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Article

The Decomposition Dynamics and Substrate Component Potential of Biomass from the Seagrass Posidonia oceanica (L.) Delile

by
Giandomenico Amoroso
1,
Alessia Cozzolino
1,
Mohamed Idbella
1,
Giuseppina Iacomino
1,
Riccardo Motti
1,* and
Giuliano Bonanomi
1,2
1
Dipartimento di Agraria, Università degli Studi di Napoli Federico II, Via Università 100, 80055 Napoli, Italy
2
Task Force on Microbiome Studies, Università degli Studi di Napoli Federico II, 80138 Napoli, Italy
*
Author to whom correspondence should be addressed.
Horticulturae 2024, 10(1), 58; https://doi.org/10.3390/horticulturae10010058
Submission received: 27 November 2023 / Revised: 3 January 2024 / Accepted: 4 January 2024 / Published: 6 January 2024
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Abstract

:
Posidonia oceanica, a Mediterranean Sea seagrass, generates significant litter debris in coastal regions, adversely affecting tourism. To manage this debris, we explored two strategies: (i) promoting in situ decomposition by introducing ligninolytic microbes from forest microbiomes and (ii) utilizing P. oceanica debris as a substrate for ornamental and horticultural species. To achieve this goal, we conducted a one-year experiment using litter bags in mesocosms that simulate in situ conditions, evaluating the second strategy with different application rates (10%, 30%, 50%, and 100%) of fresh and decomposed P. oceanica debris mixed with peat or soil, with or without mineral fertilizer. The results revealed the resistance of P. oceanica necromass to decomposition; in fact, the introduction of forest microbiomes led to a major decomposition rate, albeit with constrained practical applicability. Utilizing P. oceanica debris as a substrate without fertilizer resulted in a modest growth response relative to the application rate, particularly accentuated in horticultural species compared to ornamental ones. Mineral fertilizer alleviated adverse effects at lower application rates; however, a notable decline in growth was observed at the 100% application rate. At application rates of 10% and 30%, certain crops demonstrated improved growth compared to the control. In summary, our study suggests the potential use of raw P. oceanica debris as a growth substrate within the 10% to 50% dosage range.

1. Introduction

Posidonia oceanica (L.) Delile is a seagrass species belonging to the Posidoniaceae family, classified in the Magnoliophyta division. This species is endemic to the Mediterranean Sea, covering almost 50,000 km2 of its seabed [1,2]. P. oceanica is characterized by long, ribbon-like leaves connected to the rhizome via a leaf sheath [3]. A dense network of rhizomes anchors P. oceanica to the seabed, creating extensive meadows in coastal areas with leaf production ranging between 0.3 and 2 kg dry weight m−2 per year [4]. P. oceanica has evolved several remarkable adaptations to survive in the challenging marine environment. Its long, flexible leaves allow it to sway with the motion of the water. Like other marine plants, P. oceanica has adaptations to utilize and respond to different wavelengths of light. The leaf structure maximizes light absorption, and its photosynthetic pigments are optimized for absorbing the available blue and green light [5]. Light availability varies with depth, and the survival and growth limits for P. oceanica depend on factors such as water clarity and other environmental variables. Typically, these limits span from near beach level down to approximately 40 m [6]. Moreover, P. oceanica possesses specific adaptations for withstanding saltwater, such as effective ion regulation and the capacity to prevent the accumulation of excess salt in its tissues [7].
From an ecological standpoint, P. oceanica meadows play a fundamental role in Mediterranean marine ecosystems, significantly contributing to the health and sustainability of coastal ecotone ecosystems [8]. These seagrass meadows support diverse communities, including epiphytic organisms, serving as nurseries and providing food for invertebrates, vertebrates, and juvenile fish [9,10]. Furthermore, P. oceanica roots stabilize the seabed, preventing coastline erosion from wave forces [11]. P. oceanica also acts as a substantial carbon sink, mitigating climate impact by absorbing and storing carbon dioxide (CO2) from the atmosphere. At the end of its approximately one-year life cycle, leaf turnover occurs, leading to leaves and rhizomes detaching due to early autumn weather. These detached parts reach the beach, forming egagropiles (spherical fiber aggregates) and deposits of detritus known as banquettes [12]. These accumulations, growing in height, redefine the beach morphology and can protect it from winter storms [6,13]. The presence of this debris promotes dune plant germination due to its moisture and nutrient-holding capacity. Additionally, this debris is considered a major nitrogen source for foredune plants [14]. Simultaneously, water nutrient content, water temperature, NaCl concentration, and sediment oxygen levels influence decomposition rates, which can extend over several years before measurable degradation [15]. This slow decomposition process is attributed to the biochemical composition of plant tissues, including high C/N and C/P ratios and substantial content of polysaccharides, particularly lignin, tannin, and phenolic acids [2,16]. A recent study by [17] reported that brown litter decomposes at a slower rate than green P. oceanica leaves. However, both substrates exhibit high resistance to decomposition, and the addition of exogenous nitrogen has little effect on their decay rate.
The decomposition of banquettes poses several issues, including the release of unpleasant odors like hydrogen sulfide in anaerobic conditions and the emergence of beach flies [4]. Consequently, banquettes have a negative impact on citizens and bathers due to their unattractive appearance in coastal areas. In Italy, the legislation offers various approaches for managing P. oceanica, acknowledging its impact on beach usability for tourism purposes. The options include treating P. oceanica debris as waste, prioritizing composting sites, burying and/or relocating it within the source beach, or returning it to the sea. All these options require the screening of sand (for re-paving the source beach) and urban waste, with the exception of landfilling, as mandated by [18]. A common practice involves municipalities and beach managers using heavy machinery to remove P. oceanica, unfortunately leading to the elimination of beach sediment. Most local authorities dispose of P. oceanica as waste, combining the debris with generic urban beach waste [6,12,19].
In recent years, various alternative uses for P. oceanica have been explored, such as potential feedstock for biofuel, animal fodder production, biochar, cellulose for packaging, and a substitute for non-sustainable substrates like peat after composting [12,20]. However, these activities pose challenges for small villages due to the absence of specialized industrial structures [17]. This study examines two strategies for the sustainable and efficient management of beached P. oceanica debris. We tested the hypothesis that introducing a microbiome obtained from forest litter, potentially hosting ligninolytic fungi, could significantly accelerate the degradation process, reducing the reliance on industrial composting. To achieve this, we conducted a one-year experiment using litter bags in mesocosms that mimic in situ conditions. The second strategy evaluated the viability of P. oceanica debris as a growth substrate for ornamental and horticultural plants directly without undergoing expensive processes like composting. This assessment underscores its potential for eco-friendly applications, including integration into urban gardens and green spaces. Two types of P. oceanica debris were selected based on their degradation status: recently beached P. oceanica (referred to as fresh P. oceanica) and P. oceanica, that had been beached and accumulated for two years (decomposed P. oceanica). Various experiments were conducted, including different application rates of P. oceanica debris to peat or soil, with and without the addition of mineral fertilizer, using ornamental species (Santolina chamaecyparissus and Senecio vira-vira) or horticultural species (Cucurbita pepo, Lactuca sativa, Ocimum basilicum, and Solanum lycopersicum). The specific aims of the study were as follows:
(1)
Assess the capability of forest microbiome to accelerate the degradation of P. oceanica debris;
(2)
Identify the optimal application rate of P. oceanica debris to support the growth of ornamental and horticultural species;
(3)
Understand the need for mineral fertilization for efficient use of P. oceanica debris as a growing substrate.

