黄土区封育和放牧草地物种多样性和地上生物量对氮素添加的响应

Responses of species diversity and aboveground biomass to nitrogen addition in fenced and grazed grassland on the Loess Plateau

氮素添加对草地生态系统地上生物量具有普遍促进作用,但对解释引起物种多样性丧失的机制仍存在较大争议。本研究以黄土高原半干旱区长期封育和连续放牧草地为对象,通过设置不同氮素添加水平来探索两种典型的草地管理方式下群落地上生物量、物种多样性、群落组分结构对氮素添加的响应及其异同以及物种多样性与地上生物量的关系,并研究这些响应的潜在生态学机制。结果显示,1)氮素添加显著提高长期封育草地地上生物量的同时降低了物种多样性;放牧草地群落地上生物量、物种多样性对氮素添加的响应与封育草地相同,但其响应程度均小于封育施肥草地;2)草地群落物种多样性随地表凋落物生物量增加而降低,随植被透光率增加而升高;3)封育施肥草地Shannon-Wiener多样性指数与地上生物量存在显著线性负相关关系,而放牧施肥草地的物种多样性与地上生物量不存在显著的相关关系。以上结果表明放牧能够减缓施肥对物种多样性的负效应,在轻微降低物种多样性的前提下显著提高群落地上生物量,是一种合理的草地管理方式。

Nitrogen addition can increase the aboveground biomass of the plant community in grasslands. How-ever, it can also lead to a loss of species diversity, but the mechanisms involved in this process are unclear. The objective of this study was to explore the effects of nitrogen addition on aboveground biomass, species diversi-ty, community composition, and the relationship between species richness and aboveground biomass in long-term fenced grassland and grazed grassland on the semi-arid Loess Plateau. We focused on the differences and similarities in the grassland responses under these two typical management systems, and tried to identify the the ecological mechanisms underlying these responses. We added nitrogen at different rates [0 , 5, 10,20,40, 80 g CO（NH2）2/（m2 · yr）] to long-term fenced grassland and grazed grassland. After 3 years of treatment, we measured aboveground biomass, calculated species diversity indexes （species richness, Shannon-Wienerindex, and Pielou index） and analyzed the relationship between species diversity and productivity using regres-sion analyses. The results showed that, in the long-term fenced grassland, nitrogen addition significantly in-creased the aboveground biomass and decreased species diversity, while the grazed grassland showed similar but weaker responses to nitrogen addition. The species diversity in fenced and fertilized grassland decreased as the litter biomass increased, and also increased with greater light penetration. In comparison, species diversity in grazed and fertilized grassland decreased slowly and did not change significantly with greater light penetration. The Shannon -Wiener index was significantly negatively correlated with above-ground productivity in fenced and fertilized grassland, but no such relationship existed in grazed grassland. These results indicated that graz-ing can relieve the negative effect of fertilization on species diversity. Therefore, grazing is a reasonable grass-land management method to retain species diversity while increasing the community aboveground biomass by fertilization.