## 4 Discussion
### Underlying Mechanism Explaining the Effects of Land-Use Pattern on Ecosystem CUE  
The relationships between CUE and biological or non-biological environmental factors have been extensively studied at various temporal and spatial scales. Previous research has already promoted deep understanding of how artificial experimental temperature increases change the ecosystem CUE by influencing soil organic carbon release [24; 34; 36]. Also, using an experimental approach, [PERSON] et al. [41] studied the influence of drought on CUE, which suggested that severe drought restrained the respiration of plants and soil microbial of ecosystems, causing the decrease of Re, and finally decreased the ecosystem CUE. This finding indicated that the CUE decrease of the ecosystem was caused by a water shortage. These studies have already promoted the construction of more accurate models, which has helped to advance the discussion and prediction of the influence of climate change on ecosystem CUE and the carbon budget in different future scenarios. However, it remains difficult to summarize the influence of community plant functional type and biome type change on CUE [42; 43], as traditional vegetation type variations are barely correlated with their different CUE at large spatial scale. This situation suggests that we may achieve novel findings from different vegetation categories [44], such as land-management and -use patterns. In our study, we also focused on the environmental factors that influenced ecosystem CUE and the regulation mechanisms. With reference to [PERSON] et al. [41] on the influence of drought on CUE, and other research on the influence of microbial biomass, diversity or forest age on CUE, it has generally been indicated that the Re variation correlates with CUE change [34; 36; 45]. This helps us to focus on revealing the process governing the influence of Re on CUE, and analyzing how the relationship between Re composition changes with CUE variation under different steppe land-use patterns. Our results indicated that, even though the GPP of grazed and grazing-excluded ecosystems showed no significant difference (Figure 4), CUE and NPP significantly varied between the two ecosystems (Figures 4 and 7). This phenomenon may be attributed to the significant gap of respiration between grazed and grazing-excluded ecosystems (Figure 6), which suggests that a substantial change of respiration may be the key influence on ecosystem CUE, correspondingly causing a severe impact on carbon fixation and productivity [23; 46; 47]. The linear regression analysis between CUE and Re or the Re components also supported this assumption, since our results showed that Re and its components (Ra, Rab, Rb and Rh) were extremely significantly correlated with CUE (Figure 8, \(p<0.001\)). The change of Re explained 84% of CUE change, which is consistent with previous studies [34; 36]. It is more noteworthy that the changes of Ra were able to explain a large proportion (97%) of the variation of ecosystem CUE (Figure 8b). This may be because NPP represents the remainder after Ra is consumed from GPP, while ecosystem CUE is the ratio between NPP and GPP. Consequently, the accurate estimation of Ra and its change should inherently be highly related to the change of CUE. According to our results, the variation of ecosystem CUE may be primarily influenced by the change of the Ra consumption of plants themselves. The high correlation of Rab to CUE (91%, Figure 8c) was assumed to rely on the Ra regulation mechanism mentioned above, as in steppe ecosystems, Rab was the major part of Ra and the intensity (Figure 6) and proportion of Rr to Ra [24; 48] were both low. Though Rh and Rb were both significantly correlated with CUE (Figure 8e,f), we contend that this correlation is not a universal phenomenon. This is because we surmised the stronger Rh and Rb in grazing-excluded steppe and cultivated steppe had different causes.