Soil microbial CO2 fixation plays a significant role in terrestrial carbon sink in a dryland ecosystem: A four-year small-scale field-plot observation on the Tibetan Plateau

Assessment of the global terrestrial carbon (C) sink remains uncertain, and the uncertainty is largely derived from dryland ecosystems. Here we investigated the uncertainty and dynamics of gross primary productivity (GPP) by distinguishing the contributions of soil microbial primary producers and plants to CO2 fixation during four sequential growing seasons in a fragile dry grassland on the Tibetan Plateau. The results demonstrated that soil microbial GPP consistently accounted for a high proportion of plant GPP (18.2%), and both exhibited similar seasonal patterns during the four-year observation. Soil microbial GPP demonstrated a much greater interannual variation (76.1%) than plant GPP (15.1%), indicating that the interannual GPP uncertainty could be largely from microbial primary producers. Regression analysis indicated that plant GPP had higher sensitivity (demonstrated by slope) than soil microbial GPP to both soil water content and temperature. The GPP ratio of soil microbes to plants also demonstrated a clear seasonal change, and peaked in July in the four-year observation, with a minimum interannual variation (6.8%). The GPP ratio enhanced with increasing soil water content (P < 0.001), but did not correlate with soil temperature. Our findings suggest the great potential of soil microbial GPP, and challenge the plant-oriented models of terrestrial C estimation, which account for plant GPP but ignore soil microbial GPP. Thus, a more robust framework needs to incorporate both soil microbial and plant GPPs for accurately assessing C balance. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

The study found that soil microbial primary producers contribute significantly to CO2 fixation in a fragile dry grassland on the Tibetan Plateau, with a high proportion of plant gross primary productivity (GPP). Soil microbial GPP exhibited greater interannual variation than plant GPP, suggesting that the uncertainty in interannual GPP could be largely derived from microbial primary producers.