2. Materials and Methods

2.1. Study Site and Substrate Collection

The Posidonia oceanica (L.) Delille debris used in the study was collected in June 2022 on a seashore in the village of Agropoli, near Salerno, Southern Italy (40°21′16″ N 14°21′16″ E). Two different types of P. oceanica leaves were sampled based on their status: the “fresh P. oceanica”, i.e., the leaves of P. oceanica beached for less than two months and collected along the shoreline, and “decomposed P. oceanica”, i.e., the debris beached and stored by means of mechanical vehicles on the seashore for at least two years (Figure 1). A total of thirty kilograms per type of P. oceanica debris were taken from ten different sampling points and then transported to the laboratory located in the Department of Agriculture in Portici. The collected material was placed in a ventilated chamber at a constant temperature of 35 °C for ten days until it reached a constant weight.

2.2. P. oceanica Debris Chemical Analysis

The pH and salinity (EC) of P. oceanica debris were measured. For this purpose, we used two instruments: the METTLER TOLEDO pH meter for pH and the BASIC 30 CRISON conductivity meter for salinity. Measurements were performed on 2 dm3 dry weight samples of both types of P. oceanica mixed in 3 dm3 of distilled water inside a 5-L beaker after 24 h of stirring at 200 RPM. This process was repeated three times, and the averages were calculated based on the measurements.
The same P. oceanica debris was characterized in our recent study [17] for the following parameters: organic carbon, total nitrogen, C/N ratio, sodium, cellulose, lignin, and by 13C CPMAS NMR to assess the chemistry of organic carbon. Further chemical analyses, encompassing major nutrients and heavy metals, were conducted to enhance our understanding of the chemical profile of the utilized P. oceanica (Table 1).

