Rhizosphere Soil Bacterial Communities of Continuous Cropping-Tolerant and Sensitive Soybean Genotypes Respond Differently to Long-Term Continuous Cropping in Mollisols

Abstract

The continuous planting of soybeans leads to soil acidification, aggravation of soil-borne diseases, reduction in soil enzyme activity, and accumulation of toxins in the soil. Microorganisms in the rhizosphere play a very important role in maintaining the sustainability of the soil ecosystem and plant health. In this study, two soybean genotypes, one bred for continuous cropping and the other not, were grown in a Mollisol in northeast China under continuous cropping for 7 and 36years in comparison with soybean-maize rotation, and microbial communities in the rhizosphere composition were assessed using high-throughput sequencing technology. The results showed that short- or long-term continuous cropping had no significant effect on the rhizosphere soil bacterial alpha diversity. Short-term continuous planting increased the number of soybean cyst nematode (Heterodera glycines), while long-term continuous planting reduced these numbers. There were less soybean cyst nematodes in the rhizosphere of the tolerant genotypes than sensitive genotypes. In addition, continuous cropping significantly increased the potential beneficial bacterial populations, such as Pseudoxanthomonas, Nitrospira, and Streptomyces compared to rotation and short-term continuous cropping, suggesting that long-term continuous cropping of soybean shifts the microbial community toward a healthy crop rotation system. Soybean genotypes that are tolerant to soybean might recruit some microorganisms that enhance the resistance of soybeans to long-term continuous cropping. Moreover, the network of the two genotypes responded differently to continuous cropping. The tolerant genotype responded positively to continuous cropping, while for the sensitive genotype, topology analyses on the instability of microbial community in the rhizosphere suggested that short periods of continuous planting can have a detrimental effect on microbial community stability, although this effect could be alleviated with increasing periods of continuous planting.

Keywords: Mollisol; continuous cropping; network; rhizosphere microorganisms; soybean.

Figure 1

Rhizobacterial community Shannon index (A) and Chao1 richness (B) of different treatments. One-way ANOVA, n=12, p<0.05.

Figure 2

Constrained Principal coordinate analysis (CPCoA) based on Bray-Curtis dissimilarities of 16S rRNA diversity in the rhizosphere.

Figure 3

Relative abundance of the phylum of different treatments.

Figure 4

Ternary plots depicting compartments showing the distributions of community differentiation among different cropping systems of tolerant (A) and sensitive (B) genotypes. The position of each circle depends on its contribution to the total relative abundance. Colored circles represent OTUs that enriched in different cropping systems, which the green for the rotation cropping system; red and green for 7- and 36-year continuous cropping systems, respectively. The gray circles represent the OTUs that are not significantly enriched within a given range.

Figure 5

The relative abundance of the core microbiome for tolerant (A) and sensitive (B) genotypes.

Figure 6

Network analysis of the rhizosphere bacterial community from different treatments. Nodes represent OTUs coded by phyla and are scaled by the number of connections (node degree). Connections are plotted at r>0.8 (positive correlation, red) or r<−0.8 (negative correlation, blue) and p<0.05.

Figure 7

A common sub-network view with the “driver” node highlighted. All nodes belonging to the same community have been randomly assigned a similar color. Gray OTU nodes represent nodes that are present in both cases, but interact directly with the common subnetwork in either the case or control group. The size of the nodes was proportional to their NESH scores, and a node was colored red if its spacing between the control and study groups increased. Large and red nodes are particularly important “drivers.” CK VS 7 (36; tolerant or sensitive), network comparison between soybean-corn rotations and 7 (36) years continuous cropping for tolerant or sensitive genotype.