亚高山森林溪流镉储量与分配的动态变化特征

镉(Cd)是一种有害重金属元素,能够伴随溪流水体流动和物质沉积影响下游流域的生态环境安全,但缺乏必要关注。为了解森林溪流Cd储量及其分配的动态变化特征,以岷江上游亚高山森林集水区的溪流为研究对象,在长度10—50、50—150、150—260m区间内各选取5条典型溪流,研究Cd元素在亚高山森林-溪流-河流集合生态系统中的迁移过程。结果表明:亚高山森林溪流的Cd储量介于2.57—128.46mg/m~2之间,主要储存于沉积物中;森林溪流上、中、下游的Cd储量没有显著差异;森林溪流的Cd储量以秋季凋落物高峰期最高,春季凋落物高峰期最低;森林溪流的上、中、下游Cd储量均在秋季凋落高峰最高,上、中游在春季凋落高峰最低,下游在非凋落高峰最低;凋落物的Cd储量与溪流水文特征密切相关。可见,亚高山森林溪流Cd储量动态具有季节性变化和一定的自净能力,这些结果为进一步了解Cd元素在水-陆生态系统的生物地球化学循环提供了新的角度。

Dynamic characteristics of cadmium storage and distribution in the subalpine forest streams

Cadmium (Cd) is one of hazardous heavy metal elements with high toxicity, which can not only accumulate in plant tissues, soils and water, but also migrate from one ecosystem to another ecosystem. In the forest region, the Cd absorbed by trees might be delivered to forest stream by plant debris, surface runoff and percolating water, and deposit in stream sediment or migrate to the butted river. As a result, an investigation on the dynamic characteristics of Cd storage and distribution in the forest stream is benefit to understand the Cd biogeochemical linkages between forest and the butted river. However, little information is available on forest stream Cd storage dynamics although the emission, deposition and migration of Cd between industrial ecosystem and natural ecosystem has been widely investigated in industrial districts. Moreover, the subalpine forest located in the upper reaches of Yangtze River plays crucial roles in holding and cleaning freshwater resource, and conserving soil and water, as a consequence of which an investigation on the biogeochemical linkage between forest and forest streams can provide with key scientific basis for forest management at watershed scale. In order to understand the Cd biogeochemical linkage of the subalpine forest region with the butted river ecosystem, Cd concentration in water, sediment and plant debris were therefore measured in fifteen subalpine forest streams located in the upper reaches of Minjiang River from September 2015 to August 2016. Here, fifteen forest streams were classified into five typical subalpine forest streams in the range of length 10-50 m, 50-150 m and 150-260 m, respectively. The Cd storage in the subalpine forest streams ranged from 2.57 mg/m² to 128.46 mg/m², and was stored mainly in the stream sediment. No significant difference of Cd storage was observed among the upper, middle and lower reaches of subalpine forest streams. The highest Cd storage in the subalpine forest stream ecosystems was found in Autumn litter peak, and the lowest value in secondary litter peak in Spring, implying that the litter in Autumn is one of the most important Cd input pathways. Moreover, the highest Cd storages in the upper, middle and lower reaches of forest stream ecosystem were also observed in Autumn litterfall peak, and the lowest values varied with the reaches. The lowest Cd storages in the upstream and midstream of forest stream ecosystems were found in secondary litterfall peak in Spring, while that in the downstream was observed in non-litterfall peak. Additionally, litter Cd storage in the subalpine forest streams was significantly correlated with stream hydrological characteristics, and the relationships of Cd storage in the sediment with stream hydrological characteristics varied with stream positions. In brief, the Cd storage in the subalpine forest stream ecosystem had clear seasonality, and the potential of water self-purification. These results provided a new perspective for further understanding of the biogeochemical cycle of Cd in aquatic-terrestrial ecosystems, and for managing subalpine forest at catchment level.