2.3. Decomposition Experiment

The litter bags method in mesocosms was adopted for the decomposition experiment [21]. A 1 mm2 mesh wire was employed to construct 60 litter bags, each measuring 15 × 15 cm and containing 10 g of fresh P. oceanica (dry weight) weighed with the OHAUS TS series precision balance (±0.01 g). Litter bags were placed inside plastic containers measuring 80 × 50 cm, filled with a 40 cm layer of fresh P. oceanica debris. Subsequently, the containers were relocated to the department’s greenhouse to emulate in situ conditions, characterized by daily average environmental parameters measured with an OFYLIA 4.0-C sensor, indicating a temperature range of 14 °C to 28 °C and a daylight intensity of 1100 µmol/s/m2, corresponding to an average obtained from measurements taken on ten sunny days at 12:00 a.m. Additionally, the daily light integral calculated throughout the experiment yielded a result of 20 moles/m2/day. To achieve the microbial inoculum, 500 g of forest floor litter belonging to Fagus sylvatica L. forest, Mediterranean mixed forest (Quercus sp., Fraxinus ornus L., Castanea sativa Miller, Alnus cordata (Loisel.) Duby, Ulmus sp.), and Quercus ilex L. Forest were collected. The forest litter microbiome was previously characterized by [22] by means of the next-generation sequencing approach. Each litter type was poured into the beaker with 2 L of sterilized distilled water and stirred for 3 h. The slurry obtained was sieved (2 mm mesh) to remove coarse particles. Subsequently, the slurry was added to the mesocosms (500 mL each) that were covered with a shade cloth. Subsequently, watering was conducted at 15-day intervals, dispensing two l of water per container each time. Since the day of inoculation, five litter bags per treatment were retrieved after 90, 180, and 360 days of incubation. Afterward, the samples were placed in a ventilated oven set at 45 °C for five days, after which they were opened, and the remaining mass was quantified.

2.4. The Use of P. oceanica as a Growth Substrate

To test the performance of P. oceanica biomasses as a substrate for plant growth, two experiments were conducted in a greenhouse of the Department of Agriculture using both ornamental and horticultural plants. Based on the usual cultivation methods currently used in nursery farms, different growing substrates like soil or peat were provided depending on the type of plants. For ornamental species, P. oceanica debris was mixed with peat at different application rates, in terms of dry weight, namely 10%, 30%, 50%, and 100%. Instead, for horticultural species, the debris was mixed with sandy soil, as previously characterized by [23]. In detail, the sandy soil comprises 79.3% sand, 19.4% silt, and 1.3% clay. The additional properties include a 3.08% organic matter content, a pH of 7.53, an electrical conductivity (EC) of 1.1 dS/m, an NH4 (ammonium) concentration of 0.19 mg/kg, a total nitrogen (N) content of 16.8 g/kg, and a phosphorus pentoxide (P2O5) content of 96.65 mg/kg. The utilized peat is classified with a H5 value on the von Post Humification Scale, with properties comprising a pH of 5.87, electrical conductivity (EC) of 0.95 dS/m, NH4 content of 58.3 mg/kg, N (as Kjeldahl nitrogen) of 0.98 g/kg, P of 0.93 mg/kg, K of 1.96 mg/kg, and Ca of 1.45 mg/kg. It is widely recognized that the density of P. oceanica is particularly low, surpassing even that of peat (Table 2). As a result, the addition of P. oceanica into both soil and peat caused a reduction in bulk density proportionate to the amount added to the mixture.
Overall, three horticultural plants were selected, i.e., Cucurbita pepo L., Ocimum basilicum L., and Solanum lycopersicum L., and two ornamental species, i.e., Santolina chamaecyparissus L., and Senecio vira-vira Hieron. The experimental design for each species (Table 3) resulted in a total of 27 pots for each species (two types of P. oceanica × four P. oceanica application rates × three replicates + three control replicates with substrate only).
In a second experimental setting, to simulate the usual agronomic practices currently in use, the same experimental design was conducted for horticultural species but included the application of a ternary fertilizer Nitrophoska® Special 12-12-17, in dosage N (60 kg/Ha), P (60 kg/Ha), and K (85 kg/Ha) producted by EuroChem Agro Spa, Cesano Moderno (MB), Italy. In this case, L. sativa was chosen instead of O. basilicum because of the lack of availability in the period involved. After fertilization, which was carried out in pre-sowing, abundant irrigation followed to encourage the dissolution of the fertilizer. Pots with a diameter of 16 cm were used to set up the experiment. For transplanting, three seedlings per pot were placed in holes 5 cm deep. Post-sowing, irrigation was supplied at a frequency of twice a week in quantities of 200 mL per pot. Plants were allowed to grow to maturity (end of cycle for ornamentals and before harvest for horticultural plants), removing weeds periodically. At the end of the growth cycle (i.e., 90 days for Solanum lycopersicum. Santolina chamaecyparissus, Senecio vira-vira, and 60 days for Cucurbita pepo, Lactuca sativa L., and Ocimum basilicum) and after harvesting, the dry weight of both the above- and below-ground parts of each plant, as well as the flowers/inflorescences and fruits were quantified after drying in a ventilated oven set at 45 °C for five days.

