天然杜鹃林林窗扰动对土壤重金属含量的影响

林窗扰动是森林更新和演替过程的重要干扰类型,影响土壤的物质循环。为了解林窗扰动对贵州百里杜鹃国家森林公园天然杜鹃林土壤重金属分布的影响,以自然形成小林窗表层土壤(0~10cm)为对象,林下土壤为对照,测定土壤重金属(As、Cd、Cr、Fe、Hg、Mn、Ni、V、Pb和Zn)含量,采用随机森林模型和冗余分析等方法对土壤重金属的空间分布、污染源识别及其影响因素进行研究,运用潜在生态风险指数法评价杜鹃林潜在生态风险。结果表明:1)小林窗表层土壤中Hg、Cd的平均含量分别超过国家Ⅱ级标准2.78、1.53倍,而林下分别超过2.52、1.45倍;小林窗样点土壤Cd含量超标率高于林下27%。小林窗扰动对天然杜鹃林[6]下坡位表层土Cr含量影响显著,下坡位小林窗土壤Cr含量显著>林下。中坡位、上坡位、中坡和缓坡小林窗土壤Pb、Zn含量均>林下,不同坡位和坡度小林窗土壤Mn和Ni含量均>林下;2)冗余分析表明,Mg、海拔、Ca是解释天然杜鹃林土壤重金属分布最重要的环境因子。随机森林模型重要性分析表明,Ni是影响天然林小林窗与林下土壤重金属分布最重要的重金属因子,Cr次之;3)小林窗土壤重金属单项(除Ni外)与综合潜在生态风险>林下,小林窗与林下土壤Hg和Cd风险均较高,其余重金属处于低生态风险。所以,在百里杜鹃天然杜鹃林的保护和管理过程中,应重视Hg、Cd污染的防治以及小林窗扰动对重金属分布的影响。

Effects of Gap Disturbance on Soil Heavy Metal Content in Natural Rhododendron Forest

Gap disturbance plays an important role in the processes of regeneration and succession of forests,and influences the capability of soil material circulation.To understand the effects of gap disturbance on the distribution of soil heavy metals in the natural Rhododendron forest of the Baili Rhododendron National Forest Park,Guizhou Province,we took topsoil (0-10 cm) of natural small gaps as the main research object and topsoil of closed canopies as control groups to determine the content of soil heavy metals (As,Cd,Cr,Fe,Hg,Mn,Ni,V,Pb and Zn).The Random Forest and redundancy analysis methods were used to study the spatial distribution of soil heavy metals,and to identify the pollution sources and their influencing factors,and to assess the potential ecological risks in Rhododendron forest by Potential Ecological Risk Index Method.The results indicated that 1) the average contents of Hg and Cd in topsoil of small gaps exceeded the standard values 2.78 and 1.53 times as Grade II of national environmental quality standards for soils,respectively,while those of closed canopies were 2.52 and 1.45 times.The over-standard rate of Cd in soil of small gap plots was 27% higher than those of closed canopy.The small gap disturbance had significant effects on topsoil content of Cr in downslope natural Rhododendron forest,the soil Cr content in the small gaps of the downslope was significantly higher than that of closed canopies.The soil contents of Pb and Zn in the small gap of mid-slope position,up-slope position,mid-slope and gentle-slope were higher than those of closed canopies,and the contents of Mn and Ni in soil of the small gaps were higher than those of closed canopies at different slope positions and slopes.2) RDA analysis showed that the Mg,elevation and Ca were the most important environmental factors for explaining the distribution of heavy metals in soil of the natural Rhododendron forest.Importance analysis of random forest model indicated that Ni was the most important influence factor for the distribution of heavy metals in soil of small gaps and closed canopies in the natural Rhododendron forest,followed by Cr.3) The single index (except for Ni) and comprehensive potential ecological risk of soil heavy metals in the small gaps were higher than those of closed canopies,and soil Hg and Cd reached higher risk level in the small gaps and closed canopies,while the remaining heavy metals were at low ecological risk.In one word,during the protection and management process of natural Rhododendron forest,more attention should be paid for the prevention and control of Hg and Cd pollution,and the influence of small gap disturbance on the distribution of heavy metals.