ALBERT

Description: Colluvial deposits consisting of silts and loams were detected in several climatologically different areas of NE Tibet (3200-3700 m a.s.l.). Layering, distinct organic content and low content of coarse matter as well as location in the relief revealed an origin from low-energy slope erosion (hillwash). Underlying and intercalated paleosols were classified as Chernozems, Phaeozems, Regosols and Fluvisols. Fifteen radiocarbon datings predominant on charcoal from both colluvial layers and paleosols yielded ages between 8988 ± 66 and 3512 ± 56 uncal BP. Natural or anthropogenic factors could have been the triggers of the erosional processes derived. It remains unclear which reason was mainly responsible, due to controversial paleoclimatic and geomorphic records as well as insufficient archaeological knowledge from this region. Determinations of charcoal and fossil wood revealed the Holocene occurrence of tree species (spruce, juniper) for areas which nowadays have no trees or only few forest islands. Thus large areas of NE Tibet which are at present steppes and alpine pastures were forested in the past.

Description: Vast areas on the Tibetan Plateau are covered by alpine sedge mats consisting of different species of the genus Kobresia. These mats have topsoil horizons rich in rhizogenic organic matter which creates turfs. As the turfs have recently been affected by a complex destruction process, knowledge concerning their soil properties, age and pedogenesis are needed. In the core area of Kobresia pygmaea mats around Nagqu (central Tibetan Plateau, ca. 4500 m a.s.l.), four profiles were subjected to pedological, paleobotanical and geochronological analyses concentrating on soil properties, phytogenic composition and dating of the turf. The turf of both dry K. pygmaea sites and wet Kobresia schoenoides sites is characterised by an enrichment of living (dominant portion) and dead root biomass. In terms of humus forms, K. pygmaea turfs can be classified as Rhizomulls mainly developed from Cambisols. Wet-site K. schoenoides turfs, however, can be classified as Rhizo-Hydromors developed from Histic Gleysols. At the dry sites studied, the turnover of soil organic matter is controlled by a non-permafrost cold thermal regime. Below-ground remains from sedges are the most frequent macroremains in the turf. Only a few pollen types of vascular plants occur, predominantly originating from sedges and grasses. Large amounts of microscopic charcoal (indeterminate) are present. Macroremains and pollen extracted from the turfs predominantly have negative AMS 14C ages, giving evidence of a modern turf genesis. Bulk-soil datings from the lowermost part of the turfs have a Late Holocene age comprising the last ca. 2000 years. The development of K. pygmaea turfs was most probably caused by an anthropo(zoo)-genetically initiated growth of sedge mats replacing former grass-dominated vegetation ('steppe'). Thus the turfs result from the transformation of pre-existing topsoils comprising a secondary penetration and accumulation of roots. K. schoenoides turfs, however, are characterised by a combined process of peat formation and penetration/accumulation of roots probably representing a (quasi) natural wetland vegetation.

Description: The Tibetan highlands host the largest alpine grassland ecosystems worldwide, bearing soils that store substantial stocks of carbon (C) that are very sensitive to land use changes. This study focuses on the cycling of photoassimilated C within a Kobresia pygmaea pasture, the dominating ecosystems on the Tibetan highlands. We investigated short-term effects of grazing cessation and the role of the characteristic Kobresia root turf on C fluxes and belowground C turnover. By combining eddy-covariance measurements with 13CO2 pulse labeling we applied a powerful new approach to measure absolute fluxes of assimilates within and between various pools of the plant-soil-atmosphere system. The roots and soil each store roughly 50% of the overall C in the system (76 Mg C/ha), with only a minor contribution from shoots, which is also expressed in the root:shoot ratio of 90. During June and July the pasture acted as a weak C sink with a strong uptake of approximately 2 g C/m**2/ in the first half of July. The root turf was the main compartment for the turnover of photoassimilates, with a subset of highly dynamic roots (mean residence time 20 days), and plays a key role for the C cycling and C storage in this ecosystem. The short-term grazing cessation only affected aboveground biomass but not ecosystem scale C exchange or assimilate allocation into roots and soil.

Description: In the Eastern Central Himalaya (Langtang Valley) and its southern declivity (Helambu), palynological and phytosociological studies were carried out with the aim of reconstructing the natural vegetation pattern and elucidating the development of replacement communities between the upper cloud forest belt (2000-4000 m a.s.l.), and the alpine belt (ca. 4000-5100 m a.s.l.). According to palynological studies and historical information, the remote area under study was first influenced by man during the 15th century, presumably by hunters who set fire to the forests which were dominated by Quercus semecarpifolia, Tsuga dumosa, Abies spectabilis and Betula utilis. Pteridium aquilinum increased and Juniperus decreased as a result of the fires being set. The only permanent settlement of the upper Langtang Valley (Langtang village) is situated at 3450 m a.s.1. at the transition between the middle and upper cloud forest belt and was founded during the 17th century. A Rhododendron lepidotum dwarf shrub appeared as the first replacement community for the forest. From the continuous influence of fire, forest clearance and grazing pressure, the Rhododendron shrub was partly replaced by pastures and nitrophlous communities in places near the settlement. Permanent settlements never existed at and above the timberline. Here, the anthropo-zoogenous influence is indicated by plants favored by fire or grazing. Under natural conditions, the altitudes between 3000 and 3500 m a.s.l. seem to have been occupied by more Quercus semecarpifolia than was expected based on the present day situation. In addition to this, it was possible to trace a late increase of Abies spectabilis (19th century). The forests at the timberline were dominated by Betula ufilis before and after the beginning of human impact.

Description: Kobresia pygmaea pastures on the Tibetan Plateau face destruction while storing 2.5% of the global soil organic carbon (SOC). Representative soil samples were taken from soils of six successional degradation stages (S0 – S5) at Nagqu close to the KEMA research station on the Tibetan Plateau at 4,484 m a.s.l. in late summer 2013 and 2015. SOC stocks decrease from intact pastures (S0) to severely degraded stages (S5) by 70%. The dataset indicates SOC and nitrogen (N) losses but also includes changes in their isotopic composition. Furthermore, pH, CEC, element contents and the lignin monomers (vannillyl, syringyl, and cinnamyl (VSC) were quantified and point towards a change of soil properties. These changes affect the microbial community structure of bacteria and fungi analyzed via terminal restriction fragment length polymorphism analysis (t-RFLP). Illumina MiSeq sequencing data of the microbial community are available under the project number PRJNA626504 at the NCBI. Functional implications of these shifts in microbial community structure were assessed by identifying pronounced alterations in enzyme activities for β-glucosidase, xylanase, urease, alkaline phosphatase and phenol oxidase. Structural and functional shifts as adaption to SOC losses define a “tipping point” where the magnitude of changes in the soil-microbiome interaction of one of the world's largest grassland ecosystem exceeds a threshold that not only alters this system permanently, but which also diminishes its function as CO2-sink triggering cascading climate effects up to the global scale.