2.5. Data Analysis

A two-way ANOVA was run for the biomass of the cultivated species (root, shoot, flowers, and fruits), with P. oceanica type (decomposed and fresh) and concentration percentage (0%, 10%, 30%, 50%, and 100%) as independent variables. Pairwise differences for each treatment level were statistically evaluated with the Tukey post hoc test, with significance assessed at p < 0.05. In addition, shoot/root ratios were calculated, and a one-way ANOVA was applied accordingly, evaluating differences in statistical significance with Tukey’s post hoc test, p < 0.01. The statistical analysis was performed with IBM SPSS Statistics. A two-way ANOVA was also applied to evaluate significant differences in the percentage of mass loss of P. oceanica via decomposition over time according to different inocula.

3. Results

3.1. Posidonia Chemical Analysis

Fresh P. oceanica exhibited higher pH and salinity mean measurements, with a pH of 8.13 and a salinity of 1.2 mS/cm, while the decomposed P. oceanica leaf showed a pH of 7.75 and a salinity of 0.9 mS/cm.

3.2. Decomposition of P. oceanica Debris

Factors under analysis, such as incubation time, forest inoculum, and their interaction, significantly influence the decomposition of P. oceanica debris (Table 4). In detail, a progressive increase in mass loss of P. oceanica was observed after 90, 180, and 360 days for all treatments. Notably, compared with the control not inoculated, the three treatments with forest inoculum promote the degradation process, more remarked with the addition of F. sylvatica and Q. ilex (Figure 2). Specifically, after 360 days of decomposition, the inoculum from F. sylvatica, Q. ilex, and mixed woodland increased mass loss of 13%, 9.9%, and 4%, compared to the control, respectively.

3.3. P. oceanica as a Growth Substrate without Mineral Fertilizer

Regarding ornamental species, S. vira vira showed a good ability to grow at all application rates except in pure P. oceanica treatments (Figure 3 and Figure 4). S. chamaecyparissus provided better results with a 10% and 30% application rate of decomposed and fresh P. oceanica (Figure 2 and Figure 3). In addition, S. chamaecyparissus shows a concentration-dependent effect, with growth reduction as the application of fresh P. oceanica increases. Overall, very low growth in 100% fresh and 100% decomposed P. oceanica has been observed for both species.
With respect to horticultural species grown without fertilizer, a stepwise decreasing growth trend compared to control from 10% to 100% in decomposed P. oceanica appears evident for both S. lycopersicon and C. pepo, while in fresh P. oceanica, a negative result is observed in all percentages except for the 10% application rate (Figure 5 and Figure 6). Furthermore, all vegetable species exhibited marked growth reduction compared to control with fresh P. oceanica at concentrations ranging from 30% to 100%. In the case of O. basilicum, on the other hand, growth was similar to control at all concentrations of decomposed P. oceanica, except for 100%. Differently, with fresh P. oceanica, there was a gradual growth reduction from 10% to 100% application rate. In addition, in decomposed P. oceanica, there was similar production of inflorescence with control within the 10% to 50% concentration range. However, at 100% decomposed P. oceanica, flowering was absent. Notably, inflorescence was developed only in fresh P. oceanica at the 10% concentration.

3.4. P. oceanica as a Growth Substrate with Mineral Fertilizer

Concerning the species cultivated with the addition of fertilizer, in comparison to the control, L. sativa exhibited a production similar to the control with decomposed P. oceanica, while a substantial reduction was recorded only with fresh debris at 100% application rate (Figure 7). Notably, S. lycopersicum and C. pepo recorded an increase in production compared to control with decomposed P. oceanica across all concentrations but also with 10% and 30% application of fresh debris. C. pepo floral biomass and S. lycopersicum fruit biomass were also higher in soil amended with decomposed P. oceanica compared to the control. Finally, all three species showed similar responses to amendment with fresh P. oceanica, showing a progressive reduction in growth from 10% to 100% application rate.

3.5. Shoot-to-Root Ratios

The shoot-to-root ratios of the cultivated species exhibited distinct responses. Both ornamental species, S. vira vira and S. chamaecyparissus, experienced a reduction in the ratio with increasing P. oceanica application rate. O. basilicum maintained a consistent trend in shoot-to-root ratio, while L. sativa demonstrated a higher ratio in treatments with decomposed P. oceanica compared to fresh P. oceanica. Lastly, in C. pepo and S. lycopersicum, the fertilized treatments yielded higher results than their unfertilized counterparts (Table 5).

