Dual Effects and Driving Mechanisms of Urban Expansion on Aboveground and Belowground Biomass in China from 2000 to 2020

Urban expansion has profoundly altered ecosystem carbon dynamics, yet its associations with changes in aboveground biomass (AGB) and belowground biomass (BGB) remain insufficiently understood at the national scale. This study investigated urban expansion areas in 352 Chinese cities from 2000 to 2020 using 1-km-resolution AGB and BGB datasets. By integrating temperature, precipitation, potential evapotranspiration, and atmospheric CO2 concentration, we developed the Enhanced Aboveground Biomass Deviation Index (EAGB) and Enhanced Belowground Biomass Deviation Index (EBGB) to identify areas showing positive and negative biomass responses during urban expansion and to explore their spatial patterns and driving mechanisms. Between 2000 and 2020, China’s urban land expanded by 52,635 km2, corresponding to an average annual expansion rate of 4.1%. Areas showing positive biomass responses during urban expansion accounted for 22,376 km2 (42.5%) for AGB and 23,569 km2 (44.8%) for BGB, mainly concentrated in East and Central China. In contrast, areas showing negative biomass responses covered 5233 km2 (9.9%) for AGB and 3911 km2 (7.4%) for BGB, primarily distributed in Northwest and Southwest China. During urban expansion, biomass declines were more frequently observed in rapidly expanding small- and medium-sized cities in Northwest and North China, whereas larger cities generally exhibited more widespread biomass gains. Climatic factors, particularly precipitation and atmospheric CO2 concentration, together with human activities, were the dominant drivers of the observed spatial heterogeneity. Moreover, the dominant driving mechanisms shifted from the combined influence of climatic factors and human activities during 2000–2010 to the combined influence of climatic and topographic factors during 2010–2020. These findings provide a new framework for assessing biomass responses during urban expansion and offer scientific support for urban land-use optimization and ecological conservation.