氮沉降强度和频率对羊草叶绿素含量的影响

氮沉降强度和沉降频率是决定其对陆地生态系统影响的重要决定因素。我们结合当前世界上各地区的氮沉降状况，设计了包括9个氮沉降梯度的长期控制实验，并将氮沉降分为两种沉降频率（一年2次和每月1次）、草原管理方式分为封育和割草两种。本文主要基于上述实验平台的优势植物（羊草）叶片叶绿素含量来探讨氮沉降方式（强度和频度）和草原管理方式（封育和打草）对草地生态系统结构和功能的影响。实验结果表明：1）氮沉降显著增加了植物叶片叶绿素含量（P < 0.001）；2）每月一次模拟氮沉降处理的植物叶绿素含量显著低于一年两次氮沉降的处理（P = 0.026）；3）在相同的氮沉降强度处理下，打草地相对于封育草地具有更高的叶绿素含量（P = 0.012）；4）羊草叶绿素含量与其叶片氮浓度显著正相关（P < 0.001）；5）羊草叶绿素含量与该植株高度极显著正相关（P < 0.001）。根据上述结果我们可以看出一年一次或一年两次的模拟氮沉降（类似于施肥处理或低频率的氮素添加实验）可能会夸大真实氮沉降对草地生态系统结构和功能的影响，今后在外推类似实验结论时应更加谨慎。此外，氮沉降下打草管理有利于增加了植物叶片叶绿素含量，可提高植物的光合作用，因此在未来氮沉降加剧状况下，打草可以保持或提高内蒙古草地生产力，有利于该地区草地的可持续利用。

Nitrogen deposition and Leymus chinensis leaf chlorophyll content in Inner Mongolian grassland

Nitrogen (N) is a key element for plant growth of terrestrial ecosystems, and it is a commonly limiting factor for grassland productivity in semiarid grasslands. Nitrogen deposition has increased 2-5 times since 1950s. Some studies have documented that N deposition has positive effects on primary production or negative effects on biodiversity, soil nutrition etc; however, the results of previous studies are still inconsistent or even controversial. We designed a simulation nitrogen deposition experiment involved 9 N deposition gradients in Inner Mongolian grassland in 2008. Moreover, the experiment includes 2 N deposition frequencies (applied twice a year and applied every month equally) and 2 grassland management regimes (fenced grassland and mowing grassland). In this paper, we used the Chlorophyll content (SPAD value) of Leymus chinensis leaf, the dominant plant species in the region, in order to explore the effect of N deposition intensity and frequency and grassland management regimes on the structure and function of grassland ecosystem in Inner Mongolia. The results showed that 1) N deposition significantly increased leaf chlorophyll content; 2) the content of leaf chlorophyll in low N deposition frequency treatment was significantly higher than that in high N deposition frequency treatment; 3) the content of leaf chlorophyll was higher under mowing than under fenced, 4) leaf N concentration was highly consistent with leaf chlorophyll content, and 5) there existed positive linear relationship between plant height and leaf chlorophyll content. Totally, our finding first explored the effect of N deposition intensity and frequency and grassland management regimes on the structure and function of grassland ecosystem (in view of leaf chlorophyll content) in Inner Mongolia, and found that the low N deposition frequency (once per year or twice per year, as fertilization) should be magnified the effect of elevated N deposition in future. Therefore, we should be cautious to extend the results come from these low N deposition frequency to larger scale. Moreover, N deposition increased the leaf chlorophyll content and favored the primary productivity, and therefore can be better facilitated the grassland sustainable development in Inner Mongolia.