1. Introduction
With rapidly intensifying climate change and an increasing global population, food security is a serious global scale issue [
1]. The frequent occurrence of extreme weather events and imbalanced food supply patterns exacerbate the challenges that impact economic and social development worldwide [
2,
3]. To narrow the gap between food supplies and conventional open-field production, greenhouse technologies have been employed for the cultivation of crops and horticulture plants that can improve the production of off-season products [
4]. Greenhouse cultivation has the capacity to finely control temperatures and microclimates to facilitate the year-round growth of horticulture plants to compensate for climatic instabilities in the field [
5]. However, greenhouse technologies are a significant energy-intensive agricultural industry [
6]. In the pursuit of high crop yields, surplus inputs of fossil-based mineral fertilizers can have deleterious effects on soil, leading to environmental contamination in greenhouse cultivation systems [
7]. Hong et al. [
8] suggested that excessive nutrient accumulation in plant root zones and downward leaching via irrigation result in soil and groundwater contamination in greenhouse cultivation systems. Furthermore, increased soil salinity, decreased crop yields, and continuous cropping obstacles (CCO) result in bottleneck issues for the development of the vegetable industry in greenhouses [
9,
10].
In seeking an alternative system to replace conventional greenhouse cultivation, researchers have invested more effort into potential alternatives. The application of agro-based spent mushroom compost substrates has been shown to be a beneficial alternative approach for the cultivation of crops and horticultural plants in greenhouse systems [
11]. Spent mushroom compost (SMC) substrates are an effective cultivation media for vegetable production [
12], which can reduce nitrogen loss in greenhouse systems [
13]. The spent compost substrate consisted primarily of residual fungal mycelium, disintegrated lignocellulosic biomass, various nutrients, as well as organic matter and enzymes. Further, it had a low bulk density, loose texture, good air permeability, and nutrient retention. Thus, the SMC can be employed to bioremediate contaminated soil, and enhance its health [
14] by improving the physical structure of the soil and ecological environment for soil microorganisms [
15]. Furthermore, the SMC also contains a high organic matter content, phosphorus, and potassium, as well as trace elements that are required for plant assimilation and utilization [
16]. Owing to its low level of toxic elements, strong absorption capacity, and enhanced soil aeration and water retention capacities, SMC has been utilized as a soil amendment that contributes to improving soil quality, agronomic efficiency, and environmental safety [
15,
17]. In addition, it contains several biologically active compounds such as therapeutically valuable polysaccharides and antibacterial peptides. These, together with its antibacterial properties protect plants against pathogens; thus, minimizing the incidence of plant diseases [
18,
19].
Under conventional greenhouse conditions, horticultural systems are subject to reduced soil organic matter, declining soil quality and fertility, and decreased microbial biomass [
20,
21,
22]. Further, increased soil salinity, nutrient leaching, and soil compaction can result in continuous cropping obstacles (CCOs), which inhibit the prospects for increased production in greenhouse systems [
8,
10,
23]. Although the use of SMC substrates as a biofertilizer, soil amendment, and for the bioremediation of pollution have attracted much attention [
15], its role as a growth media for the cultivation of crops and horticultural plants in greenhouses is not well understood [
11]. Unal [
12] suggested that the application of SMC as a cultivation media exhibited positive impacts on the quality of tomato seedlings, plant growth, available organic matter, and nutrient uptake of plants [
24,
25]. Thus, there is a need to determine exactly how SMC influences the growth of tomato plants in greenhouses, inclusive of soil microbial communities and enzyme activities under different cultivation durations.
For this study, we investigated the impacts of SMC under varying cultivation timelines on physicochemical soil properties, soil enzyme activities, and the composition of microbial communities. We hypothesized that under greenhouse conditions: a) the application of agro-based SMC as growth media enhances the contents of major nutrients, enzyme activities, and microbial alpha-diversity in contrast to continuous mono-cropping soil; b) soil physicochemical properties and enzyme activities decrease with longer cultivation timelines; c) the composition of bacterial communities are more similar than fungal communities between the growth substrate and surrounding soil along with longer cultivation timelines; d) abundant beneficial microbes can be enriched by the SMC substrate and surrounding soil, which contribute to soil amelioration and improved plant growth.
4. Discussion
Spent mushroom compost (SMC) substrates, which comprise a recycled and reutilized waste products in mushroom cultivation, have been recognized as an organic material source and soil amendment for greenhouse vegetable farming [
11]. In this experiment, we found that SMC substrates had more abundant nutrients than continuous mono-cropping soil (soil CK) and the soil surrounding the substrate (
Table 1). Particularly, in the greenhouse the total N and total P contents were significantly higher in the SMC substrate than in the surrounding soil [
57,
58]. These results reinforced that the application of SMC substrates can provide a suitable growth medium for horticultural crops [
59], and can enhance the crop yields in greenhouse farming [
11]. Meanwhile, the physicochemical soil properties of the SMC substrate were significantly decreased along with the duration of cultivation (
Table 1). It was proved that nutrient reduction and the depletion of the SMC substrate was accompanied by plant growth with longer cultivation duration [
60]. Nevertheless, the physicochemical soil properties were significantly higher in the SMC substrate than in the surrounding soil (
Table 1). Lou et al. [
61] proved that the SMC substrate applied to agricultural land enhanced the soil organic matter and nutrient contents, while it reduced the total N leaching. These results demonstrated that the SMC substrate was an efficient alternative growth medium for the cultivation of horticultural crops compared to typical greenhouse soil.
