3.2 Cluster analysis of SOC and TN by multivariate regression trees
Both soils organic carbon contents and total nitrogen were divided into four groups. The largest group was around 3.32 g/kg (145 sites), when precipitation was lower than 371.5 mm. Meanwhile, air temperature and altitude were higher than 1.49 oC and 2793 m (Figure 3). The second group was approximate 6.48 g/kg including 52 sample sites when air temperature was lower than 1.49 oC. SOC contents were relative higher about 13.7 g/kg when precipitation was 371.5 mm.
The largest group included 161 sample sites approximate 3.48 g/kg of total nitrogen when precipitation was under 371.5 mm, and temperature was higher than 1.49 oC. Meanwhile, altitude threshold was 3655 m. Total nitrogen was nearly 12.8 g/kg at higher altitude. Furthermore, soil total nitrogen seemed higher under higher altitude and lower temperature conditions.