4. Discussion

4.1. Decomposition of P. oceanica Debris

P. oceanica tissues exhibit resistance to decomposition, as indicated by [17], and our current study confirms these findings. The debris lost less than 35% of its initial mass after one year of incubation, with factors such as high salinity, elevated lignin concentration, and a high C/N ratio contributing to their poor decomposability. Leaf tissues, in particular, show a predominant concentration of high salt levels compared to other plant parts, being a primary factor for the slow decomposition rate, resulting in debris accumulation on beaches and underwater environments [4,24]. Elevated concentrations of sodium (Na) have phytotoxic effects on higher plants and simultaneously hinder the activity of saprotrophic microbes crucial for decomposition processes [17]. In our study, chemical analyses of P. oceanica revealed only a marginal salinity disparity between decomposed and fresh specimens, despite prolonged exposure of decomposed P. oceanica to rainfall and increased distance from the coastline. This is attributed to rain facilitating leaching by decreasing salinity, while marine aerosols contribute sufficient salts, preventing a strong distinction between fresh and decomposed P. oceanica. This aligns with the findings of [4], where aegagropils under similar conditions as decomposed P. oceanica exhibited lower values compared to other residues, represented in our case by fresh P. oceanica [4].
The decomposition process is significantly influenced by the chemistry of organic carbon tissue, where polysaccharides predominate, along with a substantial portion composed of recalcitrant phenolic compounds, particularly lignin [3,17]. In P. oceanica litter, nitrogen and phosphorus concentrations are exceptionally low, resulting in low C/N and C/P ratios, causing nutrient starvation for saprotrophic microbes [25,26]. These ratios further increase over the process, following a negative exponential pattern [27,28]. However, a recent study by [17] revealed that the exogenous addition of nitrogen to lower the C/N ratio is ineffective in promoting the decomposition rate of such plant tissue. Additionally, the decomposition process, especially when debris is stored in large stocks, could be further impeded by abiotic conditions, including anoxic environments and elevated temperatures, altering nutrient dynamics and affecting the activity of heterotrophic bacteria [16,29].
In our study, we, for the first time, utilized forest microbiomes to introduce ligninolytic fungi, aiming to accelerate the in situ decomposition of P. oceanica. Our results indicated the highest degradation with an inoculum of F. sylvatica forest microbiome, likely due to the prevalence of soft rot fungi, recognized as characteristic decomposers of deciduous wood, including that of F. sylvatica [30]. Soft-rot fungi exhibit strong adaptability to various pH levels, temperature ranges, and limited oxygen conditions compared to other fungi. Moreover, they are among the most effective agents for lignin degradation within mixed microbial populations, possessing ligninolytic enzymes like lignin peroxidase, manganese peroxidase, and laccase [31,32]. A significant degree of decomposition, albeit less than with the F. sylvatica microbiome, was observed using the Q. ilex forest microbiome, consistent with the findings of [33]. This emphasizes the involvement of microorganisms associated with peroxidase activity, acting as early colonizers and catalysts of lignin degradation. Notably, the fungus Marasmius quercophilus Pouzar demonstrates the ability to degrade various polymers, including cellulose, lignin, and the abundant tannins found in Q. ilex wood, via its complex enzymatic activity [34]. While the application of these forest microbiomes significantly expedited the degradation process, the observed increase in the decomposition rate is quantitatively limited, suggesting that on-site decomposition might have limited usefulness. Overall, in situ decomposition does not appear to be a sufficient method for effectively managing P. oceanica deposition under terrestrial conditions. Future studies may explore combining selected microbiomes with nutrient additions (e.g., N, P, and Ca) to identify methodologies capable of rapidly decomposing the tissues of this species.

