Soil bacterial community structure responses to precipitation reduction and forest management in forest ecosystems across Germany

Abstract

Soil microbial communities play an important role in forest ecosystem functioning, but how climate change will affect the community composition and consequently bacterial functions is poorly understood. We assessed the effects of reduced precipitation with the aim of simulating realistic future drought conditions for one growing season on the bacterial community and its relation to soil properties and forest management. We manipulated precipitation in beech and conifer forest plots managed at different levels of intensity in three different regions across Germany. The precipitation reduction decreased soil water content across the growing season by between 2 to 8% depending on plot and region. T-RFLP analysis and pyrosequencing of the 16S rRNA gene were used to study the total soil bacterial community and its active members after six months of precipitation reduction. The effect of reduced precipitation on the total bacterial community structure was negligible while significant effects could be observed for the active bacteria. However, the effect was secondary to the stronger influence of specific soil characteristics across the three regions and management selection of overstorey tree species and their respective understorey vegetation. The impact of reduced precipitation differed between the studied plots; however, we could not determine the particular parameters being able to modify the response of the active bacterial community among plots. We conclude that the moderate drought induced by the precipitation manipulation treatment started to affect the active but not the total bacterial community, which points to an adequate resistance of the soil microbial system over one growing season.

Fig 1. Ratio of the absolute soil water content between reduced precipitation and control subplots.

The absolute water content in the upper 20 cm of the soil was estimated using the forest-hydrological model LWF- Brook90 for all three exploratories (S = Schorfheide; H = Hainich; A = Schwäbische Alb).

Fig 2. NMS ordination plots of the bacterial community structure obtained from reduced precipitation (R) and control (C) subplots.

Soil samples were taken from nine plots of the three exploratories in September 2012. The community structure of the metabolically active (b, c) and the total (a) bacteria were analysed by T-RFLP (a, b) and tag-pyrosequencing (c). Statistically significant correlations (p < 0.05) of soil characteristics (C_org: soil organic carbon; N_t: total nitrogen) and understorey parameters (richness: species richness and H’: Shannon’s diversity index of the understorey plant community) were indicated by arrows. For plot ID see Table 1.

Fig 3. Distribution of bacterial phyla across the different plots and treatments.

The main phylum (Proteobacteria) is subdivided in classes. For plot ID see Table 1, left bar—control, right bar—reduced precipitation subplot.

Fig 4. Differences in the number of phylotypes at the genus level.

The bacterial community was compared between the reduced precipitation (R) and the control (C) subplots of conifer intensive (left) and beech unmanaged (right) plots. (a—Schorfheide, b—Hainich, c—Schwäbische Alb). For plot ID see Table 1.

Fig 5. Phylotypes with abundance shifts between the reduced precipitation and the control subplots.

Shown groups had a relative abundance of more than 1% of the bacterial community and increased or decreased by more than 50% due to reduced precipitation. For plot ID see Table 1.