Biochar Amendment Stimulates Utilization of Plant-Derived Carbon by Soil Bacteria in an Intercropping System

Abstract

Plant-derived carbon (C) is considered fundamental to understand the interaction between rhizosphere microbes and plants in terrestrial ecosystems. Biochar soil amendment may enhance plant performance via changing soil properties or microbial diversity in the rhizosphere. However, our knowledge of how plant-microbiome associations respond to biochar amendment remains rather limited. Herein, ¹³CO₂ steady-state labeling combined with DNA stable-isotope probing was used to characterize soil bacterial communities in the rhizosphere contributing to the utilization of plant-derived C. The diversity of bacteria active in the utilization of root exudates was determined after biochar amendment in a legume-based intercropping system (Vicia faba L., with Zea mays L.). The results showed the biochar application not only changed the bacterial community structure and diversity in the rhizosphere, but also altered bacterial members actively assimilating plant-derived C. There were more labeled species in the biochar-amended soils than the control soils. Compared with the control, the biochar amendment increased the relative abundances of Firmicutes and Bacteroidetes members (i.e., Bacillus, Clostridium, Sporomusa, Desulfosporosinus, and Alicyclobacillus) while decreasing the abundances of Proteobacteria members (e.g., Methylobacterium and Sphingomonas) utilizing plant-derived C. In contrast, slow-growing species of the phyla Acidobacteria, Planctomycetes, and Gemmatimonadetes were barely labeled. The bacteria found stimulated by the biochar amendment are known for their ability to fix nitrogen, solubilize phosphorus, or reduce iron and sulfur, which may potentially contribute to the "biochar effect" in the rhizosphere. This study is the first to provide empirical evidence that biochar amendment can alter the soil bacterial community assimilating plant-derived C; this may have consequences for nutrient cycling and improving plant performance in intercropping systems.

Keywords: 13CO2 steady-state labeling; biochar; intercropping; plant-derived carbon; rhizosphere microbes; stable-isotope probing.

FIGURE 1

A conceptual diagram for the study design, methods and objective.

FIGURE 2

Isotopic signatures of δ¹³C (‰) and ¹³C-DNA concentration (ng g^–1) in the control and biochar treatments after 35-day of continuous labeling. Bars are means ± SE. Significant differences between treatments were tested by one-way ANOVA with Fisher’s LSD test.

FIGURE 3

Effect of the biochar amendment on soil bacterial communities in the rhizosphere. (A) Taxonomic composition. Significant differences between treatments were tested by one-way ANOVA with Fisher’s LSD test, different letters indicate that significance at P < 0.05 level. (B) Ordination of Bray-Curtis dissimilarities (k = 4; stress = 0.04).

FIGURE 4

Principal coordinate ordinations (PCoA) ordination based on Bray-Curtis dissimilarities of operational taxonomic units (OTUs) in the heavy (>1.70 g ml^–1) gradient fractions of the control and biochar treatments. The size of the density legend represents the range of gradient fraction OTU profiles. The size of a point corresponds to its fraction density, while the distance between points represents the similarity in microbial community composition.

FIGURE 5

Log₂-fold changes in the relative abundance of OTUs (¹³C-labeled vs. unlabeled) in the heavy (>1.70 g mL^–1) gradients for the control and biochar treatments. All the OTUs passed the 35%-sparsity threshold (i.e., OTUs were found in at least 35% fractions) of the heavy fractions. Each circle shows a single OTU, and circles denote proportion fold-changes that had an adjusted P-value below a false discovery rate of 10%.

FIGURE 6

Total responsive OTUs in each bacterial phylum that responded significantly (i.e., with BH-adjusted P-values < 0.1) in the heavy gradient fraction of the control (A) and biochar (B) treatments.

FIGURE 7

Log₂-fold changes of the relative abundance of OTUs in response to biochar (¹³C-labeled vs. unlabeled) or control (¹³C-labeled vs. unlabeled) for the top six abundant bacterial phyla. Color of circles indicate unique, shared and non-reposed OTUs between the control and biochar treatments.

FIGURE 8

Differentially abundant OTUs between the labeled vs. unlabeled heavy (>1.70 g ml^–1) gradient fractions for the control and biochar treatments at the genus level. Highly enriched OTUs with a significance of P < 0.01 are shown. The g_ indicates unclassified microbe at the genus level; the circle size represents the mean normalized counts of the OTUs across all samples.