4.2. P. oceanica Debris as Growing Substrate

The debris of P. oceanica has been utilized in various applications over the years, and more recently, it has been suggested as a promising growth substrate after composting [35]. In this study, we explored another potential strategy for managing P. oceanica debris by using this seagrass, either fresh or decomposed, as a growth substrate for both ornamental and horticultural plants. The experiments involving ornamental species showed favorable outcomes in the 10% to 50% range of application rates for both fresh and decomposed P. oceanica compared to peat. Moreover, S. vira-vira exhibited superior growth compared to S. chamaecyparissus under controlled conditions. For S. chamaecyparissus, a concentration-dependent trend was observed in fresh P. oceanica, where growth diminished with increasing percentages of P. oceanica. In a study on the growth of ornamental species by [36], a substrate composed of P. oceanica compost-based and decontaminated dredged sediments was studied, recommending a utilization concentration ranging from 30% to 70% of P. oceanica. Our findings align with such recommendations, proposing an application ratio ranging from 10% to 50% for these species. Notably, both Senecio and Santolina genera are tolerant to high salt concentrations [37,38].
In the case of horticultural species without the addition of exogenous mineral fertilizers, optimal results were observed at a 10% ratio for both fresh and decomposed P. oceanica, except for O. basilicum, which showed favorable outcomes even at 30% and 50% ratios of decomposed P. oceanica. There was a concentration-dependent trend noted for O. basilicum in fresh P. oceanica and for S. lycopersicum and C. pepo in decomposed P. oceanica, where growth declined with increasing percentages of P. oceanica. Overall, poor growth and a lack of fruiting were observed, likely due to macronutrient deficiencies associated with P. oceanica substrates characterized by very high C/N and C/P ratios [17]. According to stoichiometric theory, when organic debris decomposes, microbes may actively take up nutrients, especially nitrogen, from the organic matrix to meet their nutrient requirements. When the litter C/N ratio exceeds 30, as is the case with P. oceanica litter, microbes could cause nutrient starvation, reducing plant root growth [39]. For these reasons, nutrients can be provided by P. oceanica-based compost, as composts have the potential to enhance soil organic matter content, supply nutrients and growth regulators, and mitigate the production of phytotoxic compounds [40,41]. Furthermore, in accordance with [41], the optimal percentage ratio for the application of P. oceanica-based compost was found to be 30%, aligning with our results favoring a low ratio application, specifically 10%.
We explored an alternative method for supplying macronutrients by incorporating mineral fertilizers into the P. oceanica substrate to enhance plant development and productivity. Our findings indicate that S. lycopersicum and C. pepo exhibit significant growth compared to the control, with the exception of C. pepo at a 50% decomposed P. oceanica ratio and both species at 50% to 100% fresh P. oceanica ratios. Conversely, L. sativa plants displayed reduced growth compared to the control, particularly in fresh P. oceanica, maintaining a consistent trend in both decomposed and fresh P. oceanica, except at a 100% fresh P. oceanica ratio. We recommend optimal concentration ranges for both decomposed and fresh P. oceanica, including 10% and 30% application ratios for S. lycopersicum and C. pepo. For L. sativa, all P. oceanica percentage ratios are suggested, except for 100% fresh material. Notably, successful fruit production was observed in S. lycopersicum, with good results in all percentage ratios except for 100% fresh P. oceanica. Fertilization yielded better results in the substrate with decomposed P. oceanica compared to that with fresh P. oceanica. This is likely because plant growth is constrained not only by nutritional deficiency but also by a phytotoxic effect resulting from the combined impact of salinity and metabolites released during the decomposition process. The analysis of the shoot-to-root ratio for all species considered in the experiment provides an eco-physiological perspective on the obtained results. The root, responsible for various functions, including mineral nutrition, plays a crucial role in supplying essential mineral elements for the growth of the shoot [42]. Numerous studies have investigated plant responses to nutrient availability over the years. It was demonstrated that deficiencies in nutrients such as N, P, and K lead to an increase in root biomass and a decrease in shoot-to-root ratios [43,44]. Additionally, shoot-to-root ratios tend to rise with increasing fertilizer concentration [45]. Our results, presented in Table 5, align with these findings. Both decomposed and fresh P. oceanica have been identified as effective substrates for plant growth, with a particular emphasis on the decomposed type, as previously asserted by [46]. Their study underscored the viability of utilizing P. oceanica residues without composting, noting no observed phytotoxicity effect. However, a phytotoxic effect may arise at high concentrations of Na, as it competes with other cations in the soil, thereby reducing the cation exchange capacity [47]. Contrarily, according to [48], moderate concentrations of Na (6 dS m−1) during a growth experiment of S. lycopersicum did not influence cation availability for plants. Instead, they enhanced fruit size and increased vitamins C, E, and lycopene via the activation of specific metabolite pathways. Future studies will aim to explore the ability of P. oceanica substrates to yield high-quality fruits, as these substrates may constrain the absorption of NO3 by plants. Furthermore, additional studies will be required to assess aspects related to the presence of heavy metals at high concentrations in P. oceanica-based substrates [49,50].

