Altitudinal Change in Distribution of Soil Carbon and Nitrogen in Tibetan Montane Forests

Knowledge of vertical patterns of soil organic C (SOC) and soil total N (STN) can improve our understanding of the dynamics of SOC and STN along a profile and the potential response of SOC and STN to climate change. However, such knowledge is limited. In this study, three common Tibetan forest types, dominated by Abies, Pinus, and Picea species, were sampled across a large altitude gradient (1700-4300 m asl). Vertical depths of SOC and STN in these forests were determined, and their changes with altitude, aspect, and gradient and the associated regulating factors were analyzed using structural equation modeling (SEM). Species shift along the altitude gradient has a significant effect on SOC and STN depth patterns, partly inducing the lack of a trend in SOC and STN vertical distributions across the entire altitude range. For an individual genus, the SEM model demonstrated species-dependent altitudinal changes in patterns that were shallower with increased altitude for Abies and Pinus, but no change was detected for Picea. Greater altitudinal increases in SOC and STN stocks in the topsoil relative to the deeper layer were correlated with increased depths for Abies and Pinus. Eastern and southern aspects had a greater proportion of topsoil SOC and STN than western and northern aspects for Abies and Pinus. Gradient had a limited effect on SOC and STN vertical distribution across the three genera. Our results indicate a significant change in SOC and STN depth patterns associated with species change and the topographic effects on these vertical patterns.