Microbial community succession and trait changes during decomposition under drought

Abstract:

Microbial-driven decomposition is a fundamental process in global biogeochemical cycles. Drought associated with climate change can affect decomposition, yet the response of microbial decomposers to drought is poorly understood. Here we asked how experimental drought in California grassland and shrubland ecosystems affects microbial composition, functional potential, and decomposition rates of litter substrates. We hypothesized that drought would alter microbial allocation patterns, resulting in reduced investment in resource acquisition and lower rates of litter decomposition. In situ decomposition was monitored over 18 months using litter bags placed on the soil surface, and temporal shifts in microbial genomic and phenotypic traits were linked to changes in litter chemistry. Grass litter decomposed more quickly than shrub litter, whereas drought had a smaller impact on decomposition rates than expected. We observed that microbial taxonomic and functional composition was distinct across vegetation types and precipitation treatments early in decomposition. Particularly in grassland communities, early stages of decomposition were dominated by fungi with increased potential for carbohydrate degradation. As microbial succession progressed, bacterial diversity increased with bacterial taxa targeting a wider variety of substrates including proteins, lipids, and aromatic compounds in addition to carbohydrates. Early-stage drought tolerance traits were mostly linked to osmotic acclimation, shifting to synthesis of capsular and cell envelope substances at later stages, which could reflect a shift in dominance from fungi to bacteria in the microbial community. Community-level biomass, respiratory quotient, and decomposition capabilities were not significantly affected by drought. Contrary to our prediction, stress tolerance may not trade off against resource acquisition potential in microbial communities that have experienced long-term drought. We conclude that adaptive processes resulting in drought tolerance may mitigate expected negative effects of moisture limitation on surface litter decomposition.