Relationship between decomposition of cellulose strips and chemical properties of humus layer in natural boreal forests

The relationships between the decomposition of cellulose strips buried completely within the humus layer (Of + Oh) for two 1-year periods and humus layer properties were studied at forested sites with minimal anthropogenic influence. The 18 study sites are clustered in four catchments located between 61° and 69° N in Finland. The stands were composed of varying proportions of Scots pine, Norway spruce and deciduous species. Of the 23 humus layer properties studied, significant correlations were found between decomposition and soil pH, effective cation exchange capacity, base saturation, C/N ratio, the concentrations of P, Mn, Pb, Zn and Cu. The relationships, however, were inconsistent except for heavy metal concentrations which were positive in all cases. Decomposition was more strongly and consistently correlated to tree stand characteristics (mean height and stem volume) than to the humus layer properties. Both the decomposition of cellulose strips and the concentrations of heavy metals were intercorrelated with climate and the stand characteristics. Partial correlation analysis revealed that the relationships between decomposition and humus layer heavy metal concentrations were more strongly affected by the tree stand characteristics than by climate. After the statistical elimination of the effects of stand characteristics, no significant positive correlations between decomposition and humus layer heavy metal concentrations remained and, in some cases for total and extractable Zn, the correlations became significant and negative. Results from litterfall chemistry (nutrients and heavy metals) also underlined that any specific effects of humus layer heavy metal concentrations on decomposition may be masked if the effects of stand characteristics are not accounted for. We conclude that the relationships between decomposition and soil chemical properties can not be elucidated without considering stand structure and composition of the forest stand.