## 4 Discussion
### What Are the Major Consequences of LULC for Ecosystem Functions in the Future?  
The dynamics of cropland has an important impact on agro-ecosystem functions, such as food production and soil nutrient. We found that long-term agricultural land reclamation and farming practices have reduced the soil nutrient. Many studies have shown that in the southeastern part of the APTZNC, SOM drops by 0.248 g kg\({}^{-1}\) per decade, TN decreases by 2.54 mg kg\({}^{-1}\) per decade [79], and in the northeast of the APTZNC, SOM in black soil drops by 0.31 g kg\({}^{-1}\) per decade [54]. After 130 years of development, the SOM content was maintained at a stable level [54]. In our study, the rate of loss in SOM is slightly higher in the next 80 years under the A1B and B1 scenario, which could have resulted from the increase in ERO in the future and the increase in the reclamation area. The main reasons are as follows: Firstly, the agricultural activities have accelerated the loss of SOM. In agricultural land, farming activities, such as reclamation, tillage, and weeding, have improved the soil microbial living environment, making it easier to decompose vegetation, roots, and residues, thereby accelerating the mineralization of organic carbon [80, 81]. Secondly, due to the exposed surface of cropland, especially in the windy spring, the surface SOM is vulnerable to wind, leading to ERO [82]. Thirdly, because of the harvest, the input of SOM is reduced [83]. It indicates that after a certain period of cultivation, the cropland with low SOM may fall to a land use mode that is not suitable for continued cultivation. Therefore, in the evaluation and planning of land use, the loss of soil nutrient content caused by continuous tillage is an important indicator to be considered.  
The ERO across APTZNC was in a mildly eroded level in 2010, whereas that in cropland reached a severe level. When vegetation coverage reduced, the surface roughness also reduced correspondingly [84, 85]. In addition, the reduced amount of underground roots could barely fix the soil and prevent ERO [86, 87]. In the areas with lower SOM and TN, the soil aggregate structure is susceptible to water erosion and wind erosion, which is then stripped and transported [88, 89]. For example, in the A2 and B2 scenarios, croplands account for 33.3% and 26.0%, respectively, resulting in a large averaged ERO in the APTZNC. Not only is the cropland under human disturbance, but the bare land also contributes a lot of ERO, especially in the vicinity of Horqin sandy land, which suffers the most serious soil erosion under all of the four scenarios.  
The minimum NPP appears in cropland under various scenarios, especially under the A2 scenario. Firstly, the cropland under the A2 scenario is mainly distributed in the relatively dry and barren southwestern region, where there is the desert steppe, and the adaptability of crops is poor compared to grass under this kind of condition. Therefore, after the expansion of the cropland, the averaged NPP in the APTZNC is expected to reduce. Secondly, the built-up area expands with a reduction in forest area, resulting in the smallest averaged NPP under the A2 scenario. It should be noted that under the B1 scenario, NPP of cropland is the largest in 2100, because the cropland is found in sporadic patches primarily in the northeastern regions with better water and nutrient conditions. Moreover, the proportion of forest and grassland is much larger, contributing to a higher averaged NPP in the APTZNC.