Sorghum and Hemp Responses to Plant Growth-Promoting Microorganism Inoculation in Metal-Contaminated Dredged Sediment: A System-Level Assessment Under Environmentally Relevant Outdoor Pot Conditions
1Faculty of Sciences, University of Novi Sad, Trg Dositeja Obradovića 3, 21000 Novi Sad, Serbia; marko.solic@dh.uns.ac.rs (M.Š.); nina.djukanovic@dh.uns.ac.rs (N.Đ.); tamara.apostolovic@dh.uns.ac.rs (T.A.); irinaj@dh.uns.ac.rs (I.J.); dragana.tamindzija@dh.uns.ac.rs (D.T.); marijana.kragulj@dh.uns.ac.rs (M.K.I.)
2Institute of Field and Vegetable Crops, National Institute of the Republic of Serbia, Maksima Gorkog 30, 21000 Novi Sad, Serbia; ivana.bajic@ifvcns.ns.ac.rs (I.B.); stanko.milic@ifvcns.ns.ac.rs (S.M.); tijana.zeremski@ifvcns.ns.ac.rs (T.Z.)
*Correspondence: snezana.maletic@dh.uns.ac.rsAbstract
Metal-contaminated dredged sediments represent heterogeneous environmental matrices in which remediation responses are frequently constrained by elevated background metal loads and complex geochemical conditions. Within such systems, phytoremediation has been discussed as a nature-based management approach whose outcomes depend on plant biomass, internal metal allocation, and context-dependent interactions between plants and sediment. The present study evaluated whether bacterial and fungal plant growth-promoting microorganisms (PGPMs) were associated with changes in plant metal uptake and internal allocation in Sorghum bicolor L. and Cannabis sativa L. grown in dredged sediment collected from the Bega Canal. An outdoor pot experiment was conducted under environmentally relevant conditions, including bacterial and fungal inoculation treatments alongside non-inoculated controls, with plant responses to Cr, Ni, Cu, Zn, As, Cd, and Pb characterized using concentration- and mass-based uptake metrics, root–shoot partitioning, and sediment geochemical assessment based on pseudo-total concentrations and BCR sequential extraction fractions. Across treatments, plant responses were largely governed by intrinsic species traits and biomass production, while PGPM-associated effects remained modest and variable. Root-dominated metal retention and limited translocation were evident irrespective of species, consistent with a phytostabilization-type response rather than systematic extraction. Absolute metal uptake accounted for only a minor fraction of total sediment metal pools, underscoring the importance of interpreting concentration-based indices jointly with mass-based metrics when evaluating system-scale responses. Altogether, the findings indicate that under the investigated outdoor dredged sediment pot conditions, PGPM inoculation acts primarily as a context-specific modulator of plant responses rather than a driver of enhanced phytoremediation performance, reflecting the central role of intrinsic plant traits and stabilization-oriented processes in complex sediment systems.
1. Introduction
Sediments in inland waterways act as long-term sinks for potentially toxic elements, reflecting the combined effects of historical industrial, municipal, and diffuse anthropogenic inputs. Routine maintenance dredging redistributes these sediments, turning a largely sequestered contamination reservoir into a material that requires active management once removed from the aquatic system. Handling such dredged materials is additionally complicated by their pronounced physicochemical heterogeneity, mixed contamination profiles, and the disturbance of geochemical equilibria upon excavation. These challenges are further amplified by the large volumes of material generated during maintenance dredging, the costs associated with transport, treatment, and disposal, and regulatory constraints governing the safe reuse or final management of contaminated sediments. Consequently, remediation or beneficial reuse of dredged sediments is constrained by both environmental risk concerns and practical limitations arising from their complex composition [1,2,3,4].
Such complexity has direct implications for remediation performance, as the physicochemical heterogeneity of dredged sediments gives rise to spatially variable metal pools and constrains the predictability of treatment outcomes. Within these matrices, metals are distributed among multiple solid phases and binding environments, leading to uneven availability and heterogeneous response patterns even under nominally identical treatment conditions. Accordingly, remediation responses documented in simplified or highly controlled experimental systems may not translate directly to complex dredged sediments, where context-dependent behavior often plays a central role [5,6,7,8].
