Indicator-based diagnosis of carbon metabolism in production–living–ecological spaces: Land-use transitions, ecological network relationships, and terrain-constrained carbon flows in the Nanling Mountains, China

Land-use changes among production–living–ecological (PLE) spaces are critical drivers of regional carbon metabolism. Although these changes have been extensively examined in urban agglomerations and plains, their carbon metabolic mechanisms in mountainous areas remain inadequately explored. In this study, ecological network analysis (ENA) was used to identify ecological interactions among PLE spatial categories and evaluate the utility of the carbon metabolism network. In addition, the topographic constraints on major negative carbon flow pathways were examined. The results reveal the following. (1) Ecological spaces continuously dominated the Nanling Mountains during the full study period. From 2000 to 2020, total carbon emissions increased from 14.46 million t C to 25.18 million t C, whereas total carbon sinks remained relatively stable. (2) The carbon metabolism network was dominated by competitive and control/exploitation relationships. The overall utility values for 2000–2010 and 2010–2020 were 0.690 and 0.815, respectively, both below 1.0. In the latter period, competitive relationships decreased, whereas mutualistic relationships increased; this indicates a slight improvement in network relationships, although negative network effects still predominated. (3) The conversion of forest ecological spaces and agricultural production spaces into manufacturing production spaces, particularly F → M and C → M transitions, was the main driver of negative carbon flows. These flows were spatially concentrated in low-elevation, gentle-slope, and low-relief areas, suggesting that terrain suitability constrains carbon source expansion. The 52 counties in the study region were classified into four categories. This study demonstrates that, in ecologically dominant mountainous regions, stable carbon sinks do not automatically offset the rapid growth in carbon emissions caused by localized development. The spatial clustering of negative carbon flows along topographically suitable corridors provides a clear direction for differentiated, low-carbon territorial spatial management.