The spatial factor, rather than elevated CO₂, controls the soil bacterial community in a temperate Forest Ecosystem

Abstract

The global atmospheric carbon dioxide (CO₂) concentration is expected to increase continuously over the next century. However, little is known about the responses of soil bacterial communities to elevated CO₂ in terrestrial ecosystems. This study aimed to partition the relative influences of CO₂, nitrogen (N), and the spatial factor (different sampling plots) on soil bacterial communities at the free-air CO₂ enrichment research site in Duke Forest, North Carolina, by two independent techniques: an entirely sequencing-based approach and denaturing gradient gel electrophoresis. Multivariate regression tree analysis demonstrated that the spatial factor could explain more than 70% of the variation in soil bacterial diversity and 20% of the variation in community structure, while CO₂ or N treatment explains less than 3% of the variation. For the effects of soil environmental heterogeneity, the diversity estimates were distinguished mainly by the total soil N and C/N ratio. Bacterial diversity estimates were positively correlated with total soil N and negatively correlated with C/N ratio. There was no correlation between the overall bacterial community structures and the soil properties investigated. This study contributes to the information about the effects of elevated CO₂ and soil fertility on soil bacterial communities and the environmental factors shaping the distribution patterns of bacterial community diversity and structure in temperate forest soils.

FIG. 1.

Principal-coordinate analysis (PCoA) of the soil bacterial community structures associated with 10-year CO₂ treatment and 1-year N treatment using the sequencing-based data set (a) and the DGGE-based data set (b). Each point in the PCoA graph represents a sample associated with a treatment. A near distance of two points in the graph indicates a small ecological distance between the communities. AC, ambient CO₂; AN, ambient nutrient; EC, elevated CO₂ (200 μmol mol⁻¹ above ambient CO₂); EN, elevated nutrient (fertilized at a rate of 11.2 g N m⁻² year⁻¹); DGGE, denaturing gradient gel electrophoresis.

FIG. 2.

Summary of multivariate regression tree (MRT) analysis to illustrate the relative contributions of the categorical or quantitative environmental variables in influencing soil bacterial diversity (a and c) and structure (b and d) derived from the sequencing-based data set (a and b) and the DGGE-based data set (c and d). Solid lines represent the response variables which can be explained by the environmental variables through multivariate regression tree analysis, and line width is proportional to the percentage of the explanation, while dashed lines represent the contributions of the environmental variables for which the variation of the response variables is negligible. DGGE, denaturing gradient gel electrophoresis.

FIG. 3.

Negative linear relationships between C/N ratio and sequencing-based genotypic richness (a) and Shannon index (b), positive linear relationships between total N and rarefied genotypic richness (c) and Shannon index (d), and negative linear relationships between C/N ratio and DGGE-based genotypic richness (e) and Shannon index (f). Each point on the graph represents a sample associated with CO₂ and N treatments. AC, ambient CO₂; AN, ambient nutrient; EC, elevated CO₂ (200 μmol mol⁻¹ above ambient CO₂); EN, elevated nutrient (fertilized at a rate of 11.2 g N m⁻² year⁻¹); OTU, operational taxonomic unit.