Metal-contaminated soils and sediments may be managed using conventional physical, chemical, and thermal treatment methods, such as excavation and replacement, capping or containment, soil washing, solidification/stabilization, immobilization, electrokinetic remediation, and vitrification. Ex situ approaches can provide relatively rapid contaminant removal or risk reduction, particularly in localized or highly contaminated materials, but they may also require intensive material handling, substantial energy or reagent inputs, and high costs, while potentially altering the physical, chemical, or biological properties of the treated substrate. In situ approaches can reduce excavation and transport requirements, although their effectiveness may be constrained by metal speciation, treatment duration, and the long-term stability of the treated material. For dredged sediments, method selection is further complicated by large material volumes and by the need to balance contaminant reduction with disposal, beneficial reuse, and long-term environmental safety. Accordingly, lower-input biological approaches remain relevant as complementary alternatives, particularly where risk reduction, stabilization, ecological compatibility, and resource-efficient management are prioritized over rapid contaminant removal [4,6,9].
In this context, phytoremediation has been proposed as a nature-based approach for managing metal-contaminated soils and sediments, relying on plant–substrate interactions under prevailing environmental conditions rather than intensive physical or chemical interventions. Depending on system properties and management objectives, phytoremediation is commonly discussed in terms of phytoextraction and phytostabilization, which differ in their emphasis on contaminant removal versus immobilization within the substrate. Across complex matrices, phytoremediation outcomes have, however, been described to depend less on tissue concentration metrics alone and more on plant biomass production, root retention, and internal metal partitioning. Accordingly, in substrates such as dredged sediments, phytoremediation has been discussed primarily in terms of modulation and stabilization of metal behavior rather than consistent removal from the system [10,11,12,13,14].
Against this background, plant growth-promoting microorganisms (PGPMs), including both bacterial and fungal inoculants, have been explored as potential modifiers of plant–metal interactions in contaminated soils and sediments. At a more conceptual level, such microorganisms have been discussed in relation to their influence on rhizosphere-mediated processes, including nutrient availability, root development, and metal availability and partitioning. However, reported PGPM effects on metal uptake and internal plant metal handling have been highly variable across studies, with responses shown to depend on substrate properties, plant species, and cultivation conditions. Together, this variability highlights the importance of evaluating PGPM responses within realistic systems that reflect the complexity of contaminated substrates [15,16,17,18,19].
The present study was designed to evaluate whether bacterial and fungal PGPM inoculation was associated with changes in metal uptake and internal metal allocation in plants grown in contaminated dredged sediment under environmentally relevant conditions. To address this goal, the study was conducted as an outdoor pot experiment, with Sorghum bicolor L. and Cannabis sativa L. selected as model plant species. Plant biomass production and root–shoot metal partitioning were examined to characterize system-level patterns in plant metal handling under these conditions.
To capture system-level responses, the study incorporated concentration-based and mass-based metrics of plant metal uptake with indicators of internal metal allocation between roots and shoots. Sediment metal pools were quantified to provide environmental context for plant responses, rather than to serve as direct predictors of metal uptake or internal partitioning. By using real contaminated dredged sediment and a system-level evaluation of plant metal handling, the study builds on previous PGPM phytoremediation research implemented under simplified or highly controlled conditions. The study was not designed to infer mechanistic causality or to demonstrate systematic enhancement, but rather to assess context-dependent responses at the system level under environmentally relevant outdoor pot conditions.
2. Materials and Methods
2.1. Sediment Origin and Characterization
The sediment used for this study originated from a section of the Bega Canal in Serbia, which constitutes part of the Danube–Tisa–Danube (DTD) hydrosystem connecting the Danube and Tisa rivers. This canal network has been subjected to long-term inputs from industrial effluents, municipal wastewater, and diffuse agricultural runoff, resulting in the progressive accumulation of heavy metals in bottom sediments [20].
For the present study, dredged sediment was collected from a confined disposal area containing material removed during previous maintenance dredging of the Bega Canal. The disposal area is located near Srpski Itebej, close to the Serbian–Romanian border, extending approximately between 45°34′48.91″ N, 20°45′28.01″ E and 45°34′50.03″ N, 20°45′30.86″ E. Approximately 500 kg of bulk sediment material was collected from ten points across the disposal site using an excavator, combined to obtain a representative bulk composite sample, transported by truck to the experimental facility under ambient conditions, and mechanically homogenized prior to use. The homogenized sediment was employed as the growth substrate for all treatments.
