降水量对川西北高寒草甸牦牛粪分解速率的影响

青藏高原高寒草甸的降水年际变化很大,而且在未来全球气候变化的背景下可能进一步增大.为了研究降水量变化对川西北高寒草甸牦牛粪分解的潜在影响,通过塑料膜覆盖(去除降水)、模拟额外增水(增水量为实际降水量的50％)和对照(野外无干扰情形)形成3种不同的水分处理.测定了牛粪的含水量、干重及氮磷含量,并统计了主要分解者(手捡法获取粪食性甲虫及其幼虫)的种类和数量.结果发现:覆盖处理显著地降低了牛粪含水量、牛粪中的粪食性甲虫数量与牛粪分解量,但增水处理并没有显著影响牛粪的分解进程,可能与实验期间降水量较高有关.在牛粪分解前期,粪食性甲虫数量与牛粪含水量之间正相关,而牛粪分解量与粪食性甲虫数量之间正相关,说明牛粪含水量的高低影响了粪食性甲虫数量,进而决定了牛粪分解量.但是,降水量的人为增加或减少均没有显著地改变牛粪中的全氮和全磷含量.总体上看,在高寒草甸的放牧生态系统中,降水量的减少将会降低牛粪的分解速率、延长牛粪分解过程,从而可能会降低生态系统的生产力水平.

Effect of rainfall regimes on the decomposition rate of yak dung in an alpine meadow of northwest Sichuan Province, China

The alpine meadow of the Tibetan Plateau is the fifth largest rangeland of China. The estimated population density of yaks is about 10/hm²; the recorded dung density is about 3600 pats/hm² on average and sometimes can reach 5900 pats per hm², occupying about 24% of the total grassland area in summer grazing pastures. The decomposition rate of yak dung is therefore of great importance to ecosystem primary productivity and nutrient cycling of the alpine meadow. Previous studies have demonstrated that the decomposition rate is largely dependent upon both abiotic factors such as temperature and rainfall and biotic factors including microbial biomass and the abundance of beetles, flies, and many other decomposers. Changes of the abiotic factors may affect the dung physical properties and dung the decomposer abundances, collectively influencing the rate of the dung loss rate. However, the effect of the abiotic factors on the dung decomposition has scarcely been examined in relation to the decomposer abundances. The amount of rainfall fluctuates greatly between years and the magnitude of the interannual variation may become greater in the future warmed world of the Tibetan Plateau. In this study, in order to determine the potential effect of rainfall regimes on the decomposition rate of yak dung and associated underlying mechanisms, we artificially manipulated rainfall amount to conduct an experiment involving three treatments including yak dung pats covered by plastic sheet (rainfall exclusion), sprayed with additional water (50% more than natural rainfall), and exposed to natural rainfall (as control) in an alpine meadow of northwestern Sichuan Province. For each treatment, there were 30 replicates that were sampled for 6 times (on the third, fifth, eighth, thirteenth, eighteenth, and twenty-fifth days after starting the experiment), each having five replicates. Water content, the number of coprophagous beetle adults and larvae that were hand-collected, and the dung dry weight were surveyed for 6 times, total nitrogen and phosphorus concentrations of the dung pats were measured for three times (On the starting day and the fifth and twenty-fifth days). Significant reductions in dung water content, weight loss, and the number of beetle adults were found in the treatment of rainfall exclusion, but rainfall addition did not change these variables possibly because of the high amount of rainfall during the experiment period, i.e. the rates of dung loss and nutrient release were indistinguishable between the control and the treatment with additional water. During the early stages of the experiment (particularly in the first five days), water content was positively correlated with the number of beetle decomposers, which was in turn positively correlated with the rate of dung loss. This indicates that the change of rainfall amount might have indirectly affected the dung decomposition rates via directly changing the decomposer abundance. However, the manipulation of rainfall amount did not lead to significant differences in total nitrogen and phosphorus concentrations of yak dung among the three treatments. This suggests that nutrient release from dung to soil was proportional to the dung loss during the experiment. In conclusion, reduction of rainfall amount may slow down dung decomposition and nutrient cycling in the study alpine meadow ecosystems and hence perhaps reduce the ecosystem primary productivity over the long run.