Enhanced ecosystem carbon sink in shrub-grassland ecotone under grazing exclusion on Tibetan plateau.

• Shrub - grassland ecotone acts as a net CO 2 sink, with the maximum NEE occurring in July. • Shrub - grassland ecotone degradation diminishes the carbon sequestration capacity, and grazing exclusion enhanced carbon sequestration capacity, particularly in heavily degraded ecotone. • Soil moisture and soil temperature are pivotal drivers influencing NEE. • NEE was negatively correlated with soil temperature during April to June and September to October, but was positively correlated with soil temperature during July to August. • NEE displays a consistent negative correlation with soil moisture during April to October. Grassland degradation is a prevalent issue at the interface of shrubland and grassland ecosystems, particularly susceptible to environmental changes. To counteract the degradation, grazing exclusion has been proposed as a dominant restoration strategy on the Tibetan plateau, the world's largest and highest plateau. However, the impact of degradation and subsequent restoration on the carbon dynamics consist of carbon sink / source capacity of shrub-grassland ecotones remain unclear. Therefore, in this study, using a transparent chamber (0.8 m in length × 0.8 m in width × 0.6 m in height) attached to an infrared gas analyzer, the in situ field investigation was performed to measure the gross ecosystem primary productivity (GEP), ecosystem respiration (Re), net ecosystem CO 2 exchange (NEE), and environmental factors over a four-year continuous period in lightly and heavily degraded shrub-grassland ecotones during recover process on the Tibetan Plateau. Our result showed that: 1). Shrub-grassland ecotone acts as a net CO 2 sink, with the maximum NEE occurring in July during the growing season. 2). Ecotone degradation diminishes the carbon sequestration capacity. Specifically, the annual GEP, Re, and NEE rates decrease by 14.1 %, 7.3 %, and 27.2 %, respectively, in the heavily degraded ecotone compared to the lightly degraded counterpart. 3). Grazing exclusion positively influenced the carbon sequestration capacity, particularly in heavily degraded ecotone, which leads to a substantial increase in the annual GEP, Re, and NEE rates of 15.4 % and 33.4 %, 12.3 % and 20.9 %, 22.25 % and 64.9 % for lightly and heavily degraded ecotones, respectively. 4). A piecewise structural equation model reveals that soil moisture and soil temperature are pivotal drivers influencing NEE. 5) Correlation analysis shows that NEE was negatively correlated with soil temperature during April to June and September to October but was positively correlated with soil temperature during July to August. In contrast, NEE displays a consistent negative correlation with soil moisture throughout the growing season. This research provides new insights on grassland degradation and restoration for carbon sequestration in alpine shrub-grassland ecotones and highlights their implications for sustainable management of alpine ecosystems. [ABSTRACT FROM AUTHOR]