LB 38-319 - The warming effects on soil microbial communities are amplified in cool seasons

The ongoing rise in global surface temperature is expected to accelerate soil C decomposition and release CO2 from soils (i.e., soil respiration, RS) by lowering the energy threshold required for microbial heterotrophic respiration (Rh). While there is ample literature on the unidirectional responses of soil microbial communities to long-term warming, only a few have considered the effect of seasonal variation. Given that cold climates have been shown to possess higher climatological temperature sensitivities of soil C and RS than warm climates, we hypothesized that this variability in temperature sensitivity will have important consequences for microbial responses to warming, i.e., the responses of microbial community composition, functional traits, and C decomposition potentials to warming will depend on the time of year, with a higher magnitude of warming effects in colder seasons than in warmer seasons.Here, we investigated the temporal dynamics of soil microbial communities under experimental warming (+3°C) in a tallgrass prairie ecosystem in Oklahoma, USA over a consecutive 12 months. We used GeoChip 5.0 to analyze functional genes associated with C decomposition and 16S ribosomal RNA gene-based amplicon sequencing to profile microbial taxonomic compositions and estimate community average rRNA operon (rrn) copy number, a community-level functional trait representing microbial life strategy. We also utilized a gene-informed terrestrial ecosystem model (gMEND) to examine the response of microbial carbon use efficiency to warming.We showed that the interplay between warming and time altered (p < 0.050) community taxonomic and functional compositions. During the cool months, warming induced a soil microbiome with markedly higher genomic potential for C degradation, community-level ribosomal RNA operon (rrn) copy numbers, and microbial metabolic quotients, suggesting that warming stimulated fast-growing microorganisms that accelerated carbon degradation. Modeling analyses showed that warming reduced the temperature sensitivity of microbial carbon use efficiency (CUE) by 28.7% when the monthly average temperature was low, resulting in lower microbial CUE and higher heterotrophic respiration (Rh) potentials. As Rh is a crucial component of RS, we further revealed that microbial functional traits, together with soil temperature, moisture, nitrate content, and gross primary productivity modulated RS in cool months more strongly than in warm months through structural equation modeling.Collectively, our findings reveal distinct characteristics of microbial functional traits under warming in cool months, challenging common practice in climate change studies to collect soil samples only in the growing season, and advancing our mechanistic understanding of the seasonal pattern of RS sensitivity to warming.