Effect of grazing on C pools and fluxes in Tibetan grassland revealed by 13C labeling
2 Faculty of Geography, University of Marburg
3 Institute of Soil Science, Leibniz University Hannover
P 1.15 in Ecosystem Function
Within the Qinghai-Tibetan Plateau (TP), which is one of the largest geomorphological units on the Eurasian continent, the Kobresia pastures represent the worlds largest montane-alpine ecosystem. The Kobresia pastoral ecosystems, appeared to be quite stable during the last centuries or millenia of nomadic animal husbandry. Since 1959 land use changes (sedentarisation programs) lead to pronounced overgrazing and land degradation with consequences for the vegetation and soils. The Kobresia and other grassland species are probably increasingly replaced by blue-green algae and crustose-lichens (crust) which may alter the carbon (C) dynamics and have an impact on the regional and global C cycles. This led to the hypothesis that the C dynamics, C stock and soil respiration differs on these crust areas compared to grass-grown. Dissolved organic carbon (DOC) like root exudates provides the main source of C for soil microorganisms and may be the most dynamic component of soil C. Root exudates as a more dynamic C pool could react more sensitively to changes and reveal these. The objectives of the present study were to compare the C dynamics of components of root exudates (glucose, glycine and acetic acid), C stock and soil respiration between two contrast patches of vegetation: (i) blue-green algae and crustose-lichens (crust) and (ii) grass-grown with predominantly Kobresia and Stipa species (without crust), in a montane-alpine grassland (Kobresia-Stipa pasture) on the TP.
After addition of 13C labeled root exudates (glucose, glycine and acetic acid), which representing the main groups of low molecular weight organic substances (LMWOS) in soil, we followed the subsequent evolution of 13CO2 (decomposition of 13C-labelled substances and metabolites), fate of 13C in soil and uptake of LMW dissolved organic carbon (DOC) by plants, during 29 days after labeling.
The substances differed concerning decomposition, plant uptake and remain in soil. Differences in C dynamics were found between the two contrasts patches of vegetation developed under contrast land use. Between a third and two thirds of exudates remained in soil after 29 days of labeling. A proportion of these is stabilized and improves the soils in physical and chemical sense.