Distribution and mineralization of organic carbon and nitrogen in forest soils of the southern Tibetan Plateau

•OC content did not vary among the aggregate size classes and forest type.•The mineralization of OC and N were not affected by forest types.•The mineralization showed similar dependence on soil OC and N between forest types.•N addition did not affect the mineralization of OC.

The forests of the Tibetan Plateau store large amounts of soil organic carbon (OC) but are among the most vulnerable and sensitive ecosystems to environmental change. The lack of knowledge regarding the distribution and turnover of OC and nitrogen (N) in Tibetan Plateau forest soils limits the ability to predict how this ecosystem will respond to climate change. In this study, we collected mineral soils from coniferous and broadleaf forests on the southern Tibetan Plateau and measured OC and N contents in both bulk soils and water-stable aggregates. We also determined the mineralization of OC and N in bulk soils and examined the effects of N addition on OC mineralization. Our objectives were to investigate the distribution and mineralization of soil OC and N in various forest types and to determine the stability of OC following N addition. Our results showed that OC and N in macroaggregates contributed 76% of the OC and N in bulk soils. Forest type did not affect the OC or N contents of either bulk soils or aggregates. Similarly, OC and N mineralization and their relationships with soil OC or N contents were similar between broadleaf and coniferous forests, indicating that soil OC and N distribution and turnover were insensitive to forest type. Nitrogen mineralization was dominated by ammonification in these forest soils. Nitrogen addition did not affect OC mineralization or its relationship with soil OC or N contents. These results indicate that the OC contents of forest soils of the southern Tibetan Plateau are relatively insensitive to N addition.