The physicochemical properties and contamination status of the dredged sediment have been previously described in detail [21] and provided the foundation for the present pot experiment. Baseline sediment characteristics and heavy metal concentrations were determined again prior to the pot experiment to confirm its suitability and to obtain an independent dataset for the current study.
As indicated by the values shown in Table 1, the sediment was alkaline, sandy loam in texture, contained moderate organic matter and measurable nutrient contents, and was characterized by a relatively high cation exchange capacity and non-saline conditions.
2.2. Pot Experiment Design
The pot experiment was conducted outdoors under natural environmental conditions in Rimski Šančevi, Serbia, employing dredged sediment as the growth substrate and two test species, forage sorghum (Sorghum bicolor L.) and hemp (Cannabis sativa L.). Certified seeds of S. bicolor cv. NS Džin and C. sativa cv. HELENA were obtained from the Institute of Field and Vegetable Crops, Novi Sad, Serbia, with detailed seed-lot quality parameters provided in Table S1. These species were selected as fast-growing, high-biomass annual crops with documented tolerance to metal-contaminated conditions and relevance for phytoremediation and phytomanagement of contaminated soils and sediments, providing two model plant systems for assessing PGPM-associated responses under the same dredged sediment conditions [21,22,23,24].
Each pot was loaded with 5 kg of air-dried and homogenized sediment that had been prepared immediately prior to the experiment. Five seeds were sown per pot, and upon germination, the seedlings were thinned to retain a single plant per pot. The experimental design encompassed three treatments for each species—control (no inoculant; S or H), bacterial inoculation (S/B or H/B), and fungal inoculation (S/F or H/F)—with each treatment established in triplicate. Both plant-growth-promoting inoculants were supplied as liquid commercial formulations, including a bacterial PGPR-based product (GROUNDFIX) and a fungal inoculant (MYCOFRIEND), and were applied once just before sowing. The inoculants were not experimentally formulated or optimized within this study; instead, the manufacturer-specified field-equivalent quantities (4 and 0.5 L/ha, respectively) were used. These quantities were diluted in distilled water to obtain an adequate working volume and uniformly sprayed over the sediment prior to pot filling. The cultivation period lasted 10 weeks, from 25 April 2024 to 3 July 2024, and was selected based on plant development during the experiment and previous experience from comparable contaminated-sediment cultivation studies [21,25].
For the cultivation period, key meteorological parameters were obtained from Meteostat [26], using data from the Novi Sad–Rimski Šančevi meteorological station, the nearest measuring station to the experimental site, and are summarized in Table S2. Because the experiment was conducted outdoors, air temperature, precipitation, and light exposure reflected naturally occurring meteorological conditions rather than controlled laboratory or greenhouse settings. During the 10-week cultivation period, the average air temperature was 20.9 °C, with daily mean temperatures ranging from 9.5 to 29.3° C. Cumulative precipitation amounted to 163.7 mm, with precipitation recorded on 28 days. The average daylight and dark periods were approximately 15 h and 9 h, respectively, with an average sunshine duration of approximately 480 min/day. Pots were irrigated with tap water as needed, according to plant developmental stage and prevailing outdoor conditions, together with visual assessment of substrate moisture. Irrigation was applied carefully to maintain plant growth while avoiding excessive wetting and visible leaching from the pots; however, substrate moisture was not continuously instrument-monitored, and leachate was not collected or chemically analysed.
At harvest, each plant was removed from its corresponding pot and processed as an individual biological replicate. Shoots and roots were separated and thoroughly rinsed with distilled water to remove adhering particles. The plant material was then dried at 40 °C, weighed, and subsequently ground to a fine powder prior to chemical analysis.
Prior to sowing, sediment allocated for each treatment was sampled to obtain a composite pre-harvest sample, derived by combining representative subsamples from the homogenized sediment batch. At harvest, sediment from the three replicate pots within each treatment was collected and pooled to generate one post-harvest composite sample per treatment. This approach was used to obtain an integrated treatment-level sediment sample in a matrix characterized by microscale heterogeneity and to provide a geochemical context for plant responses. Accordingly, post-harvest pseudo-total concentrations and BCR fractions were interpreted descriptively as treatment-level sediment data, rather than as explanatory factors for differences observed among plant biological replicates. All pre- and post-harvest sediment samples were air-dried, ground, and sieved prior to chemical analysis.
