Effects of Spent Mushroom Substrate Treated with Plant Growth-Promoting Rhizobacteria on Blueberry Growth and Soil Quality

Abstract

Spent mushroom substrate (SMS) is the residual biomass generated after harvesting the fruitbodies of edible fungi. It is produced in large quantities and contains abundant nutrients. Plant growth-promoting rhizobacteria (PGPR) are a group of plant-associated microorganisms known for their ability to enhance plant growth, improve disease resistance, and boost soil quality. In this study, three PGPR strains with the highest plant growth-promoting potential were selected based on their ability to grow effectively in SMS extract. The SMS substrates were mixed with PGPR solutions and sterile water to establish a batch culture system. The mixture was initially incubated at 28 °C for 3 days, followed by continuous aerobic decomposition in a ventilated environment for 180 days. Based on the quality analysis of the PGPR-treated SMS, the 54-day treatment for transplanting blueberry seedlings was selected. The PGPR-treated substrates showed significantly higher TN, HN, and AP than controls (p < 0.05), suggesting a potential role of PGPR in enhancing nutrient availability. Alpha diversity index analysis revealed significant differences in microbial diversity between the PGPR-treated substrates and the control. Furthermore, the PGPR-treated substrates significantly influenced plant growth characteristics, soil nutrient content, and rhizosphere microbial diversity. Enhanced plant growth characteristics were strongly correlated with increased soil nutrient levels, suggesting a potential link between rhizospheric microbial communities and plant growth performance. This study provides a novel approach and experimental framework for the utilization of SMS and the development of PGPR-based biofertilizers, offering valuable insights into sustainable agricultural practices.

Keywords: PGPR; SMS; blueberry; rhizosphere.

Figure 1

Growth kinetics of PGPR strains in different media. (a) Absolute growth curves (Log10-transformed OD₆₀₀). (b) Normalized growth profiles (Log10(ODt/OD0)). Notes: T1, T2, T3 represent strain identifiers; SMS: spent mushroom substrate extract; CK: control medium (beef extract peptone). Data represent mean ± SD (n = 3).

Figure 2

Effects of PGPR inoculation on physiological indices of blueberry plants: (a) Survival rates of transplanted plants after 45 days; (b) Plant height (cm); (c) Chlorophyll concentration (total Chl, mg/g FW, fresh weight). Notes: Survival (%) = (Number of surviving seedlings)/25 × 100 (%). 25 represents the initial number of blueberry seedlings transplanted per pot, and there were three pots for every treatment. Plant height was measured on day 45 for all blueberry seedlings survived. The eighth leaf from four selected seedling with 10 cm height in each treatment was harvested. Note: Dots of the same color represent biological repetitions of the same treatment. The treatment represented by the color of the points in figures b and c is consistent with that in Figure a.

Figure 3

Nutritional element contents in rhizosphere soils of blueberry plants under different treatments. Experimental groups: PGPR-inoculated (T1, T2, T3) Control group: Nutrient soil alone (labeled TC1) and sterilized SMS mixed with nutrient soil at a 1:4 ratio (labeled TC2). (a) Organic carbon (OC). (b) Total nitrogen (TN) contents; (c) Hydrolysable nitrogen (HN); (d) Total phosphorus (TP); (e) available phosphorus (AP); (f) Total potassium (TK); (g) Available potassium (AK). Notes: Data expressed three replicates per treatment. Abbreviations: OC, organic carbon; TN, total nitrogen; HN, hydrolysable nitrogen; TP, total phosphorus; AP, available phosphorus; TK, total potassium; AK, available potassium. Note: Dots of the same color represent biological repetitions of the same treatment. The treatment represented by the color of the points in (b,c) is consistent with that in (a).

Figure 4

Microbial community analysis in blueberry rhizosphere. (a) Phylum-level bacterial composition heatmap. (b) Phylum-level fungal composition heatmap. (c,d) Bacterial α-diversity assessed using Chao1 (species richness) and Shannon (diversity) indices. (e,f) Fungal α-diversity assessed using Chao1 (species richness) and Shannon (diversity) indices. (g,h) Phylogenetic β-diversity based on unweighted UniFrac distances. (i,j) Principal coordinates analysis (PCoA) of bacterial (i) and fungal (j) communities.

Figure 4

Microbial community analysis in blueberry rhizosphere. (a) Phylum-level bacterial composition heatmap. (b) Phylum-level fungal composition heatmap. (c,d) Bacterial α-diversity assessed using Chao1 (species richness) and Shannon (diversity) indices. (e,f) Fungal α-diversity assessed using Chao1 (species richness) and Shannon (diversity) indices. (g,h) Phylogenetic β-diversity based on unweighted UniFrac distances. (i,j) Principal coordinates analysis (PCoA) of bacterial (i) and fungal (j) communities.

Figure 5

Correlation heatmap integrating PGPR traits, plant growth, soil elements, and rhizosphere microbiota. Note: Chl: total chlorophyll, OC: The organic carbon contents, TN: total nitrogen contents, HN: hydrolysable nitrogen contents, TP: total phosphorous contents, AP: available phosphorous contents, TK: total potassium contents, and AK: available potassium contents. Simpson B and Shannon B were α diversity index of rhizosphere bacteria, and Simpson F and Shannon F were α diversity index of rhizosphere fungi. ** was as significantly associated at 0.01 level (bilateral). * was as significantly associated at 0.05 level (bilateral).