Temperature drive the altitudinal change in soil carbon and nitrogen of montane forests: Implication for global warming

As a matter of co-varying environmental and biogeochemical factors, controversy remains regarding the altitudinal patterns in soil organic carbon (SOC) and soil total nitrogen (STN) stocks of mountain forests. Here, three dominant montane forests in southern Tibet, namely Abies, Pinus and Picea forests, were heavily sampled from ca. 1700 to 4300 m asl to examine the altitudinal patterns in SOC and STN stocks and the co-varying effects of mean annual temperature (MAT), mean annual precipitation (MAP), tree species and soil type. Overall, SOC and STN stocks in the top 100 cm increased linearly by 54 Mg C/ha and 5 Mg N/ha per 1000 m altitude increase, respectively. The altitudinal trends resulted partly from the tree species shift with altitude, because Pinus forests dominated at lower altitudes (1709 to 3585 m asl) and had generally lower SOC and STN stocks than Abies forests, which dominated at higher altitudes (2615 to 4217 m asl). SOC and STN stocks increased with altitude in Abies (most pronounced) and Pinus forests, whereas no clear altitudinal trend was observed in Picea forests. Soil type was another controlling factor. SOC and STN stocks under similar soil types for each tree species showed universally increasing trends with increasing altitude. All the observed altitudinal trends in SOC and STN stocks were primarily controlled by decreasing MAT with increasing altitude. Our results therefore emphasize that soil C and N of montane forest may be sensitive to future global warming, especially at higher altitudes, currently dominated by Abies forest.