Quantification of Extreme Climate Index Contributions to Grassland Carbon Sink of Sanjiangyuan, Tibetan Plateau: Effects of Pastoral Agriculture on Ecosystem Respiration and Carbon Management Implications

Grassland ecosystems on the Tibetan Plateau play a critical role in the global carbon cycle, however, the quantitative influence of extreme climate events on their carbon sink dynamics remains insufficiently understood. This study focused on the Sanjiangyuan (Three-River-Source) region, a representative alpine pastoral area, employed net ecosystem productivity (NEP) estimation, Theil–Sen trend analysis, the coefficient of variation, the Hurst index, and ridge regression modeling to quantify the spatiotemporal characteristics of grassland carbon source/sink dynamics and the contributions of 13 extreme climate indices during 2000–2024. The results indicate that the regional mean NEP increased at a rate of 1.49 g C m−2 a−1, where 90.78% of the area functioned as a carbon sink, reflecting relatively weak ecosystem respiration and dominant vegetation carbon absorption. However, Hurst index analysis reveals that 85.68% of regions exhibit an inverse sustainability trend, suggesting a potential shift from carbon sinks to carbon sources in the future. This implies enhanced ecosystem respiration and the possible replacement of carbon sink functions by carbon source functions. The ridge regression analysis demonstrated that the extreme temperature indices, particularly the warm days (TX90p, 38.1% relative contribution), cool nights (TN10p), and warm spell duration (WSDI) indices, were the dominant drivers of NEP variation. These findings provide adaptive management strategies were proposed: in highly variable and inversely persistent regions, regulating grazing intensity, optimizing fencing management, and restoring degraded grasslands should be implemented to mitigate excessive respiration-related carbon emissions and maintain carbon sink stability, a scientific basis for optimizing pastoral agricultural carbon management and ecosystem respiration regulation under intensifying climate extremes on the Tibetan Plateau.