Differential Responses of Arbuscular Mycorrhizal Fungal Communities to Long-Term Fertilization in the Wheat Rhizosphere and Root Endosphere

Abstract

Arbuscular mycorrhizal fungi (AMF) provide essential nutrients to crops and are critically impacted by fertilization in agricultural ecosystems. Understanding shifts in AMF communities in and around crop roots under different fertilization regimes can provide important lessons for improving agricultural production and sustainability. Here, we compared the responses of AMF communities in the rhizosphere (RS) and root endosphere (ES) of wheat (Triticum aestivum) to different fertilization treatments, nonfertilization (control), mineral fertilization only (NPK), mineral fertilization plus wheat straw (NPKS), and mineral fertilization plus cow manure (NPKM). We employed high-throughput amplicon sequencing and investigated the diversity, community composition, and network structure of AMF communities to assess their responses to fertilization. Our results elucidated that AMF communities in the RS and ES respond differently to fertilization schemes. Long-term NPK application decreased the RS AMF alpha diversity significantly, whereas additional organic amendments (straw or manure) had no effect. In contrast, NPK fertilization increased the ES AMF alpha diversity significantly, while additional organic amendments decreased it significantly. The effect of different fertilization schemes on AMF network complexity in the RS and ES were similar to their effects on alpha diversity. Changes to AMF communities in the RS and ES correlated mainly with the pH and phosphorus level of the rhizosphere soil under long-term inorganic and organic fertilization regimes. We suggest that the AMF community in the roots should be given more consideration when studying the effects of fertilization regimes on AMF in agroecosystems. IMPORTANCE Arbuscular mycorrhizal fungi are an integral component of rhizospheres, bridging the soil and plant systems and are highly sensitive to fertilization. However, surprisingly little is known about how the response differs between the roots and the surrounding soil. Decreasing arbuscular mycorrhizal fungal diversity under fertilization has been reported, implying a potential reduction in the mutualism between plants and arbuscular mycorrhizal fungi. However, we found opposing responses to long-term fertilization managements of arbuscular mycorrhizal fungi in the wheat roots and rhizosphere soil. These results suggested that changes in the arbuscular mycorrhizal fungal community in soils do not reflect those in the roots, highlighting that the root arbuscular mycorrhizal fungal community is pertinent to understand arbuscular mycorrhizal fungi and their crop hosts' responses to anthropogenic influences.

Keywords: arbuscular mycorrhizal fungi; co-occurrence network; long-term fertilization; organic matter; wheat.

FIG 1

Principal coordinate analysis (PCoA) analysis on the arbuscular mycorrhizal fungal (AMF) community based on Bray-Curtis distance and results of PERMANOVA testing the effects of compartment and fertilization on AMF communities. Control, no fertilization; NPK, mineral fertilizer only; NPKS, mineral fertilizer plus wheat straw; NPKM, mineral fertilizer plus cow manure.

FIG 2

Fertilization treatment significantly affects the arbuscular mycorrhizal fungal (AMF) communities in the rhizosphere soil (RS) and root endosphere (ES). (A) Canonical analysis of principal coordinates (CAP) based on Bray-Curtis distances constrained to fertilization treatment. (B) AMF operational taxonomic unit (OTU) richness at a sequencing depth of 10,000 reads per sample among different fertilization treatments in the RS and ES. Significant effects are indicated with different lowercase letters, as detected using Wilcoxon rank-sum tests. (C) Relative abundance of the main AMF families in the RS and ES across four treatments. For abbreviations, see the legend to Fig. 1.

FIG 3

Effects of fertilization on arbuscular mycorrhizal fungi (AMF) co-occurrence networks on the rhizosphere soil (RS) and root endosphere (ES). (A) Co-occurrence subnetworks of different fertilization treatments in the RS and ES, respectively. The subnetworks of four fertilization treatments were visualized to show the significant associations (r > 0.60, P < 0.05) between AMF operational taxonomic units (OTUs) filtered from the overall co-occurrence networks of each compartment. Each dot represents an OTU, different colors represent different families, the node size represents the degree of the node, and edges denote significant correlations between OTUs. (B) Network connectivity as represented by node degrees for different fertilization treatments in the RS and ES, respectively. Different lowercase letters indicate statistically significant (P < 0.05) differences as detected using Wilcoxon rank-sum tests. For abbreviations, see the legend to Fig. 1.

FIG 4

Structural equation model (SEM) showing the relationships among fertilization treatments, rhizosphere soil chemical properties, root nutrients, arbuscular mycorrhizal fungal (AMF) diversity, and the network complexity of AMF communities in the rhizosphere soil (RS) and root endosphere (ES), respectively. Each box represents a variable in the model, whereas the number above each arrow represents the value of the standardized path coefficient. Blue lines indicate positive effects, red lines indicate negative effects, and gray lines indicate no significant correlation. The final models fit the data well, as assessed using the maximum-likelihood method.