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

我们之前对不同年龄的大块植被、土壤和羊毛的分析表明，在大约。在20世纪60年代和90年代发生重大社会经济变化后不久，面积为500 x 300平方公里的C4丰度增加了。这创造了一个明显的空间结构，特别是在大城市周围100公里的范围内，C4丰度增加，随着与城市距离的增加呈指数下降。该提案将(i)继续并扩展我们过去对羊毛（+粪便）样本的研究，但将其扩展到林分尺度、西林流域尺度和国家尺度；（ii）通过分析植被，有助于对观察到的现象进行机械理解。分析干旱梯度、放牧梯度和地形梯度符合MAGIM第二阶段的目的。它将主要检验目前的C4丰度是由两个相反的趋势造成的假设：空气中二氧化碳浓度增加引起的减少和过度放牧引起的增加。过度放牧是由（区域差异的）社会经济条件造成的。因此，这两种趋势形成了C4丰度减少或增加的复杂区域格局。过度放牧对C3/C4竞争的主要影响是改变了植被的供水量。

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.