Decadal warming-induced shifts in plant community composition and biomass allocation enhance alpine soil carbon accrual

Warming is expected to increase both soil organic matter decomposition and plant growth in alpine ecosystems, leading to an uncertain fate of soil carbon (C) stocks in these temperature-vulnerable ecosystems. There are very few empirical data over decadal timescales to address this uncertainty. Here, we conducted a 10-year warming experiment in which surface soil C stocks, together with C inputs (plant production) and outputs (microbial respiration), were measured each year under ambient and elevated temperatures in an alpine grassland. We observed that the decadal warming enhanced soil C stocks, particularly in the late stages of the experiments, due to warming-induced increases in plant C inputs. The increase in soil C stock was mainly due to the following three mechanisms. First, plant C input significantly increased under warming by shifting plant community composition towards grass dominance that had taller plant height and higher belowground productivity and allocation. The mechanisms were also related to the higher temperature optimum of grasses compared to non-grass species. Second, abundant precipitation and humid environments facilitated positive responses of ecosystem carbon uptake to warming. Third, ecosystem carbon fluxes showed optimal temperatures and were able to thermally adapt to climate warming, which benefit ecosystem carbon uptake. The above findings revealed the key response mechanisms of soil C stocks in alpine ecosystems to long-term climate change, enriched the understanding of the feedback relationship between the carbon cycle and climate change, and provided important parameters and experimental evidence for carbon cycle models.