生长条件对苔藓硫含量和硫同位素组成指示大气硫沉降的影响

Influences of Growing Condition on Sulfur Concentration and δ(^(34)S) of Mosses for Indicating Atmospheric Sulfur Deposition

对相同大气沉降和土壤背景但生长条件不同的苔藓w(S)和δ(34S)进行了分析.土生苔藓w(S)高于石生苔藓,表明土壤对苔藓w(S)指示大气硫沉降有影响.受树冠遮挡的苔藓w(S)偏低反映了树冠吸收会削弱冠下的大气硫输入,而不同类型树冠对大气硫的截留吸收程度不同,这可能是造成树冠下方苔藓w(S)存在差异的主要原因,但随树冠厚度变化无明显规律.石生苔藓δ(34S)与大气硫δ(34S)吻合,而土生苔藓δ(34S)明显受土壤硫的影响而偏正.此外,树冠下苔藓δ(34S)偏高还与树冠吸收大气硫的过程中发生34S歧视有关,苔藓δ(34S)随树冠厚度增加而偏正,进一步证实了树冠的长期吸收会使沉降到树冠下的大气硫源δ(34S)偏正.因此,受生长条件歪曲了的苔藓w(S)和δ(34S)不能用于解释大气硫沉降的变化和来源,开阔地的石生苔藓比土生苔藓和受树冠遮挡的苔藓更具有大气指示意义.

Sulfur content and δ（34S） of mosses under different growing conditions but with the same atmospheric deposition and soil background were investigated. Sulfur content of soil-growing mosses was found significantly higher than that of epilithic mosses, showing that soil substratum has influence on using mosses sulfur for indicating atmospheric sulfur deposition. Mosses under canopies have generally lower sulfur content than those at open sites, indicating that canopy retention has caused a significant reduction of atmospheric sulfur inputs. Besides, mosses have varying sulfur content under different canopies, which was related to the retention capacities of canopies, but no correlation was seen between mosses sulfur and canopy thickness. ^（34 S） of epilithic mosses was very close to that of atmospheric sulfur deposition, while less negative δ（^34 S） for soil-growing mosses was significantly influenced by soil-derived sulfur. Moreover, δ（^34 S） values of mosses under different canopies were related to the ^34S discrimination during canopy retention of atmospheric sulfur deposition, which was confirmed by the positive linear relationship between canopy thickness and mosses δ （^34 S）, suggesting that long-term canopy absorption of atmospheric sulfur will make the ^（34S） of atmospheric sulfur more positive. Consequently, sulfur content and δ（^34S） of mosses distorted by growing conditions could not be used to interpret the level and sources of atmospheric sulfur deposition, and epilithic mosses at open sites were more valuable and reliable than those grown on soil and shed by canopies in atmospheric sulfur deposition research.