Author Contributions

Conceptualization, G.A. and G.B.; methodology, G.B.; software, G.A. and A.C.; validation, G.B., R.M. and M.I.; formal analysis, G.B.; investigation, G.A. and A.C.; resources, G.I.; data curation, G.B. and M.I.; writing—original draft preparation, G.A. and A.C.; writing—review and editing, G.A. and G.B.; visualization, R.M.; supervision, G.B.; funding acquisition, G.B. and R.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Dipartimento di Agraria, Università degli Studi di Napoli Federico II, under the project “VALORIZZAZIONE AGRONOMICA DEI RESIDUI DI POSIDONIA OCEANICA MEDIANTE L’UTILIZZO DI MICROBIOMI SELEZIONATI”.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Images of Posidonia oceanica: (A) study site with beached fresh P. oceanica; (B) artificial dump of decomposed P. oceanica litter; (C) detail of fresh P. oceanica; (D) and decomposed P. oceanica. Pictures by A. Cozzolino (A), G. Bonanomi (B) and G. Amoroso (C,D).
Figure 1. Images of Posidonia oceanica: (A) study site with beached fresh P. oceanica; (B) artificial dump of decomposed P. oceanica litter; (C) detail of fresh P. oceanica; (D) and decomposed P. oceanica. Pictures by A. Cozzolino (A), G. Bonanomi (B) and G. Amoroso (C,D).
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Figure 2. Decomposition of P. oceanica not inoculated or inoculated with different microbiomes. Values are average ± standard deviation; statistical details are in Table 4.
Figure 2. Decomposition of P. oceanica not inoculated or inoculated with different microbiomes. Values are average ± standard deviation; statistical details are in Table 4.
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Figure 3. Biomass of ornamental species cultivated in fresh and decomposed P. oceanica substrate. Different letters (lowercase letter = shoot; uppercase letter = root) indicate statistically significant differences within each substrate type (ANOVA Tukey test; p < 0.05).
Figure 3. Biomass of ornamental species cultivated in fresh and decomposed P. oceanica substrate. Different letters (lowercase letter = shoot; uppercase letter = root) indicate statistically significant differences within each substrate type (ANOVA Tukey test; p < 0.05).
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Figure 4. Selected images of S. chamaecyparissus (A) and S. vira vira (B) cultivated in a substrate characterized by different application rates of decomposed P. oceanica substrate. Pictures by G. Amoroso.
Figure 4. Selected images of S. chamaecyparissus (A) and S. vira vira (B) cultivated in a substrate characterized by different application rates of decomposed P. oceanica substrate. Pictures by G. Amoroso.
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Figure 5. Biomass of horticultural species cultivated in fresh and decomposed P. oceanica substrate without the addition of mineral fertilizers. Different letters (lowercase letter = shoot; uppercase letter = root; italics = flowers) indicate statistically significant differences within each substrate type (ANOVA Tukey test; p < 0.05).
Figure 5. Biomass of horticultural species cultivated in fresh and decomposed P. oceanica substrate without the addition of mineral fertilizers. Different letters (lowercase letter = shoot; uppercase letter = root; italics = flowers) indicate statistically significant differences within each substrate type (ANOVA Tukey test; p < 0.05).
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Figure 6. Selected images of horticultural species cultivated in a substrate characterized by varying loading ratios of P. oceanica substrate; (A) L. sativa cultivated in a substrate characterized by different application rates of fresh P. oceanica; (B) S. lycopersicum in the control and grown in pure fresh or decomposed P. oceanica; (C) O. basilicum cultivated in a substrate characterized by different application rates of decomposed P. oceanica without addition of mineral fertilizers. Pictures by G. Amoroso.
Figure 6. Selected images of horticultural species cultivated in a substrate characterized by varying loading ratios of P. oceanica substrate; (A) L. sativa cultivated in a substrate characterized by different application rates of fresh P. oceanica; (B) S. lycopersicum in the control and grown in pure fresh or decomposed P. oceanica; (C) O. basilicum cultivated in a substrate characterized by different application rates of decomposed P. oceanica without addition of mineral fertilizers. Pictures by G. Amoroso.
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Figure 7. Biomass of horticultural species cultivated in fresh and decomposed P. oceanica substrate in presence of mineral fertilizer. Different letters (lowercase letter = shoot; uppercase letter = root; italics = flowers/fruits) indicate statistically significant differences within each substrate type (ANOVA Tukey test; p < 0.05).
Figure 7. Biomass of horticultural species cultivated in fresh and decomposed P. oceanica substrate in presence of mineral fertilizer. Different letters (lowercase letter = shoot; uppercase letter = root; italics = flowers/fruits) indicate statistically significant differences within each substrate type (ANOVA Tukey test; p < 0.05).
