Seasonal Dynamics of Ecosystem Carbon Exchange and Their Influencing Factors in Grasslands of Different Degrees of Salinization in Northern China

Soil pH plays a critical role in shaping the structural composition and functional dynamics of grassland ecosystems. The seasonal dynamics of carbon exchange and the factors influencing them in grassland ecosystems along saline-alkaline gradients remain unclear. In this study, saline-alkaline grasslands in northern China were classified into four gradients based on soil pH: mild salinization (pH = 8.36 ± 0.01), moderate salinization (pH = 9.21 ± 0.06), severe salinization (pH = 9.92 ± 0.04), and extreme salinization (pH = 10.49 ± 0.01). Ecosystem carbon exchange (net ecosystem carbon exchange (NEE), ecosystem respiration (ER), and gross ecosystem productivity (GEP)), as well as related biotic and abiotic factors, were investigated in the years 2023 and 2024. Results indicated that extreme salinization significantly reduced NEE, ER, and GEP, whereas no significant differences were observed in these carbon flux components between moderate and severe salinization levels. In 2024, NEE, ER, and GEP exhibited seasonal dynamics; compared to the early (May) and late (September) periods, greater differences were observed during the middle (June-August) period, particularly across varying salinization gradients. Significant negative correlations were observed between soil temperature, root-to-shoot ratio (R/S) and NEE, ER, and GEP, while above-ground and below-ground biomass were significantly positively correlated with NEE, ER, and GEP. Soil moisture exhibited a significant quadratic relationship with both ER and GEP. Importantly, results showed that the R/S explained the greatest variation in carbon fluxes. In summary, as salinization increased, carbon exchange capacity declined significantly, particularly under conditions of extreme salinization, where the R/S emerged as the primary regulatory factor.