若尔盖高寒草甸粪堆大小对牦牛粪分解速率的影响

Impacts of Dung Pat Size on the Decomposition Rate of Yak Dung in a Zogie Alpine Meadow

放牧牦牛是若尔盖高寒草甸最重要的草地利用方式,放牧牦牛过程中会产生大量的大小（重量）差异明显的牛粪堆,牛粪的及时分解与否对草地生态系统的养分循环有重要影响。为了探明粪堆大小对牛粪分解速率的影响,调查了牛粪堆大小的野外分布格局,同时,采用人为模拟粪堆大小的实验方法,设置8种不同鲜重（50、100、250、500、700、1 000、1 400和2 000 g）的牛粪堆处理,检测了牛粪的主要分解者功能群（粪甲虫）数量并测定了牛粪分解速率。方差分析结果表明,粪堆大小对牛粪分解速率和甲虫密度均有显著影响。回归分析结果显示,牛粪分解速率与粪堆大小之间呈现曲线（二次方程）关系,即中等大小（1 000-1 400 g）粪堆的分解速率最大;粪堆大小与粪甲虫密度（单位重量牛粪中的甲虫数量）之间存在相似的二次方程关系;粪甲虫密度与牛粪分解速率之间呈现线性关系。野外调查结果显示,鲜重在600-1 500 g的粪堆数量占总数的近50%。以上结果表明,粪堆大小与牛粪分解速率之间的非线性关系可能是牛粪分解者（粪甲虫）对粪堆大小长期适应的结果。

There is a large variation in dung pat size even from a single yak species in the Zogie Alpine Meadow, a typical yak grazing grassland. Thus, dung decomposition rate is important to the nutrient cycling in this grazing ecosystem. To examine the size effect on decomposition rate of yak dung, we conducted a field experiment i nvolving eight size classes of circular dung pats differing in fresh weight（i.e. 50、 100、 250、 500、 700、 1 000、 1 400 and 2 000 g）, each class having four replicates. At the end of the experiment, we examined dung loss rate and beetle density for each dung pat. The relative dung loss rate（g g-1d-1）showed a curvilinear（quadratic）relationship with dung pat size, with the highest rate being in the middle-sized dung pats（1 000-1 400 g）. Similar relationships were observed between beetle density（individuals per dung mass）and dung pat size. Significant positive linear relationships were observed between beetle density and dung loss rate. In addition, an independent field survey showed that about 50% of yak dung pats ranged from 600 g to 1 500 g in fresh mass in the study alpine pasture.These results suggest that the non-linear relationship between dung pat size and dung loss rate might be due to the long-history evolution and adaptation of the within-dung detritivores（dung beetles）to the frequency distribution of dung pat sizes.