Functional shift in soil microbiome with decline in photosynthetic carbon input signifies rapid decrease of soil organic carbon during alpine grassland degradation

Abstract

How photosynthetic carbon input regulates the microbial processes involved in carbon incorporation into soil organic carbon (SOC) and its stabilization during grassland degradation remains unclear. We utilized ¹³C to trace photosynthetic carbon incorporation into SOC and its pools, particulate (POC) and mineral-associated (MAOC) organic carbon, and carbon assimilation by soil microbes across five stages of alpine grassland degradation (S0, without grazing; S1, moderate grazing; S2–S4, light, moderate, and heavy degradation). As grassland approached S4, SOC in the top layer decreased by 53% compared with that in S3. A similar trend was observed in the middle and bottom soil layers, corresponding to a significant decrease in POC (decreased by 54, 40, and 35% in the top, middle and bottom layer, respectively) and carbon incorporation into POC (decreased by 83, 24, and 91% in the top, middle and bottom layer, respectively). A rapid decrease in MAOC was observed in S4, and carbon incorporation into MAOC decreased abruptly in the middle (10–20 cm) and bottom (20–30 cm) soil layers of S3. More than 57% of the incorporated carbon was concentrated in the top (0–10 cm) layers of S0-S4, whereas the middle and bottom layers of S3 and S4 exhibited nearly zero carbon incorporation. During degradation, fungal groups exhibited a downward trend in photosynthetic carbon assimilation, which was associated with their decreasing contribution to carbon incorporation into SOC. However, a relatively high proportion of bacteria participated in carbon assimilation at all soil depths at each stage, suggesting that more bacteria became active in decomposing the original SOC with decreasing carbon input. Our study successfully links aboveground and belowground processes which are crucial to comprehensively understand ecosystem responses to climate change and human activities.