Spatiotemporal dynamics and associations of soil organic carbon and total nitrogen in the Qilian Mountain Grasslands.

Understanding the coordinated changes in soil carbon and nitrogen is essential for evaluating ecosystem responses to environmental change, particularly in ecologically fragile alpine regions such as the Qilian Mountains. In this study, the denitrification-decomposition (DNDC) model was used to assess the spatiotemporal dynamics of soil organic carbon density (SOCD) and total nitrogen density (STND) in the 0-30 cm soil layer from 1975 to 2024. The results revealed that SOCD and STND were higher in the northern and east-central grasslands and lower in the southwestern regions. Both stocks exhibited fluctuating but overall increasing trends, with notable increases aligned with major ecological protection policies in China. To better understand the coupling of soil carbon and nitrogen, we constructed a composite indicator called soil carbon and nitrogen density (SCND) using principal component analysis. This indicator captures the synergistic accumulation of organic carbon and total nitrogen driven by shared ecological processes and was further used to explore its associations with environmental factors, enabling an integrated assessment of soil carbon-nitrogen dynamics. The results revealed that elevation and soil bulk density were the main direct drivers of carbon and nitrogen accumulation, both of which exerted negative effects, whereas the other factors acted through indirect pathways. These findings underscore the importance of topography and soil structure in regulating carbon and nitrogen dynamics. It is recommended to plant deep-rooted grass species, limit heavy machinery, and maintain long-term ecological protection to prevent declines after initial gains from interventions. In addition, the carbon-to-nitrogen (C/N) ratio showed increasing spatial heterogeneity over time, with high values in the western and central regions, where nitrogen input can be enhanced by introducing legumes or applying organic fertilizers. In the northern and southeastern areas, grazing exclusion or low-intensity grazing is recommended to promote organic matter accumulation. Vertically, the C/N ratio decreased with soil depth, indicating strong carbon and nitrogen coupling within the soil profile. Overall, this study highlights the coordinated dynamics of soil carbon and nitrogen in the Qilian Mountain grasslands, providing valuable insights for the sustainable management and resilience improvement of grasslands in this region under changing environmental conditions.