2.3. Assessment of Phytoremediation Efficiency
Phytoremediation performance was evaluated using standard soil-to-plant transfer indices. The bioaccumulation factor (BAF) was calculated separately for roots and shoots as the ratio of metal concentration in plant tissue (Cplant) to the corresponding metal concentration in sediment (Csed). The translocation factor (TF) was determined as the ratio of metal concentration in shoots (Cshoot) to that in roots (Croot). The calculations of BAF and TF were performed according to Equations (1) and (2), respectively [27,28].
2.4. Chemical and Analytical Methods
Physicochemical properties of the sediment were assessed according to the protocols described below: pH was measured potentiometrically in a 1:5 (w/v) sediment-to-deionized water suspension after 60 min of equilibration, with a glass electrode [29]. Electrical conductivity was determined in a 1:5 sediment-to-deionized water extract following a 30 min agitation period [30]. Organic matter content was quantified by the loss-on-ignition method subsequent to combustion at 550 °C [31]. Total nitrogen was determined using the Kjeldahl method, consisting of sulfuric acid digestion followed by distillation and titration [32]. Total phosphorus was quantified after extraction with ammonium lactate, with subsequent colorimetric detection using the molybdenum blue procedure and measurement by UV–Vis spectrophotometry (Shimadzu UV-1800, Shimadzu Corp., Kyoto, Japan), according to an internal laboratory protocol. Total potassium was determined after microwave-assisted digestion (Start E, Milestone, Sorisole, Italy), with subsequent measurement by flame emission spectrophotometry employing an atomic absorption spectrometer (iCE 300 series, Thermo Fisher Scientific, Cambridge, UK), as specified in Method 3500-K D [33]. Cation exchange capacity was assessed by ammonium acetate extraction (1 M NH4OAc, pH 7.0) following a method modified from van Reeuwijk [34], calculated based upon the sum of exchangeable base cations and expressed as cmol(c)/kg dry weight. Exchangeable Ca2+ and Mg2+ were quantified by graphite furnace atomic absorption spectrometry following Method 7010 [35], while Na+ and K+ were determined by flame emission spectrophotometry in accordance with Methods 3500-Na B and 3500-K D [33], respectively. Particle size distribution was evaluated by dry sieving for the sand fraction and sedimentation analysis for the silt and clay fractions [36].
Pseudo-total metal concentrations in sediment and plant samples were determined after microwave-assisted acid digestion as specified in Method 3051A [37]. The resulting digests were analyzed by atomic absorption spectrometry employing graphite furnace or flame modes, as appropriate (refer to instrumentation details above), in accordance with Methods 7010 and 7000B, respectively [35,38]. Element-specific AAS conditions and validated method-performance parameters are provided in Supplementary Table S3. The measured elements included Cr, Ni, Cu, Zn, As, Cd, and Pb. Although As is chemically classified as a metalloid, it is commonly discussed together with heavy metals in environmental contamination studies; therefore, for consistency and readability, the analysed elements are collectively referred to as “heavy metals” throughout the manuscript.
Operationally defined metal fractions in sediment were obtained by applying the BCR sequential extraction procedure following the method described in [39]. Metal concentrations in the extracts were quantified by atomic absorption spectrometry under the same instrumental conditions.
2.5. Data Analysis
Statistical analyses were performed using TIBCO Statistica, version 14.1 (TIBCO Software Inc., Palo Alto, CA, USA). All statistical procedures were performed exclusively on plant datasets comprising three biologically independent replicates per treatment. Plant data were summarised as means ± standard deviations, and individual replicate values were plotted to visualise biological variability within treatments. Non-parametric statistical methods were used as the small number of biological replicates did not permit reliable assessment of normality, and environmental datasets typically deviate from parametric assumptions. Within each plant species, treatment differences (S vs. S/B vs. S/F and H vs. H/B vs. H/F) were evaluated by means of the Kruskal–Wallis test, followed by Dunn’s post hoc comparisons with Bonferroni correction for multiple testing. Direct statistical comparisons between sorghum and hemp were not performed because the two species represent distinct plant systems rather than alternative treatment levels; therefore, cross-species differences were summarised descriptively. Statistical significance was defined as p < 0.05 [40,41,42]. Figures were constructed using OriginPro 8.5 (OriginLab Corporation, Northampton, MA, USA). The overall experimental workflow is summarized in Figure 1.
