Carbon sinks/sources' spatiotemporal evolution in China and its response to built-up land expansion.

Terrestrial ecosystem carbon sink examination in China still faces great uncertainties. Determinant analysis has focused on climate change but ignored the influence of fast urban expansion. Using remote sensing images, climate variable data, and high-resolution land use data, this research improved net ecosystem productivity (NEP) simulation model based on a large number of field observations, and investigated spatial-temporal changes of NEP. This research calculated the NEP loss caused by built-up land expansion and used geographically weighted regression (GWR) model to explore the spatial heterogeneity of the relationship between NEP growth and built-up land expansion. The results showed that China contributed a carbon sink of 0.33 Pg C per year from 2000 to 2020. Southern China had a greater capacity to sequester carbon than northern China. The carbon sink capacity of most Chinese regions increased. Built-up land expansion caused 4.95 Tg C of carbon sink loss per year, which was mainly concentrated in eastern China. In GWR model, 50.8% of regions showed negative correlations between NEP growth and built-up land expansion. These two variables were mostly positively correlated in the northwest and negatively correlated in the southeast. Consequently, this study suggests that maintaining the capacity of carbon sinks in southern provinces is important for China to meet its carbon neutrality goal.