## 4 Discussion
### Underlying Mechanism Explaining the Effects of Land-Use Pattern on Ecosystem CUE
The higher Rh and Rb of long-term grazing-excluded steppe might come from the long-term accumulation of litter and soil organic carbon, and similar results have been previously observed [45; 49]. The high level of microbe activity and Rh of cultivated area in our research could be attributed to the application of large amounts of farm manure (approximately 240 kg ha\({}^{-1}\)). Therefore, the correlation between Rh and Rb and CUE in our case might be occasional. The root respiration, as a component of Ra and Re, showed no significant correlation with CUE (Figure 8d). Previous studies have rarely considered Rr, because it is a very small proportion of Re and is hard to accurately determine [24; 50]. Similarly, we did not find a comparable conclusion on the relation between Rr and CUE. This insignificant correlation result may be attributed to the low proportion of Rr in Re and Ra, at around 6% to 20% respectively. Therefore, the mild variation of Rr could not change Ra and Re sufficiently to have a notable impact on ecosystem CUE. The change of respiration and its components mechanically explained variation of CUE under different land-use patterns. These impacts on CUE would further regulate the carbon budget and vegetation productivity of the ecosystems [51]. Moreover, a significant negative relationship existed between the LAI variation among land uses and CUE (Figure 9a, \(p<0.001\)). The LAI is a major explanatory factor for CUE changes under different land uses of steppe (91%, Figure 9a). Although it was slightly lower than Ra (Figure 8b, 97%), LAI still explained more of the CUE variation than Re (Figure 8a, 84%), whereas previous research has mostly focused on the contribution of Re [20; 24; 52]. The relation between LAI and productivity has been proven repeatedly and applied in biomass modeling with remote-sensing data [53; 54]. In addition, the positive relationship between productivity or biomass with Ra in ecosystems has been widely reported [55]. Consequently, higher LAI is commonly related to a higher productivity and biomass level, a higher Ra level, and the strong negative relationship between Ra and CUE (Figure 8b). This series of associated correlations could mechanistically explain the significant negative relationship between LAI and CUE. Our further analysis indicated that LAI was significantly positively correlated with Re, Ra and Rab (Figure 8, \(p<0.001\)) and the LAI change explained more than 90% of the variation of Re and Ra among different steppe uses. This suggests that the negative relationship between LAI and ecosystem CUE may be a general rule in the situation of land use-induced vegetation-cover change. However, it should be realized that this regulation may only be applied to specific situations (e.g., land-use change of steppe) or varied vegetation in adjacent areas. As with the conclusion of [PERSON] et al. [39], we suggest that consistent relations between varied vegetation types and their ecosystem functional traits may rarely exist across distant regions or across a traditional vegetation biome. The regulatory mechanism and our understanding of how land use and management influence ecosystem CUE must be examined over a greater range of ecosystems. This will improve the simulation of the CUE change with different land-use scenarios, thus promoting the accurate prediction and modeling of the variation of carbon budget and productivity caused by potential land-use change. Thereby, through scientific management of land use, sustainable land-use plans could be applied to optimize the regional carbon budget, which would contribute to the adaptation to and mitigation of climate change. We thank [PERSON] and [PERSON] for their discussion and helpful suggestions on the early version of the manuscript. This research was jointly funded by the National Key R&D Program of China (grant no. 2017 YFA0604801) and Natural Sciences Foundation of China (41601615). In addition, the authors thank [PERSON] for his help in improving the quality of the figures.