Nonlinearity of China's Carbon Sink Increasing and Its Nonlinear Relationship With Land Use Patterns

China's terrestrial carbon sink, quantified by net ecosystem productivity (NEP), has exhibited significant yet spatially heterogeneous growth over the past four decades, driven by climate change, land use transitions, and ecological restoration policies. However, the nonlinearity of NEP enhancement and its coupling mechanisms with dynamic land use patterns remain poorly understood. This study integrates linear trend analysis, ensemble empirical mode decomposition, and boosted regression tree (BRT) modeling to systematically unravel the nonlinear characteristics of NEP trends (1981–2019) and their landscape‐mediated drivers across four ecoregions. Key findings reveal that: (1) While 43.75% of China's land area showed a linear increase in NEP, only 13.46% exhibited monotonic growth (TrendIN), whereas 16.46% displayed trend reversals (TrendDE‐TO‐IN), highlighting dominant nonlinear dynamics. (2) Land use pattern indices (LUPI)—spanning fragmentation (PD), dominance (LPI), connectivity (CONTAG), shape complexity (AWMPFD), and diversity (SHDI)—demonstrated divergent trajectories: South China and the Tibetan Plateau (TP) experienced increasing fragmentation (PD increases) alongside declining connectivity (CONTAG decreases), while Northwest China (NWC) showed inverse patterns, reflecting region‐specific anthropogenic and ecological pressures. (3) TrendIN regions (e.g., NWC and TP) were governed by LPI in NWC and CONTAG, where threshold exceedance (slope > 0) stabilized carbon accumulation. The trend reversal regions of NEP relied on PD and AWMPFD, where initial declines in edge effects (slope < 0) preceded NEP recovery. Notably, NEP responses to LUPI gradients exhibited U‐shaped thresholds (slope = 0) in monotonically increasing regions but monotonic shifts in TrendDE‐TO‐IN zones, underscoring legacy effects of historical landscape configurations. By bridging landscape ecological theory with nonlinear trend decomposition, this study advances the understanding of how multiscale land use patterns regulate carbon sequestration, offering actionable insights for spatially adaptive land management to support China's “dual carbon” goals.