北亚热带3种森林群落对大气湿沉降重金属的调控

森林群落截留消纳沉降重金属的能力影响着森林集水区溪水输出重金属的情况,决定着小流域的水质安全。为评价北亚热带地区典型森林生态系统对大气降水中主要重金属离子的截留能力和分配特征,以浙江庙山坞林场3种典型森林群落(毛竹林、杉木林、青冈阔叶林)和森林集水区-小溪为研究对象,于2018年7月—2019年6月监测了12次降水事件,分析、比较和讨论了大气降水、林内穿透雨、树干茎流、枯透水、地表径流和集水区溪水中7种重金属(铅(Pb)、锌(Zn)、镉(Cd)、铜(Cu)、砷(As)、镍(Ni)和锰(Mn))的质量浓度和通量的动态变化。结果表明,大气降水中7种重金属,Pb、Cd、As、Cu、Zn、Mn和Ni的年均浓度分别为0.974、0.124、0.512、3.42、36.7、8.48和1.94μg/L,3种森林群落林冠层截留的干沉降重金属中Zn和Mn的比例最高,林冠层降水对重金属Mn和Zn的淋溶量极高,超过大气降水沉降量的2.78倍和54.2倍,同时截留了降水中的As和Ni;枯落物层对Mn、Cd和Zn表现出截留作用,对Cu和Ni有淋溶或释放的作用;3种森林群落的地表径流中Pb、Mn、Cd和Zn浓度明显降低,As和Ni的浓度略高于枯透水,但由于地表径流量非常小,土壤表层对降水中重金属均表现出极强的截留作用。3种森林群落对大气降水中的重金属均表现出极高的截留率,森林群落之间没有显著差异,其中青冈阔叶林的截留能力最强,杉木林较弱。而地表径流及土壤渗透水等汇集到森林集水区后,溪水中重金属Mn、Ni、Cd和Zn的浓度和通量均有大幅的增加,仅对重金属As和Cu呈现截留的作用,这可能与森林土壤重金属本底值偏高以及森林长期接收的重金属沉降在降水淋溶下从酸性土壤中大量溶出有关。

Distribution of heavy metals in precipitation by three forest communities in northern subtropical region of China

The ability of forest communities to retain, absorb and intercept heavy metals from atmospheric deposition affects the output of heavy metals from streams in forest catchment areas, and determines the safety of local water. To explore the role of forest communities in the interception and purification of heavy metals in precipitation by typical forest types in northern subtropical in China, three forest ecosystem, Phyllostachys edulis plantation, Cunninghamia lanceolata plantation, and Cyclobalanopsis glauca broadleaf forest located in Miaoshanwu stand, Zhejiang Province, were selected and monitored from July, 2018 to June, 2019. Twelve precipitation events were sampled. The water samples of precipitation, throughfall, stemflow, litterfall, surface runoff, and river water in the forest catchment were measured. The water mass and dynamic changes of seven heavy metals(Pb, Zn, Cd, Cu, As, Ni, and Mn) mass concentrations and fluxes in water samples were analyzed. The results showed that the annual average concentrations of Pb, Cd, As, Cu, Zn, Mn, and Ni in precipitation were 0.974, 0.124, 0.512, 3.42, 36.7, 8.48 and 1.94 μg/L, respectively. The leaching amount of heavy metals Mn and Zn in the canopy of the three forest communities was extremely high, exceeding 2.78 times and 54.2 times the fluxes of atmospheric precipitation, while As and Ni in the precipitation were intercepted all forest communities. The analysis on heavy metals in dry deposition attached by leaves suggested that the Zn and Mn were the largest ones intercepted by leaves in forest canopies, reaching 0.692—8.02 mg/m2 and 4.12—5.85 mg/m2, which might be the largest source of these two heavy metals in forest precipitation. The litter layer exhibited a trapped effect on Mn, Cd and Zn, and the surface layer exhibited a strong interception effect on heavy metals in precipitation. Among them, Cyclobalanopsis glauca broadleaf forest had the strongest interception capacity, and Cunninghamia lanceolata forest was weaker. The three forest communities showed huge capacity to intercept metals ions in precipitation according to the runoff analysis and appeared no significant difference among the forest communities. The Miaoshanwu forest catchment area had a large amount of leaching of Mn, Ni, Cd and Zn, and only had a trapping effect on the heavy metals As and Cu, which may be related to the higher background value of heavy metals in the soil and the higher amount of dissolved heavy metals from acidic soils in the dry deposition received by forest catchment areas.