Long-term leaf C:N ratio change under elevated CO2 and nitrogen deposition in China: Evidence from observations and process-based modeling

Climate change, elevating atmosphere CO2 (eCO2) and increased nitrogen deposition (iNDEP) are altering the biogeochemical interactions between plants, microbes and soils, which further modify plant leaf carbon‑nitrogen (C:N) stoichiometry and their carbon assimilation capability. Many field experiments have observed large sensitivity of leaf C:N ratio to eCO2 and iNDEP. However, the large-scale pattern of this sensitivity is still unclear, because eCO2 and iNDEP drive leaf C:N ratio toward opposite directions, which are further compounded by the complex processes of nitrogen acquisition and plant-and-microbial nitrogen competition. Here, we attempt to map the leaf C:N ratio spatial variation in the past 5 decades in China with a combination of data-driven model and process-based modeling. These two approaches showed consistent results. Over different regions, we found that leaf C:N ratio had significant but uneven changes between 2 time periods (1960-1989 and 1990-2015): a 5% ± 8% increase for temperate grasslands in northern China, a 3% ± 6% increase for boreal grasslands in western China, and by contrast, a 7% ± 6% decrease for temperate forests in southern China, and a 3% ± 5% decrease for boreal forests in northeastern China. Additionally, the structural equation models indicated that the leaf C:N change was sensitive to ΔNDEP, ΔCO2 and ΔMAT rather than ΔMAP and ecosystem types. Process-based modeling suggested that iNDEP was the main source of soil mineral nitrogen change, dominating leaf C:N ratio change in most areas in China, while eCO2 led to leaf C:N ratio increase in low iNDEP area. This study also indicates that the long-term leaf C:N ratio acclimation was dominated by climate constraint, especially temperature, but was constrained by soil N availability over decade scale. Temporal variation of multi-annual mean leaf C:N ratio in China (the difference between 2 time period from 1960 to 1989 to 1990-2015); (a) result by plot samples, (b) result by random forest (RF) model, (c) result by CLM5.0; (d) results from all above 3 approaches averaged by PFT. Different lowercase letters in the same color on error bars indicate significant differences across different PFTs at p < 0.05. [Display omitted] •Leaf C:N ratio decreased in southern China, but increased in western and northern China in the past 5 decades.•N deposition dominated leaf C:N ratio change in most areas, while elevated CO2 dominated in low N deposition area.•Leaf C:N ratio shifts were mo