Influence of grazing pressure on the carbon isotope composition of the grasslands of China: Spatio-temporal variations at multiple scales

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Our previous analyses of bulk vegetation, soil and wool differing in age have shown that in an area approx. 500 x 300 km² in size the C4 abundance increased shortly after major socio-economic changes in the 1960s and 1990s. This created a pronounced spatial structure with an increased C4 abundance especially within a circle of 100 km around large cities exponentially decreasing with increasing distance to the city. The proposal will (i) continue and extend our past work on wool (+ feces) samples but extend it to the stand scale, the Xilin catchment scale and the country scale and (ii) it will contribute to a mechanistic understanding of the observed phenomena by analyzing vegetation. It fits to the intention of the second phase of MAGIM to analyze aridity gradients, grazing gradients and topographical gradients. It will mainly examine the hypotheses that the present C4 abundance results from two opposing trends: a decrease caused by the increasing CO2 concentration in air and an increase caused by overgrazing. Overgrazing results from (regional differing) socio-economic conditions. Both trends thus create a complex pattern of regions with decreasing or increasing C4 abundance. A main effect of overgrazing influencing the C3/C4 competition is that it changes the water supply of the vegetation.