Climate and geochemistry interactions at different altitudes influence soil organic carbon turnover times in alpine grasslands

Soil organic carbon turnover time (τ) is a key component of the carbon cycle. However, the impact of geochemical and climatic conditions on τ remains poorly understood in high alpine regions. We investigated climatic variables [e.g. mean annual precipitation (MAP) and mean annual temperature (MAT)], as well as geochemical variables [e.g. soil total element content (Mn, Ti, Fe, Si, Al, Mg, Ca, Na, K, and P), soil clay content, and soil pH] and estimated τ in the 0–30 cm soil layer at 169 alpine grassland sites along a 3000 km-long transect on the Tibetan Plateau. We found that τ ranged from 4 to 289 years on the Tibetan Plateau and showed a decreasing trend from the northwest to southeast and an increasing trend with altitude. The estimated τ was 71⁷⁸₆₃ years (mean with 95% confidence interval) in alpine meadows, which did not significantly differ from alpine steppes (76⁸⁷₆₅ yr). Overall, using boosted regression tree analysis, geochemistry was the most important controlling factor for τ (54% of the relative effect on τ), followed by climate (36%), and altitude (10%). When examining the relative contribution of individual variables, we found that MAP was the primary predictor of τ, followed by soil Si content, and altitude. Notably, variation in τ was explained by precipitation rather than temperature. Altitude indirectly affected τ by regulating climatic and soil geochemical conditions. The direct negative effect of climate on τ was opposite the positive indirect effect of climate on τ via soil geochemistry. These results highlight the importance of considering the interactions of climatic and geochemical factors as well as hydrological conditions when predicting how carbon turnover in the soils of semi-arid alpine grasslands will respond to future climate change.