microbial succession on decomposing Scots pine root litter under two irrigation regimes

Decomposition is a major component of the carbon cycle in forest soils and understanding the underlying processes is key for budgeting carbon turnover. Decomposition rates in forests are constrained by the presence of biological mediators and the underlying environmental conditions (e.g. water availability). Identifying soil microbial key-players during decomposition combined with measuring the chemical derivatives of this process can deepen our understanding of this complex process. For this purpose, high-throughput sequencing techniques offer unique opportunities to comprehensively survey these key players under different environmental conditions and in space and time. In this study, we used a metabarcoding approach of ribosomal markers to study the succession of bacterial and fungal decomposers on decomposing root litter over two years. Litterbags containing pine roots were buried in a mature pine forest for two years and have been gradually sampled. Decomposition and the associated communities were surveyed under two different precipitation regimes, i.e. under ambient semi-arid conditions as well as under long-term irrigation. Early stages of decomposition (<1 year) were characterized by the presence of largely copiotrophic microorganisms (e.g. Proteobacteria, yeast and molds). The largely copiotrophic populations decreased at later stages of decomposition (1-2 years) and were replaced by more oligotrophic bacteria as well as specialized litter-associated (e.g. white-rot fungi, Planctomycetes) or parasitic (e.g. Pleosporales) fungi. This succession can likely be explained by the decline of easy degradable sources and a relative increase of more recalcitrant compounds such as lignin at later stages of decomposition. The effect of irrigation had minor impact on the microbial decomposition. Despite contrasting water contents, neither fine nor coarse roots experienced slower degradation under dry soil conditions. We hypothesize that – despite the pronounced differences in community composition – the lack of difference in root decomposition rates between the ambient and irrigated forest plots has been caused by a sufficient degree of functional redundancy among the resident microbial taxa. These findings have important implications under future climate scenarios, since dry condition in drought prone Alpine forest does not reduce microbial decomposition, hence limits the primary litter input of the trees.