Enzyme activities are an indicator of soil quality and participate in nutrient cycling during plant growth [
62], which can be utilized to assess plant growth medium in ecosystems in response to available nutrients and metabolic requirements [
63]. Soil alkaline phosphatases [
64], soil protease [
65], and β-Glucosidase [
66] are important enzymes for available P, N, and C cycling processes. The concentrations of these enzymes differ between substrates and surrounding soils under various cultivation durations (
Table 2), which is mainly due to differences in physicochemical soil properties, such as soil organic matter, total N, and pH, and soil enzyme activities involving Glu, ALP, and Pro enzymes (
Table 1,
Figure 5). Sinsabaugh [
67] reported that extracellular enzymes are often associated with the acquisition, transformation, and mineralization of C, P, and N in their growth medium. Among the older cultivated plants, the activities of C, N, and P cycling enzymes were reduced in the substrate medium (
Table 2). It was elucidated that crop growth and harvesting can lead to decreased enzyme activities due to the reduction/shortage of nutrients year by year in the growth medium [
68]. The β-Glucosidase activity was enhanced with longer cultivation duration, whereas the activities of protease and alkaline phosphatases reached their peak values under the Soil 1yr and Soil 3yr treatments compared to the continuous mono-cropping soil (Soil CK treatment), respectively (
Table 2). These results may help to explain how enzyme activities were influenced by crop root systems via the alteration of root system architectures [
69] and rhizosphere exudations [
70] in the growth medium, as well as the exchange of nutrients between the SMC substrate and surrounding soil. Furthermore, enzyme activities primarily originated from root secretion and microorganisms [
71], and were closely correlated with microbial communities and structures in the growth media ecosystem [
72].
Spent mushroom compost (SMC) substrates, consisting of spent mushroom compost, manure residues, and other agricultural waste, can partially or completely substitute growth media for horticultural crop production in greenhouses [
73]. Meng et al. [
74] reported that SMC substrates were a better alternative than peat-based growth media for greenhouse tomato and pepper seedlings due to higher morphological growth and lower instances of
Fusarium (of
Ascomycota phylum) pathogen infections. It is also known that SMC substrates can be employed as an alternative source of organic matter for crop growth, which contribute to increased soil microbiological activities [
75]. In this study, following multiple years of tomato cultivation, the SMC substrates still had higher microbial alpha-diversity than continuous monocropping soil (Soil CK) and non-planted SMC substrates (Sub CK) treatment (
Figure 1). This result confirmed that crop root systems can augment microbial diversity through the formation of beneficial symbionts [
76,
77]. Undergoing several years of tomato cultivation, the compositions of bacterial and fungal communities showed similar trend in the growth medium (substrate and surrounding soil) (
Figure 2). It was revealed that continuous cropping practices altered the microbial community structures and compositions in the rhizospheric soil [
23,
78], as well as in the spent mushroom substrate [
79]. The continuous cropping system is a common practice in greenhouse vegetable farming, which has adverse effects on horticultural crop yields and quality due to pathogenic diseases [
80].
Continuous cropping obstacles (CCO) induce declines in crop quality and yields and the exacerbation of diseases and pests [
81], which may be correlated with the modification of soil enzyme activities and microorganism communities [
82]. A reduction in beneficial microbes (i.e
Bacillus and
Trichoderma) and accumulation of fungal pathogens
Fusarium occurred under various continuous cropping practices in greenhouse soil [
83].
Fusarium wilt (FW) in strawberry was mainly caused by
Fusarium oxysporum fungal pathogens in greenhouses [
84]. In this study, the continuous cropping soil of tomatoes in greenhouses (Soil CK) exhibited a higher abundance of pathogenic bacteria of the
Cyanobacteria phyla and fungi of the
Ascomycota phyla(
Table 3, Figure S3). Denikina et al. [
85] reported that
Cyanobacteria taxa were associated with the diseased endemic sponge
Lubomirskia baicalensis in Lake Baikal. Challacombe et al. [
86] revealed that
Ascomycota fungi can exist as latent saprotrophs or pathogens within plant tissues based on genome and secretome analyses in arid ecosystems. In contrast, the SMC substrates had a higher abundance of bacteria (
Deinococcus-Thermus, Patescibacteria, Dependentiae, Halanaerobiaeota, and
Nitrospirae) (
Table 3,
Figure 4a) and fungi (
Basidiomycota, Rozellomycota, Mortierellomycota, Mucoromycota, Chytridiomycota, and
Olpidiomycota) (
Table 3,
Figure 4b).
