Whole-soil-profile warming does not change microbial carbon use efficiency in surface and deep soils

Significance

Although surface and deep soils will warm at nearly the same rate throughout the next century, the response of deep-soil organic carbon (C) to climate warming is still unknown. Microbial carbon use efficiency (CUE), the proportion of C ultimately assimilated into biomass, is an important driver of soil C storage. We found that the microbial CUE decreased with soil depth in an alpine grassland, and was mainly controlled by soil C availability. However, short-term (3.3 y) whole-soil-profile warming (0 to 1 m, +4 °C) did not significantly affect either soil available C or microbial CUE across soil depths. Collectively, these results highlight the role of soil C availability in controlling microbial CUE and its response to warming across the soil profile.

Abstract

The paucity of investigations of carbon (C) dynamics through the soil profile with warming makes it challenging to evaluate the terrestrial C feedback to climate change. Soil microbes are important engines driving terrestrial biogeochemical cycles; their carbon use efficiency (CUE), defined as the proportion of metabolized organic C allocated to microbial biomass, is a key regulator controlling the fate of soil C. It has been theorized that microbial CUE should decline with warming; however, empirical evidence for this response is scarce, and data from deeper soils are particularly scarce. Here, based on soil samples from a whole-soil-profile warming experiment (0 to 1 m, +4 °C) and ¹⁸O tracing approach, we examined the vertical variation of microbial CUE and its response to ~3.3-y experimental warming in an alpine grassland on the Qinghai–Tibetan Plateau. Microbial CUE decreased with soil depth, a trend that was primarily controlled by soil C availability. However, warming had limited effects on microbial CUE regardless of soil depth. Similarly, warming had no significant effect on soil C availability, as characterized by extractable organic C, enzyme-based lignocellulose index, and lignin phenol–based ratios of vanillyls, syringyls, and cinnamyls. Collectively, our work suggests that short-term warming does not alter microbial CUE in either surface or deep soils, and emphasizes the regulatory role of soil C availability on microbial CUE.