for ecosystem functions. However, the effect of long-term N addition on the relationship between species diversity and functional diversity, and the corresponding impacts on primary productivity have rarely been studied; particularly the underpinning mechanisms remain unclear. Our objective is to examine the effects and pathways of long-term N addition on species diversity, functional diversity and primary productivity in a typical steppe.
Methods A manipulative N addition experiment located in a typical steppe of the Nei Mongol grassland has been conducted for 18 years. The experimental design included seven levels of N addition rate (i.e., control, 0, 1.75, 5.25, 10.50, 17.50, 28.00 g·m^-2·a^-1) with nine replicates for each treatment. The plant functional traits of dominant species, species richness and composition, and aboveground net primary productivity (ANPP) were determined. In addition, species diversity, functional attribute diversity and community-weighted mean traits were calculated.
Important findings 1) N addition significantly reduced species richness and Shannon-Wiener index, but had little effect on functional diversity, including functional attribute diversity and community-weighted mean traits. 2) Structural equation model analyses showed that functional diversity was mainly affected by species richness, whereas the decrease in species diversity didn’t lead to the decrease in functional diversity. The community- weighted mean traits did not change under N addition, which was mainly due to the shift in functional group composition, that is, the proportion of perennial rhizome grass in plant community increased greatly under N addition. 3) The effect of N addition on ANPP was through direct pathways of species richness and shift in functional group composition, and further via an indirect pathway of community-weighted mean traits. Particularly, the community-weighted mean traits were the most important factors and explained 48% of total variation in ANPP, implying that the primary productivity is mainly determined by the biomass and functional traits of dominant species in the plant community. Our results well supported the mass-ratio hypothesis.