2.1 Changes in soil physicochemical properties
To better distinguish the changes in the soil physical and chemical properties after morel planting, we compared the soil before and after planting. Cultivation of M. sextelata and M. septimelata did not significantly affect the physical and chemical properties of the soil (Table 1). However, relatively higher soil OC, NN, AP, Ca, and K contents were observed after planting M. sextelata. Nevertheless, compared to the original soil (pH 6.46), the pH of the soil associated with both M. sextelata and M. septimelata exhibited a significant reduction of 15.3% and 13.5%, respectively.
Table 1
Alterations in soil physical and chemical properties.
Treatment
|
Yield
|
pH
|
OC (mg kg− 1)
|
NN (mg kg− 1)
|
AP (mg kg− 1)
|
Elements
|
(kg ha− 1)
|
Ca (mg L− 1)
|
K (mg L− 1)
|
Mg (mg L− 1)
|
Na (mg L− 1)
|
Primitive soil
|
-
|
6.46 ± 0.14 a
|
12.1 ± 0.94 b
|
0.33 ± 0.16 b
|
11.2 ± 3.55 c
|
2.37 ± 0a
|
4.00 ± 0.98 c
|
35.9 ± 1.34 a
|
3.17 ± 1.24 a
|
M. sextelata
|
4045 ± 60.7 a
|
5.47 ± 0.07 b
|
21.2 ± 4.59 a
|
3.28 ± 0.71 a
|
26.0 ± 4.55 a
|
2.03 ± 0.90 a
|
27.7 ± 0.61 a
|
1.40 ± 0.09 b
|
2.63 ± 0.94 a
|
M. septimelata
|
3558 ± 44.1 b
|
5.59 ± 0.04 b
|
14.6 ± 3.72 ab
|
1.17 ± 0.59 b
|
18.7 ± 2.53 b
|
1.16 ± 0.44 a
|
25.7 ± 0.76 b
|
1.57 ± 0.00 b
|
2.74 ± 1.37 a
|
Note: OC, organic carbon; NN, nitrate nitrogen; AP, available phosphorus. The data presented arplicates ± SD. The letter indicates statistical significance statistically significant differences based on the LSD Fisher test with a \(\:P<\:0.05\). |
2.2 Soil microbial community diversity and structure
Following quality filtering, 494,834 high-quality 16S rDNA sequences and 560,802 high-quality ITS rDNA sequences were obtained from six soil samples. Each sample retained more than 20 sequences, resulting in the identification of 6,483 bacterial and 1,201 fungal OTUs. Three alpha diversity indices (Coverage, Chao, Ace, and Shannon) were employed to assess the coverage, richness, and diversity of the soil microbial communities.
The growth of morels influenced the alpha diversity of bacteria and fungi, with notable differences observed between M. sextelata and M. septimelata (Fig. 1A). Specifically, the Chao, Ace, and Shannon indices of fungi and bacteria associated with M. sextelata were higher compared to those of M. septimelata, suggesting greater microbial abundance in M. sextelata. Principal component analysis (PCA) of fungal genera indicated that samples from M. sextelata and M. septimelata tended to cluster together, signifying similarity (Fig. 1B). In contrast, the PCA results for the bacterial genera displayed considerable dissimilarity (Fig. 1C). These findings suggest divergent niches between M. sextelata and M. septimelata, with M. sextelata exhibiting a broader niche and more scattered sample distribution, while M. septimelata demonstrates a narrower niche with samples concentrated in closer proximity (Fig. 1D).
Understanding the interactions between morels and these microorganisms is crucial for the successful cultivation and preservation of morel populations. The composition of the soil microbial communities following the cultivation of M. sextelata and M. septimelata is depicted in Fig. 2. The predominant fungal phyla observed were Ascomycota, Mortierellomycota, and Basidiomycota, collectively representing > 98% of the total soil fungi (Fig. 2A). Moreover, both species belonged to Ascomycota, with M.sextelata displaying a higher abundance (89.7%) compared to M.septimelata (80.0%). The dominant bacterial phyla observed include Proteobacteria, Actinobacteriota, Chloroflexi, Bacteroidota, Acidobacteriota, Firmicutes, Gemmatimonadota, Myxococcota, Methylomirabilota, and Verrucomicrobiota (Fig. 2B), collectively representing over 75% of the total soil bacteria. Actinobacteriota was the most prominent phylum associated with M. sextelata, accounting for a relative abundance of 37.7%. Conversely, Proteobacteria emerged as the predominant phylum associated with M. septimelata, with a relative abundance of 42.8%. Furthermore, the distributions of other bacterial phyla associated with M. sextelata and M. septimelata exhibited notable differences. Specifically, Actinobacteriota, Chloroflexi, and Acidobacteriota were found to be more abundant in M. sextelata compared to M. septimelata. Conversely, the abundance of Proteobacteria displayed the opposite trend, being higher in M. septimelata than in M. sextelata. The soil fungal ITS1 region and bacterial 16S rRNA genes associated with M. sextelata and M. septimelata exhibited 425 and 2976 shared operational taxonomic units (OTUs), respectively. The shared OTUs represented species common to both M. sextelata and M. septimelata, accounting for 35.3% and 45.9% of the total 16S rRNA and ITS datasets, respectively (Fig. 2A and 2B).
