Effect of seasonal soil freeze–thaw cycles on bacterial diversity in the process of fine root decomposition at different critical periods

Abstract

Abstract Soil microbial community plays a crucial role in fine roots decomposition in the forest ecosystem, and understanding the dynamics of soil microbial diversity in the fine roots will be helpful to reveal the mechanism of fine root decomposition at different critical periods. As yet, little is known about the dynamics of soil microbial diversity in the process of fine roots decomposition during the soil freeze–thaw season in the high-frigid forest ecosystem. In order to understand the effects of seasonal freeze–thaw cycles on bacterial diversity in the process of fine roots decomposition at different critical periods in cold winter, litterbags with spruce (Picea asperata), fir (Abies faxoniana) and birch (Betula albosinensis) fine roots were buried in the forest floor at altitudes of 3582 m, 3298 m, and 3023 m in different sites in the eastern Tibet Plateau on October 26, 2009. The litterbags were picked up and stored in ice box at the periods of onset of freezing (OF), deep freezing (DF), early thawing stage (ETS), middle thawing stage (MTS), late thawing stage (LTS), and early growing season (EGS) from freeze–thaw season to early growing season. DNA of bacterial in fine roots taken back was extracted immediately. Based on the PCR-DGGE technique, diverse bacterial communities in the studied fine roots were found in the whole process of fine roots decomposition. Bacterial diversity in fine roots were significantly higher at the periods of onset of freezing, middle thawing and late thawing, but significantly lower at the periods of in deep freezing and early thawing. Bacterial diversity in decomposing fine roots varied greatly with seasonal soil freeze–thaw process and tree species, but varied slightly with the studied altitudes. Regardless of tree species, higher similarity of bacterial community in fine roots was observed among different altitudes at the same period, while relatively lower similarity of bacterial community in fine roots was observed at different critical periods at the same altitude. However, the similarity of bacterial community in fine roots was always higher than 48% during the freeze–thaw season. Biodiversity indices of richness, Shannon–Wiener index, Simpson index and evenness of bacterial community were correlated significantly with average, maximum and minimum temperatures in soils during the freeze–thaw season, only except for the correlation between soil average temperature and Simpson index. The nitrogen concentration in fine roots was correlated significantly with Simpson index and evenness of bacterial community, but the phosphorus concentration was not significantly correlated with these bacterial indices. In conclusion, continuous soil freezing reduced the level of bacterial community diversity in fine roots, and the bacterial community diversity in fine roots at soil thawing period would recover the level at onset freezing period, regardless of the studied altitudes. The dynamics of bacterial community in fine roots conformed to the dynamics of root decomposition. Meanwhile, fine roots quality could also influence the level of bacterial community diversity.