## 4 Discussion  
### Effect of Different Land-Use Patterns on Ecosystem CUE  
Previous research has shown that the ecosystem CUE varies from 0.23 to 0.83, with an average CUE of 0.53 among 60 different vegetation types [20; 35], most of which were forest. For the three land-use patterns in our study, the lowest CUE occurred in cultivated steppe (0.57) and the highest was  
Figure 9: Relationships between ecosystem (**a**) CUE, (**b**) Re, (**c**) Ra and (**d**) Rab on leaf-area index (LAI). The \(R^{2}\) and significance levels of the regressions are shown; ***\(p<0.001\).  
the grazing-excluded steppe (0.74), which were within the range of previous studies, but were higher than average. We suggest that the CUE of steppe ecosystems, although under different management patterns, is typically higher than forest ecosystems, which may be attributed to their lower proportion of non-photosynthetic biomass [36]. The non-photosynthetic tissues of plant increased the ratio of respiration to gross production [21], which would finally decrease the CUE. It was previously believed that the CUE of different vegetation types varied around 0.5 [20]; however, in recent years, with the development and accumulation of observation data of ecosystem fluxes, numerous results have indicated that CUE varies considerably among different ecosystems [20; 36]. Nevertheless, models developed in the late 20 th century were based on the presumption that the ecosystem CUE was a relatively stable value around 0.4-0.6, resulting in the inconsistence between modeled and observed CUE [37].  
There was no significant difference between the CUE of _Stipa grandis_-dominant ecosystems (grazed steppe, 0.70 and grazing-excluded steppe, 0.74), while both of them are significantly higher than the wheat-dominant ecosystem (cultivated steppe, 0.57, Figure 7). These results verified the following two assumptions proposed in previous studies. First, [PERSON] et al. [17] showed that ecosystem CUE in forests is influenced by the biomass allocation difference among different vegetation types. The ecosystems with higher biomass proportion in leaves would have a higher CUE than ecosystems with a greater belowground biomass or other non-photosynthetic proportion. Previous findings also indicated that _Stipa grandis_-dominant steppe had a higher biomass proportion in leaves [12; 16]. Moreover, [PERSON] et al. [36; 37] indicated that, compared with traditional biome differences, the differences of plant traits may better explain ecosystem functional variations, including CUE. In our study, the land-use pattern change in natural steppe was its transformation to domesticated crops, substantially altering community composition and plant traits, and consequently influencing the inherent ecosystem CUE and potential other key ecosystem functional characteristics [38]. Thus, we confirmed that the changes of ecosystem species composition and, especially, dominant plant inherent traits caused by human land-use or management patterns could have a major influence on ecosystem functional properties, and such large-scale land-use and management change may influence regional or even global carbon balance and climate [39; 40].