中美土壤元素背景值调查研究中数理统计方法运用及影响

Application and Impact of Statistic Methods on the Soil Background Values in China and the United States

土壤元素背景值是统计性的范围值,是评价土壤环境质量和制定土壤环境质量标准的重要参考依据,其因土壤母质、成因以及各种地球化学作用的影响而不同.通过分析20世纪60年代至90年代中美土壤元素背景值调查研究中数理统计方法的运用,发现当时受国内外学者对土壤元素含量及统计分析方法认识的局限,中美在土壤元素背景值研究中存在一些偏态分布被当作正态分布、对称剔除偏态分布的异常值和用平均值计算偏态分布背景值等问题,可能导致所得土壤元素背景值不能很好地代表区域背景情况.2007-2013年美国再次开展全国土壤调查和土壤元素背景值研究,对数理统计方法做出了改进和优化,主要包括:①用箱式图法分析距离中间50%数值较远的异常值;②针对对数分布形态也进行了分析;③用中位数和绝对中位偏差计算偏态分布的土壤元素背景值.在研究方法有了新的认识和发展的今天,建议中国优化土壤元素背景值数理统计研究方法,再次开展全国土壤元素背景含量调查工作,并从以下两个方面加以改进:①采用箱式图剔除异常值;②针对不同的频数分布形态计算相应的统计量作为土壤元素背景值.

The background values of soil elements were generally derived from the statistical analysis of the soil survey data,which can provide an important reference for the development of soil quality standards and evaluation of soil quality.The background values are influenced by a range of factors,including parent material,soil genesis,and geochemical effect.The limitations of the knowledge of soil element contents and statistical methods proposed by domestic and foreign scholars were discussed based on the analysis of the national soil background values survey in China and the USA from the 1960s to 1990s.The statistical methods for the background values derivation were somewhat problematic in China and the USA.For example,the skewed distribution was mistaken for the normal distribution,the outliers were eliminated symmetrically in the skewed distribution,and the average values were used as the background values in the skewed distribution.Therefore,the background values obtained cannot represent the regional background contents exactly.Improvements and optimizations were made in the USA in the latest national soil background values survey and research in 2007-2013.First,the outliers away from the middle 50%of the data were analyzed by box-plot.Second,the dataset was analyzed to follow normal distribution after taking logarithm.Third,the median and the median absolute deviation were used as the background values in the skewed distribution.In view of the above deficiencies,this study proposed to optimize the mathematical statistical method and conduct further national survey on the soil background values for China.First,the outliers should be eliminated by box-plot.Second,the frequency distribution should be demonstrated before the background values obtained.Third,the different values should be used as the background values in various frequency distribution.