AbstractsBiology & Animal Science

This thesis was conducted within the scope of a graduation fellowship from the state of Baden-Württemberg, Germany (Grant No.: VI 4.2-7631.2/Baumann) in cooperation with the Depart-ment of Ecology, Peking University, Beijing. Scientists specialised in both ecology and soil science investigated the same sites, thus allowing an interdisciplinary approach to evaluate soil properties, C and N cycles as well as geomorphological processes in close connection to ecosys-tem interrelations on the Tibetan Plateau. The research sites are located along a 1,200 km long north-south transect at altitudes between 2,925 and 5,105 m ASL. Two thirds of the Tibetan Plateau is influenced by permafrost. Due to the high sensitivity to global climate warming and land use changes, permafrost degradation processes are widespread, increasing the heterogeneity of soil formation, soil hydrology, and related soil chemical processes (i.e. C and N cycling). In order to account for the resulting extremely diverse ecosystem, investigations at different spatial scales related to large-scale climate patterns were performed. The scales comprise the total main transect, the split transect into an eastern and western section, diverse catenas along distinct geomorphological relief units, and finally the single site soil profiles. The first part of this work examines C and N contents as well as portions of plant available min-eralised nitrogen in relation to their main influencing parameters. For investigations on land-scape scale, soil moisture was found to have the strongest effect on C and N cycling, followed by CaCO3-content and soil texture. Altogether, the general linear model explains 64% and 60% of the variation of soil organic carbon (SOC) and total nitrogen (NT) contents, respectively. Thereby, two aspects are important: (1) temperature variables have no significant influence and (2) indicators for soil development (i.e. CaCO3 and soil texture) are included besides commonly con-sidered ecological (i.e. moisture, temperature and biomass) parameters. It could be shown that in the highly diverse permafrost-affected ecosystem of the Tibetan Plateau, other factors than precipitation mainly control soil moisture contents and distribution, with permafrost and relief position being the most dominant parameters. Since pedogenic parameters turned out to be important predictors, the degree of soil development can be regarded as an additional control quantity, indicating higher C and N contents of topsoils with longer duration of undisturbed and stable soil development. Mineralised plant available N can be almost exclusively found as am-monium-N, which is closely related to higher soil moisture contents and frigid climate condi-tions, showing by far the highest contents in the permafrost main soil group. As nitrification is strongly temperature dependent, nitrate-N contents are correspondingly very low. The results provide clear evidence that limitation in plant available nutrients as a negative feedback to lower soil moisture is crucial for plant growth in…