Weakened priming effect along soil profile in alpine grasslands on the Tibetan Plateau

Subsoils hold a substantial reservoir of organic carbon (C), and its dynamics can be greatly influenced by fresh C inputs through priming effect, potentially altering the magnitude of soil C-climate feedback. Despite the importance of soil C dynamics in regulating this feedback, our understanding of how soil C release and the priming effect vary along the soil profile remains limited, especially in alpine grasslands on the Tibetan Plateau. In particular, the relative importance of abiotic and biotic factors, such as soil physicochemical properties, aggregate and mineral protection, substrate quantity and quality, and plant and microbial properties (e.g., microbial biomass and diversity), in mediating vertical variations in soil C release and the priming effect is still unclear. Using 1-meter-deep soil profiles from five sites on the plateau, our ¹³C isotope labeling incubation experiments revealed a significant decline in both C release and the priming effect with increasing soil depth. We found that variations in soil C release along the profile were primarily influenced by soil properties (soil moisture and pH), mineral protection (the molar ratios of amorphous Fe/Al oxides to soil organic C (SOC) and soil mineral specific surface area), and hydrolase activity. In addition, vertical variations in the priming effect were dominantly affected by soil properties (soil moisture and pH), mineral and aggregate protection (the molar ratio of exchangeable Ca to SOC and the proportion of C occluded in clay+silt fractions), and microbial properties (oxidase activity and the copy number of bacterial ribosomal RNA gene operons). These findings provide valuable insights into the complex soil C cycling across profiles and its feedback to climate change.