## 1 Introduction  
Increased greenhouse gas (GHGs) emissions from human activities have caused significant concerns around the world [1]. As the largest component in GHGs, more than 10 PgC (1 PgC = 10\({}^{15}\)gC) is released into the atmosphere each year from human-related activities such as fossil fuel combustion and cement manufacture [2]. Green vegetation, on the other hand, absorbs carbon dioxide (CO\({}_{2}\)) through a biotic process called photosynthesis, which sequesters CO\({}_{2}\) out of the atmosphere [3; 4]. However, a significant increase in GHGs emissions in the past few decades has broken this emission-absorptionbalance and led to increased atmospheric CO\({}_{2}\) concentration, which is believed to be the main cause for global warming [5]. Facing such a challenge, international cooperation has been called for to cut down carbon consumption and reduce carbon release into the atmosphere [6]. Unfortunately, the increasing trend of the atmospheric CO\({}_{2}\) concentration did not stop. Along this track, it is forecasted that the global temperature would rise more than 3 \({}^{\circ}\)C by the end of the 21 st century [6], which imposes a global catastrophic risk to human living environments. Hence, innovative work must be done to address the continuously accumulative CO\({}_{2}\) in atmosphere due to the imbalanced emissions and absorptions in GHGs. Improving carbon sequestration from vegetation may provide a prominent way to this goal. The net carbon sequestration capacity from vegetation can be reflected from the dynamics of net primary productivity (NPP), which is the net amount of solar energy converted to plant organic matter through photosynthesis in a given period [3; 7]. Terrestrial ecosystems are featured by vegetation biomes capable of sequestering varied amounts of carbon. For example, forestry usually but not necessarily produces more NPP than croplands given the same area [8]. Converting land-use types, e.g., from farmlands to forests, may help secure carbon sequestration [9]. However, it is impractical to resort to land-use transformation, because reduced farmland means less food supply, which is not acceptable to sustain a local economy. Whether it is possible to improve vegetation productivity without converting land cover use is a valuable topic that should be investigated. Furthermore, it is meaningful to know the sensitive area that possesses the most potential in the enhancement of carbon sequestration from vegetation. Natural grasslands and savannas occupy nearly half of the terrestrial globe and provide important economic and ecological services to modern societies [10]. Unfortunately, a considerable proportion of the global grasslands shows degraded vegetation, e.g., reduced vegetation productivity [11], land desertification [12], or loss of biodiversity [13]. This could be attributed to climate constraints (e.g., shortage of precipitation) [14], natural environmental constraints (e.g., soils with poor nutrients which are required by vegetation growth) [15], and human activities (e.g., overgrazing) [16]. Historical data recording carbon sequestration flux of grasslands with dependent variables such as the climatic and environmental conditions and human footprint can be analyzed to assess the human-induced impact on vegetation carbon sequestration. While the climatic constraints and environmental constraints are difficult to be lifted in a short period of time, it is relatively easier to adjust human activities such as an exclusion of grazing or the restoration of abandoned mining lands so that more carbon could be sequestered. To measure the impact on vegetation carbon sequestration from human activities, we proposed the concept of reduced carbon uptake (RCU)--the amount of carbon that should have been sequestered but was actually not due to human impacts. Then, we assessed RCU and explored the influential factors related to human activities on RCU, aiming at making informed grassland management policies.