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Table 1. Chemical traits of fresh and decomposed P. oceanica assessed by elemental analyses.
Table 1. Chemical traits of fresh and decomposed P. oceanica assessed by elemental analyses.
P. oceanica
DecomposedFresh
CSC (meq/100 g)58.045.1
P (mg/kg)133.2163.5
K (mg/kg)698.0806.9
Ca (mg/kg)31,567.025,224.7
Na (mg/kg)681.51742.0
Mg (mg/kg)4889.05017.7
N (%)1.81.1
C (%)38.938.3
C/N22.135.1
C/P2921.72342.8
B (mg/kg)3297.23120.4
Fe (mg/kg)3818.93874.2
Mn (mg/kg)35.431.9
Zn (mg/kg)116.1108.1
Se (mg/kg)0.90.7
Mo (mg/kg)0.60.5
Al (mg/kg)1665.31521.7
Cu (mg/kg)18.019.3
Pb (mg/kg)7.78.1
Hg (mg/kg)<0.1<0.1
Cr (mg/kg)6.56.5
Ni (mg/kg)20.722.4
Cd (mg/kg)0.20.3
As (mg/kg)2.52.4
Table 2. Bulk density of the substrates used in the experiment with different P. oceanica application rates.
Table 2. Bulk density of the substrates used in the experiment with different P. oceanica application rates.
Bulk Density (kg/m3)
P. oceanica Application RateMixture
SoilPeat
0%1121.21125.00
10%1009.12117.27
30%798.33100.68
50%582.7382.91
100%42.4042.40
Table 3. Experimental design concerning the use of P. oceanica as a substrate for cultivation of ornamental and horticultural species.
Table 3. Experimental design concerning the use of P. oceanica as a substrate for cultivation of ornamental and horticultural species.
Cultivation SystemSpeciesMixtureP. oceanica Application Rate
HorticolturalS. lycopersicumFresh P. oceanica-soil
Decomposed P. oceanica-soil
L. sativaFresh P. oceanica-soil
Decomposed P. oceanica-soil
O. basilicumFresh P. oceanica-soil
Decomposed P. oceanica-soil
C. pepoFresh P. oceanica-soil(100%; 50%; 30%; 10%; 0%)
Decomposed P. oceanica-soil
OrnamentalS. vira viraFresh P. oceanica-peat
Decomposed P. oceanica-peat
S. chamaecyparissusFresh P. oceanica-peat
Decomposed P. oceanica-peat
Table 4. Synthetic results of two-way ANOVA test of P. oceanica decomposition experiment. Inoculum type (F. sylvatica, Q. ilex, and mixed woodland) and decomposition time (90, 180, and 360 days) are the main factor of the analysis.
Table 4. Synthetic results of two-way ANOVA test of P. oceanica decomposition experiment. Inoculum type (F. sylvatica, Q. ilex, and mixed woodland) and decomposition time (90, 180, and 360 days) are the main factor of the analysis.
EffectdFFp-Value
Inocula37.732<0.001
Time2121.914<0.001
Inocula × time60.6270.708
Error48
Table 5. Shoot-to-root ratios were calculated for ornamental and horticultural species cultivated in both fresh and decomposed P. oceanica substrate. Different letters denote statistically significant differences within each substrate type, determined via ANOVA Tukey test (p < 0.05). Additionally, * and ** signify significant differences at p < 0.05 and 0.01, respectively, within the same species (both fertilized and unfertilized) and substrate.
Table 5. Shoot-to-root ratios were calculated for ornamental and horticultural species cultivated in both fresh and decomposed P. oceanica substrate. Different letters denote statistically significant differences within each substrate type, determined via ANOVA Tukey test (p < 0.05). Additionally, * and ** signify significant differences at p < 0.05 and 0.01, respectively, within the same species (both fertilized and unfertilized) and substrate.
P. oceanica TypeP. oceanica Application RateS. vira-viraS. chamaecyparissusO. basilicumL. sativaS. lycopersicumS. lycopersicum
Fertilized		C. pepoC. pepo Fertilized
0%8.30 a7.92 a1.28 a4.53 a5.47 abc20.41 a**2.97 c18.58 a**
decomposed10%4.55 a9.39 ab2.01 a2.39 a5.81 ab16.39 a**12.52 abc9.49 b
30%4.41 a7.76 ab2.16 a4.66 a5.81 ab 14.67 a*18.39 a9.85 b**
50%3.46 a5.43 ab2.00 a4.95 a6.76 ab11.64 a*16.22 ab9.57 b*
100%1.11 a2.61 b2.28 a3.76 a6.33 ab11.63 a 5.36 bc7.82 b
fresh10%6.99 a9.65 ab1.62 a1.78 a8.23 a16.78 a*9.92 abc10.50 b
30%3.95 a6.74 ab2.04 a2.60 a4.06 bc20.25 a**11.56 abc10.73 b
50%2.53 a7.28 ab1.88 a1.73 a3.57 bc14.14 a*8.10 bc6.42 b
100%1.01 a1.34 b3.77 a1.25 a1.96 c11.45 a*6.52 bc6.69 b
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Amoroso, G.; Cozzolino, A.; Idbella, M.; Iacomino, G.; Motti, R.; Bonanomi, G. The Decomposition Dynamics and Substrate Component Potential of Biomass from the Seagrass Posidonia oceanica (L.) Delile. Horticulturae 2024, 10, 58. https://doi.org/10.3390/horticulturae10010058

AMA Style

Amoroso G, Cozzolino A, Idbella M, Iacomino G, Motti R, Bonanomi G. The Decomposition Dynamics and Substrate Component Potential of Biomass from the Seagrass Posidonia oceanica (L.) Delile. Horticulturae. 2024; 10(1):58. https://doi.org/10.3390/horticulturae10010058

Chicago/Turabian Style

Amoroso, Giandomenico, Alessia Cozzolino, Mohamed Idbella, Giuseppina Iacomino, Riccardo Motti, and Giuliano Bonanomi. 2024. "The Decomposition Dynamics and Substrate Component Potential of Biomass from the Seagrass Posidonia oceanica (L.) Delile" Horticulturae 10, no. 1: 58. https://doi.org/10.3390/horticulturae10010058

APA Style

Amoroso, G., Cozzolino, A., Idbella, M., Iacomino, G., Motti, R., & Bonanomi, G. (2024). The Decomposition Dynamics and Substrate Component Potential of Biomass from the Seagrass Posidonia oceanica (L.) Delile. Horticulturae, 10(1), 58. https://doi.org/10.3390/horticulturae10010058

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