Effects of warming on leaf litter decomposition and soil organic carbon stability in alpine shrubs

Litter decomposition is a core process regulating the formation and stability of soil organic carbon. Under the background of climate warming, the intrinsic mechanisms by which the expansion of alpine shrubs affects carbon balance of alpine ecosystems by altering litter input characteristics (decomposition rate and chemical composition) remains unclear. We explored the effects of warming on the decomposition dynamics, carbon fraction changes of litters with different qualities and the stability of soil organic carbon, taking the leaf litters of Sorbus rufopilosa (high-quality, low C/N ratio and low lignin content) and Rhododendron lapponicum (low-quality, high C/N ratio and high lignin content) in the alpine regions of western Sichuan as the research objects. We conducted a 180-day laboratory controlled-temperature incubation experiment, with two temperature treatments: ambient temperature (10 ℃ in the daytime / 5 ℃ at night) and warming (20 ℃ in the daytime / 15 ℃ at night). The results showed that warming significantly accelerated the decomposition of the litters of two shrub species, with the decomposition rates of S. rufopilosa and R. lapponicum being increased by 47.9% and 21.0%, respectively. There was an average decrease of 9.7% and 14.7% in the non-polar extractives and acid-hydrolyzable carbohydrates and an average increase of 16.9% in the acid-unhydrolyzable residue. Warming significantly increased the content of soil dissolved organic carbon and decreased the content of soil recalcitrant organic carbon, but did not affect the content of soil total organic carbon. Under warming conditions, soil carbon stock index and carbon stock activity increased by 2.5% and 53.4% compared to normal temperature under S. rubra litters input, and increased by 3.2% and 79.5% for R. alpinum litters input, while the soil carbon stock management index decreased by 13.6% and 7.3%, respectively. In conclusion, warming accelerated the decomposition of alpine shrub litters and reduced the stability of soil organic carbon, while the input of low-quality litter could better maintain the stability of soil organic carbon in alpine meadows compared with high-quality litter.