Seasonal decoupling of climate and human activity on litter and dung decomposition across the Qinghai-Xizang Plateau

Abstract In grassland ecosystems of the Qinghai-Xizang Plateau, litter and dung decomposition are essential for nutrient cycling and productivity. Using remote sensing data, field observations, and machine learning, we examined spatiotemporal patterns of litter decomposition mass (LDM) and dung decomposition mass (DDM) from 2000 to 2020. Both showed increasing trends: during the growing season (GS), LDM rose from 76.1 to 105.8 kg ha-1 and DDM from 16.9 to 26.6 kg ha-1. During the non-growing season (NGS), LDM increased from 29.0 to 46.6 kg ha-1 and DDM from 1.3 to 4.3 kg ha-1. A consistent southeast-to-northwest gradient was observed, with highest values in the southeast (GS: LDM > 500, DDM > 100 kg ha-1; NGS: LDM > 250, DDM > kg ha-1). During the GS, climate change and human activity both promoted decomposition, positively influencing LDM and DDM across 68.7%/55.8% and 85.9%/81.7% of the total grassland area, respectively. During the NGS, human activity emerged as the near-universal positive driver (> 90% of the area), while the climate weakened substantially, revealing a seasonal decoupling of decomposition controls. Decomposition peaked at ∼4000 m elevation (GS: LDM = 366.6, DDM = 86.2 kg ha-1; NGS: LDM = 156.4, DDM = 9.9 kg ha-1), identifying mid-elevation grasslands as critical nutrient-cycling hotspots. These findings underscore the need for season- and elevation-specific grassland management to sustain ecosystem productivity on the Qinghai-Xizang Plateau.