Substrate availability and soil microbes drive temperature sensitivity of soil organic carbon mineralization to warming along an elevation gradient in subtropical Asia

Subtropical forest soil exerts a large, but uncertain effect on terrestrial carbon (C) cycling. Global warming is anticipated to alter subtropical soil C cycling but currently, there is no consensus on how warming will affect soil C at different elevations. We conducted a short-term laboratory soil warming incubation experiment (ambient temperature +4 °C) along an elevational gradient in Wuyi Mountains of southeastern China to examine the response of soil organic carbon (SOC) mineralization to rising temperatures. Soil samples were collected from three elevations (630 m, 1450 m and 2130 m), and microbial community composition was determined using phospholipid fatty acids (PLFAs). The SOC mineralization increased with rising mean annual temperature (i.e., with decreasing elevation) and with experimental warming. Unlike most other similar experimental studies, we found that the temperature sensitivity (Q₁₀) of SOC mineralization to short-term experimental warming significantly decreased with increasing elevation. We also found that temperature sensitivity of SOC mineralization in response to warming depends on substrate availability, as indicated by the significant relationship between dissolved organic carbon (DOC) and Q₁₀ values. In addition, soil microbial biomass increased significantly with increasing elevations, but was not significantly affected by short-term experimental warming. Experimental warming reduced the abundance of total PLFAs, bacteria, fungi, and actinomycetes in the low-elevation soil. Experimental warming significantly changed soil microbial community composition at low elevation, with increases in the ratios of cyclopropyl to monoenoic precursor fatty acids (cy:pre), saturated to monounsaturated fatty acids (sat:mono), and isomers to trans-isomers fatty acids (i:a), all of which are stress indicators, indicating that warming treatment increased microbial respiration rather than microbial growth, because the microbial respiration per biomass increases under environmental stress. Microorganisms likely altered their membrane fatty acid components and mass in response to changes in available C. The differences in Q₁₀ associated with short-term warming and among elevations with long-term temperature differences indicate that the effect of warming on SOC mineralization may change through time and this should be taken into account when predicting SOC mineralization in response to continual rising temperatures.