Effect of topography on the composition of soil organic substances in a perhumid sub-tropical montane forest ecosystem in Taiwan

The distribution of soil organic substances at five topographic sites was evaluated in the undisturbed mountainous forest of the Yuanyang lake ecosystem, northeastern Taiwan. The high annual rainfall there (>4000 mm) contributes to extreme acidity of the soil, which has a thick organic layer. The soil organic substances are classified as water soluble organic carbon (WSOC), microbial biomass carbon (Cmic), fulvic acids, and humic acids. There was a significant correlation between Cmic and WSOC in soil profiles along the landscape. Distribution of WSOC was consistent with total organic carbon, decreasing from the surface to the deeper horizons and increasing from the summit to the footslope. The majority of soil extractable organic substances was fulvic acids, which showed a significant downward migration to deeper mineral horizons. Fulvic acids contributed to up to 4.00% and 41.62% of total organic C in the organic and mineral layers, respectively, and the footslope site had the highest amount. Humic acids accumulated significantly in the A and E horizons of the relatively well-drained summit and shoulder sites, while only small amounts were contained in horizons of sites with poor drainage. Compared to fulvic acids, few humic acids were present in the mineral horizon. The results demonstrate that not only topography but also the intrinsic properties of each fraction of soil organic matter affects distribution and migration of organic substances. The perhumid climate conditions may also hasten differentiation of the distribution in soil profiles and the landscape.