Contrasting responses of microbial diversity and community structure in decaying root bark and xylem to N addition in an alpine shrubland

Abstract

Microbial degradation of root components (root bark and xylem) is a vital biogeochemical process that may be influenced by N deposition because N often limits microbial growth. However, there is a lack of information on microbial diversity and community structure during long-term decomposition of root bark and xylem under N addition, which seriously hinders our understanding of microbial decomposition of the root. In this study, litterbags containing roots from a dominant alpine shrub species (Sibiraea angustata) were buried in soil and sampled after 8 years of decomposition and four levels of N addition. The bacterial and fungal diversity significantly differed between root bark and xylem, due to their distinct C chemistry and water content. The microbial richness in the root bark decreased linearly with increasing N levels, whereas the opposite trend was observed in the root xylem. Additionally, high levels of N significantly altered the microbial community structure in the root bark but not in the root xylem. Despite these variations in overall microbial community patterns, N addition did not affect the root-decomposition rate and corresponding recalcitrant C in the root bark and xylem, likely due to the unchanged relative abundances of key functional guilds involved in root decomposition (i.e. litter saprotrophs, cellulolytic bacteria, and ligninolytic bacteria). Given the contrasting microbial responses to N addition in root components, our results suggest that the theory on root decomposition should consider different microbial controls in the decomposition of root bark and xylem.