Deinococcus-Thermus bacteria are highly resistant against extreme environmental stress [
87,
88]. Tian et al. [
89] demonstrated that the bacteria super-phyla
Patescibacteria contains ultra-small cells, simple membrane structures and streamlined redundant and nonessential functions to avoid phage predation, and adapt to specific stressed environments. It was reported that the
Halanaerobiaeota phyla was dominant in extremely haloalkaline environments due to its salt tolerance and anaerobic attributes [
90], while
Nitrospirae phyla were less prevalent and participated in C and N cycling between different ecosystems [
91,
92]. Further, as an important ectomycorrhizal fungi, the
Basidiomycota phyla can undergo symbiosis with host plants, which become their C sources and habitats [
93]. Both
Rozellomycota and
Chytridiomycota fungi belong to the Zoosporic phyla with motile spores, which typically play critical ecological roles in the recycling of energy and matter in food webs [
94]. As fast growing saprotrophic fungi, the
Mortierellomycota phyla are potentially influenced by soil temperature [
95] and have important biological functions for the protection of plants against pathogens [
96]. Interestingly,
Mucoromycota fungi may be utilized as a biorefinery that employs fungi for its highly versatile metabolic system, which can generate several valuable bioproducts including of pigments, polyphosphates, ethanol, organic acids, enzymes, as well as low- and high-value lipids [
97,
98]. Therefore, it was revealed in this study that the greenhouse SMC substrate model was more enriched with several beneficial microbes than the continuous cropping field model.
Compared with open-field cultivation, plastic-greenhouse cultivation is a popular agricultural production platform on a global scale [
9]. With the advantages of a prolonged growing season with a stable hospitable environment, plastic-greenhouse cultivation is preferred in many cases for the production of high value vegetables and other crops [
99,
100]. However, long-term intensive plastic-greenhouse cultivation can easily give rise to continuous cropping obstacles (CCO), which lead to deleterious changes in the composition of soil microbial communities [
9]. Interesting, this study suggested that the long-term intensive cropping of tomatoes with the SMC substrate growth medium (1-, 3-, and 7-years cultivation) in a plastic-greenhouse had more similar bacterial beta-diversity than the non-planted SMC substrates (Sub CK) (
Figure 2). It was demonstrated that long-term greenhouse vegetable cultivation with the SMC substrate growth medium altered the structures of soil microbial communities, as reported by Liu et al. [
101]. Furthermore, the soil fungi beta-diversity of surrounding soil between the different cultivation durations (1-, 3-, and 7-years cultivation) was relatively consistent, in contrast to continuous mono-cropping soil (Soil CK) (
Figure 2b). The root systems [
102] and root exudates [
103] of vegetable crops play critical roles in the compositional restructuring of soil microbial communities in the rhizosphere and surrounding soil environments, particularly under continuous mono-cropping [
104]. Unal [
11] suggested that SMC substrates were a viable alternative growth media for tomato seedling production in plastic greenhouses. Thus, SMC substrate cultivation might be a feasible strategy for the improved large scale production of horticultural vegetables and crops in greenhouses worldwide [
73].
Meanwhile, we found that the dominant taxa
Dongia and
Sphingomonas (
Proteobacteria phyla) were highly abundant in the continuous mono-cropping soil (Soil CK) (
Figure 3a). Pathogenic bacteria in the
Sphingomonas genus commonly cause brown spot disease on yellow Spanish melon fruits [
105], and rase Panax ginseng with rusty root disease that seriously affects its production [
106]. Yet, several beneficial bacterial genera (i.e.
Saccharimonadales, Nocardioides, Gaiella) were significantly dominant in the SMC substrate growth media for tomato cultivation in the greenhouse (
Figure 3a).
Saccharimonadales (
Patescibacteria phylum) was influenced by soil sugar concentrations, the high abundance of which can enhance soil alkaline phosphatase activities [
107].
Nocardioides (
Actinobacteria phylum) was responsible for the degradation of vinyl chloride (VC), which is carcinogenic to humans [
108]. Zhao et al. [
109] reported that
Nocardioides and
Gaiella were important beneficial bacteria for the suppression of
Fusarium wilt in a long-term tomato monoculture soil. Even the
Bacillus genus of important beneficial bacteria exhibited a significantly higher abundance in the surrounding soil of the 7-year cultivation (
Figure 3a) [
83]. Fungi were enriched in the SMC substrate and surrounding soil environment, which included several potentially beneficial fungal taxa of
Thermomyces that secrete glycoside hydrolase and proteases [
110] and coprophilous fungi of
Kernia genus [
111]. Further, the biodegradation fungi of the
Kernia and
Mortierella genera [
112], the core fungus of
Remersonia genus involved in humification processes [
113], bioremediation microbiomes of contaminated
Aspergillus genus environments [
114], and alkaliphilic fungus of
Sodiomyces genus were present [
115], (
Figure 3b). In contrast, the
Ascobolus [
116] and
Fusarium [
117] taxa of potential pathogens were significantly decreased during long-term cultivation with the SMC growth medium under greenhouse conditions.