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

Soil organic carbon turnover time (tau) is a key component of the carbon cycle. However, the impact of geochemical and climatic conditions on tau 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 tau 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 tau 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 tau was 71(63)(78) years (mean with 95% confidence interval) in alpine meadows, which did not significantly differ from alpine steppes (76(65)(87) yr). Overall, using boosted regression tree analysis, geochemistry was the most important controlling factor for tau (54% of the relative effect on tau), followed by climate (36%), and altitude (10%). When examining the relative contribution of individual variables, we found that MAP was the primary predictor of tau, followed by soil Si content, and altitude. Notably, variation in tau was explained by precipitation rather than temperature. Altitude indirectly affected tau by regulating climatic and soil geochemical conditions. The direct negative effect of climate on tau was opposite the positive indirect effect of climate on tau 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.