Increased ecological land and atmospheric CO 2 dominate the growth of ecosystem carbon sinks under the regulation of environmental conditions in national key ecological function zones in China.

Increased ecological land (IEL) such as forests and grasslands can greatly enhance ecosystem carbon sinks. Understanding the mechanisms for the magnitude of IEL-induced ecosystem carbon sinks is crucial for achieving carbon neutrality. We estimated the impact of IEL, specifically the increase in forests and grasslands, as well as global changes including atmospheric CO ₂ concentration, nitrogen deposition, and climate change on net ecosystem productivity (NEP) in National Key Ecological Function Zones (NKEFZs) in China using a calibrated ecological process model. The NEP in NKEFZs in China was calculated to be 119.4 Tg C yr ^-1 , showing an increase of 42.6 Tg C yr ^-1 from 2001 to 2021. Compared to the slight contributions of climate change (-8.0%), nitrogen deposition (11.5%), and reduction in ecological land (-3.5%), the increase in NEP was primarily attributed to CO ₂ (66.5%) and IEL (33.5%). Moreover, the effect of IEL (14.8 Tg C yr ^-1 ) surpassed that of global change (13.1 Tg C yr ^-1 ) in the land use change zone. The IEL-induced NEP is significantly associated with CO ₂ fertilization, regulated by precipitation and nitrogen deposition. The high values of IEL-induced NEP occurred in areas with precipitation exceeding 800 mm and nitrogen deposition exceeding 25 kg N ha ^-1 yr ^-1 . We recommend prioritizing the expansion of ecological land in areas with sufficient water and nutrients to enhance CO ₂ fertilization, while avoiding increasing ecological land in regions facing unfavorable climate change conditions. This study serves as a foundation for comprehending the NEP response to ecological restoration and global change.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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