Responses of Subsoil Organic Carbon to Climate Warming and Cooling Is Determined by Microbial Community Rather Than Its Molecular Composition

Little is currently known about how long-term climate changes modulate the relationship between soil organic carbon (SOC) molecular composition, microbial community and SOC storage and the mechanisms involved. Here, we show substantial changes in subsoil SOC in the Qinghai-Tibetan alpine grasslands over 16 years of soil warming and cooling. Warming reduced SOC content by 8.5%, whereas cooling increased it by 7.0%. Neither warming nor cooling affected plant- and microbial-derived molecular components. However, warming elevated the fungal-to-bacterial biomass ratio (F/B) and the gram-positive to gram-negative bacterial biomass ratio (G+/G-) by 15.0% and 8.6%, respectively, whereas cooling reduced them by 4.5% and 9.6%. Warming reduced SOC storage by directly increasing F/B and G+/G- and indirectly decreasing the soil carbon-to-nitrogen ratio, whereas cooling enhanced SOC storage primarily by decreasing F/B. Conventional warming experiments, which consider only climate warming and neglect cooling, may underestimate the negative impacts of warming on subsoil SOC pools in alpine grasslands.