黄土区不同恢复年限草地群落生物量及根冠比对氮添加的响应

大气氮沉降增加作为全球变化的主要环境问题之一，已引发人们的广泛关注，持续的氮沉降对草地生态系统的组成、结构和功能产生重要影响。为深入了解草地恢复进程中群落生物量和根冠比对氮沉降的响应，以黄土区3个不同恢复年限（初期12a、中期28a和后期37a）的天然草地为研究对象，通过设置6个氮添加水平，CK （0）、N1（2.34g m^-2a^-1）、N2（4.67g m^-2a^-1）、N3（9.34g m^-2a^-1）、N4（18.68g m^-2a^-1）、N5（37.35g m^-2a^-1）来测定草地群落地上生物量、地下生物量和总生物量，并计算根冠比和氮响应效率（NRE）。结果表明：（1）地上生物量在恢复中期最大，随氮添加梯度增加，地上生物量在恢复初期和恢复后期呈不显著上升趋势，对氮添加表现为非线性的正响应（ΔNRE>0），在恢复中期呈不显著下降趋势，对氮添加表现为非线性的负响应（ΔNRE<0）。（2）群落地下生物量对氮添加无显著响应，总生物量只有在恢复后期的N4添加水平下，与对照存在显著差异。（3）根冠比在恢复初期时，N3添加水平下显著高于对照和其他氮添加水平，其余恢复年限对氮添加无显著响应。综上所述，通过分析比较黄土区不同恢复年限草地群落的地上、地下及总生物量和根冠比对氮添加的响应。建议对该区域开展试点实验，实行适应性草地管理，如进行两年一次刈割或轻度放牧（2只羊/hm²），来探寻更科学有效的管理措施，使草地实现系统性恢复，进而满足生态系统容量和社会需求的变化。

Responses of grassland community biomass and root-shoot ratio to nitrogen addition in different restoration years on the Loess Plateau

As one of the main environmental problems of global change, the increase of atmospheric nitrogen deposition has aroused widespread concern. Continuous nitrogen deposition has an important impact on the composition, structure and function of grassland ecosystems. In order to deeply understand the response of community biomass and root-shoot ratio to nitrogen deposition in the process of grassland restoration, this study took 3 natural grasslands with different recovery years (early 12 years, middle 28 years, and late 37 years) on the Loess Plateau as the research object. Six nitrogen addition levels, CK (0), N1(2.34 g m^-2 a^-1),N2(4.67g m^-2a^-1),N3(9.34g m^-2a^-1),N4(18.68g m^-2a^-1),N5(37.35g m^-2a^-1)were used to measure the aboveground biomass, underground biomass, and total biomass of grassland communities, and the root-shoot ratio and nitrogen response efficiency (NRE). The results showed that:(1) The aboveground biomass was the largest in the middle stage of restoration. With the increase of nitrogen addition gradient, the aboveground biomass showed an insignificant upward trend in the early and late stages of restoration, a non-linear positive response to nitrogen addition (ΔNRE>0), an insignificant downward trend in the middle stages of restoration, and a non-linear negative response to nitrogen addition (ΔNRE<0). (2) The underground biomass of the community had no significant response to nitrogen addition, and the total biomass was significantly different from that of the control only at the N4 addition level in the later stage of restoration. (3) At the early stage of restoration, the root-shoot ratio at the N3 addition level was significantly higher than that of the control and other nitrogen addition levels, while the other restoration years had no significant response to nitrogen addition. In conclusion, the responses of aboveground, underground, total biomass and root-shoot ratio of grassland communities with different restoration years on the Loess Plateau to nitrogen addition were analyzed and compared. We suggest that pilot experiments should be carried out in this region to implement adaptive grassland management, such as biennial cutting or light grazing (2 sheep/hm²), to explore more scientific and effective management measures to achieve systematic restoration of grassland, so as to meet the changes of ecosystem capacity and social needs.