2.3 Interactions between microbial taxa in the network
To elucidate connectivity within the bacterial and fungal communities, we computed all possible Pearson correlation coefficients among the top 200 abundant OTUs in both M. sextelata and M. septimelata, constructing a co-occurrence network. Comparing the bacterial and fungal network structures of M. sextelata and M. septimelata, the network associated with M. sextelata exhibited greater complexity and a higher number of links than that of M. septimelata (Fig. 3A and 3B). Specifically, the edges connecting soil bacteria and fungi in M. sextelata numbered 1,579 and 1,813, respectively, whereas in M. septimelata, they numbered 1,124 and 1,444, respectively.
Following OTU taxonomic analysis, we identified OTU1225 and OTU439 as being associated with M. sextelata and M. septimelata, respectively. Subsequently, we assessed the relationship between the top 50 and 100 bacterial and fungal OTUs and morels (Fig. 4). Compared to M. septimelata, M. sextelata has more connections with microorganisms. Among the identified OTUs, 7 bacterial and 3 fungal species were found to negatively affect M. sextelata. Notably, Devosia, Exophiala pisciphila, and Nectriaceae showed a significant negative correlation with the abundance of M. sextelata (|r| > 0.9, \(\:P\:<\:0.05\)). Conversely, four bacterial and four fungal species exhibited significant positive effects on M. sextelata (top 50 in abundance), including Marmoricola, Vicinamibacteraceae, Ascomycota, and Chordomyces antarcticus (|r| > 0.9, \(\:P\:<\:0.05\)). However, the top 50 bacteria seemed to not affect the growth of M. septimelata and were only associated with two fungi. Therefore, we expanded the scope and established a network diagram including the top 100 microorganisms. The connections between M. septimelata and microorganisms remain limited, with only one bacterium and three fungi having a negative impact, and one fungus having a positive impact. Pyrenochaeta and Pleosporales had a significant negative effect on M. septimelata (|r| > 0.9, \(\:P\:<\:0.05\)).
2.4 Functional Prediction Analysis
FUNGuild classified and analyzed the fungal community based on microecological guilds, functionally categorizing fungi using published literature or authoritative website data 22. The primary functions attributed to the soil of M. sextelata and M. septimelata were identified as saprophytic and pathogenic bacteria, collectively constituting over 70% of the total abundance in the predictive functional analysis (Fig. 5). Specifically, the contribution rates of saprophytic bacteria and pathogens associated with M. sextelata were 58.7% and 18.3%, respectively, whereas those associated with M. septimelata were 65.9% and 4.4%, respectively. M. septimelata demonstrates a positive impact on pathogen inhibition and organic matter decomposition.
2.5 Correlation analysis
The Mantel test and correlation analysis revealed a relationship between morel production and soil physical and chemical properties (Fig. 6). Nine variables were associated with the growth of the morels. Specifically, the growth of M. sextelata was positively correlated with Na but negatively correlated with pH, OC, NN, AP, Ca, and Mg. Conversely, the growth of M. septimelata showed a significant positive correlation with Mg and P (\(\:P\:<\:0.01\)), as well as a positive correlation with AP. Furthermore, correlations among soil physical and chemical properties were observed, with NN significantly positively correlated with OC, and pH significantly positively correlated with Mg (\(\:P\:<\:0.01)\). Additionally, NN was significantly positively correlated with AP and K, while Mg was significantly negatively correlated with K and P (\(\:P\:<\:0.05\)).
Using the Spearman-based co-occurrence network, a significant correlation was observed between bacterial and fungal genera (relative abundance of the top 50 OTUs) and soil properties (Fig. 7). In this analysis, positive interactions were observed in bacterial and fungal systems, accounting for 37.2% and 60.5%, respectively (\(\:\left|r\right|\:>\:0.5\), \(\:P\:<\:0.05\)). Within the bacterial system, P accounted for 28.6%, with pH, Mg, Ca, and Na accounting for 16.7%, 16.7%, 14.3%, and 11.9%, respectively. Notably, Stenotrophomonas exhibited significant negative correlations with P (r < -0.9, \(\:P\:<\:0.05\)), while Terrabacter and Vicinamibacteraceae showed significant positive correlations with P (r < 0.9, P\(\:\:<\:0.05\)). Notably, Stenotrophomonas may contribute to an increase in soil pH, with Devosia and Rhizobium tibeticum demonstrating similar effects on pH elevation. In the fungal system, AP accounted for 26.3%, with OC, P, Mg, Na, and NN accounting for 13.2%, 13.2%, 10.5%, 10.5%, and 10.5%, respectively. The four pairs of relationships are significantly correlated with each other (\(\:\left|r\right|\:>\:0.88\), \(\:P\:<\:0.05\)). Trichothecium roseum was significantly positively correlated with Ca. P was significantly positively correlated with Chordomyces antarcticus, Myrmecridium, and Ascomycota (r < 0.88, \(\:P\:<\:0.05\)).