Abstract

Xingkai Xu, Hongyan Miao and Zhe Chen

Many unmanaged temperate and tropical old-growth forests are shown to retain their capability to sequester carbon (C) from the atmosphere, but the C sequestration mechanisms in the soil were unknown. Seven thin soil layers within a depth of 20 cm were sampled from one unmanaged tropical seasonal old-growth rain forest and three unmanaged temperate old-growth forests (e.g. Betula ermanii, Picea jezoensis, and Pinus koraiensis mainly mixed with Tilia amurensis, Fraxinus mandshurica and Quercus mongolica) in southwest and Northeast China, respectively, to study the vertical distribution and their relationships of dissolved and total organic matter, microbial biomass C and nitrogen (N), and exchangeable metals concerned in forest soils. There were larger concentrations of all C and N pools and higher pH values in uppermost soil layers under three temperate oldgrowth forests than in corresponding layers of tropical forest soil, and the concentrations in temperate forest soils decreased more with increasing soil depth. Among the three temperate forests, the soils at less than 10 cm depth under the pine and broadleaf mixed forest had the smallest total C -to -N ratio, and the largest dissolved organic N concentration occurred in the 0 - 2.5-cm soil. Contrary to the exchangeable Al and Pb, the concentrations of exchangeable Ca, Mg and Ba in the 0 to 20 cm profile soils under the three temperate old-growth forests were larger than those under the tropical old-growth rain forest, and the largest values were observed in the profile soils under the pine and broadleaf mixed forest. Hence, the concentration ratios of Ca to Al and (Ca+Mg) to Al in the temperate forest profile soils were larger than those in the tropical forest profile soils. Maximal concentrations of all tested metals (excluding Al and Pb) were observed in uppermost soil layers under the four old-growth forests and varied with tree species. According to the principal component analysis and regression analysis, the vertical distribution of dissolved organic C and organic N concentrations in the soil at the various depths is a function of the decreasing microbial activity and is partly associated with the metals of concern. Together with the soil C availability and microbial metabolic quotient reported earlier, the results would improve our understanding of the C sequestration mechanisms involved in these old-growth forest soils.