Humus forms in relation to altitude and forest type in the Northern mountainous regions of Iran

This study focuses on the morphological and chemical development of humus forms as a function of altitude and forest type in the Northern Iran. Terrestrial humus forms related to European Humus Group proposal (EHGP) were evaluated under six common forest types, i.e. Alnus subcordata (AS), Acer velutinum (AV), Fagus orientalis - Carpinus betulus (FC), Carpinus betulus (CB), Fagus orientalis (FO) and Fagus orientalis - Carpinus betulus - Parrotia persica (FCP) stands with different topographic conditions. Within 61 permanent plots, organic (OL, OF, OH), organic-mineral (Ah) and mineral (soil A 0–15; first 15 cm under Ah) layers were collected for identification and chemical analyses. Our results support the idea that altitude and species composition were the two major factors controlling the presence of the different humus forms and their chemical characteristics. While mull was mainly observed at the lowest altitude under AS, moder was usually present at middle altitude under FC and FCP and mor dominated at the highest altitude under FO and FC. Moreover, organic layer thickness also increased with altitude and was higher in the case of mor. On the contrary, the organic-mineral layer thickness was higher at the lowest elevations in the case of mull. The chemical composition of humus layers also differed among forest types. PCA analysis showed that AS and AV forest types were closely related with mull and amphi humus forms, pH, macro nutrients and soil C and N, while FO, FC, and FCP forest types were closely related with moder and mor humus forms, C:N ratio, slope and altitude.