3. Results
4. Discussion
The present study assessed the potential influence of two PGPM inoculants on the behaviour of multiple heavy metals in a contaminated dredged sediment system cultivated with Sorghum bicolor L. and Cannabis sativa L. Rather than seeking to establish consistent enhancement of metal uptake, the study was designed to examine whether PGPM inoculation was associated with changes in plant metal accumulation and internal metal distribution under environmentally relevant outdoor pot conditions. Across the investigated parameters, the results were characterised by modest effects, substantial variability, and a general lack of statistical significance at the treatment level.
Such an overall result profile is not unexpected for outdoor pot experiments conducted on inherently heterogeneous dredged sediments, where plant development, microbial activity, and metal speciation are jointly governed by intrinsic substrate properties and biological interactions [45,46,47]. Within this context, parameters describing internal plant distribution of metals provide a particularly informative framework for interpreting metal dynamics across treatments and species.
Accordingly, the Discussion adopts a pattern-oriented approach, focusing on recurring trends across complementary metrics rather than isolated treatment effects or individual metals. Emphasis is placed on internal plant distribution of metals, including root–shoot concentration patterns and translocation behaviour, while PGPM-associated differences are addressed in terms of their magnitude and context dependence. Sediment-based metrics are employed mainly for environmental background, not for establishing direct causal links with plant uptake.
Across all treatments and both plant species, internal plant distribution of metals was consistently characterized by pronounced root retention and limited translocation to aboveground tissues. Root metal concentrations exceeded those in shoots for all investigated elements, and translocation factors remained below unity throughout, indicating a conservative internal handling strategy that was largely unaffected by PGPM inoculation. This root-dominant distribution pattern suggests that internal metal allocation was principally governed by intrinsic plant control rather than treatment-associated differences. Such behaviour is commonly recognized as indicative of a phytostabilization-type response, whereby metals are preferentially sequestered in belowground tissues [22,24,48,49]. In the context of the present study, this consistent internal distribution pattern provides a more robust grounding for interpretation than absolute uptake metrics, which were more susceptible to variability in plant biomass.
While concentration-based indices consistently indicated conservative internal metal handling, mass-based metrics revealed a more nuanced picture of metal allocation within the plants. Root–shoot partitioning and absolute metal uptake are inherently influenced by plant biomass and growth differences and therefore capture quantitative aspects of metal accumulation rather than regulatory allocation processes. Importantly, the coexistence of low TFs with less pronounced root-dominated mass-based root–shoot allocation can be attributed to differences between concentration-derived and mass-based metrics, as opposed to a shift toward greater translocation to aboveground tissues. Absolute metal uptake varied across treatments and between the two plant species, largely reflecting differences in biomass production instead of uniform shifts in internal metal allocation. Taken together, these findings highlight the value of jointly considering concentration-based indices and mass-based metrics when assessing plant metal behaviour in contaminated sediment systems [10,12,14].
Against this background of strong intrinsic control over internal metal allocation, the responses to PGPM inoculation in terms of absolute plant metal uptake were generally modest and variable, without evidence of consistent or directional enhancement across species and treatments. Within this quantitative framework, PGPM-associated differences in absolute metal uptake were restricted to variation in magnitude, without indicating a systematic modification of accumulation behaviour. Notably, the direction and extent of these uptake variations differed between the two plant species, suggesting that species identity conditioned the expression of PGPM-associated differences.
Beyond treatment-related variability, species identity emerged as a dominant factor structuring absolute metal uptake, with sorghum consistently exhibiting higher uptake magnitudes than hemp across corresponding treatments. This interspecific contrast was primarily captured by differences in uptake magnitude, reflecting differences in plant biomass and growth characteristics rather than divergent metal handling strategies. Accordingly, the higher absolute uptake observed in sorghum does not imply a shift toward phytoextraction-type behaviour, but instead reflects a higher capacity for metal retention consistent with its larger biomass [11,12,13,50].
Despite the noted differences in plant metal uptake across species and treatments, the absolute amounts accumulated in plant tissues accounted for only a minor fraction of the total metal pools present in the dredged sediment, with even the highest values falling below approximately 0.2%. In this context, any measurable changes observed in sediment metal concentrations over the course of the experiment cannot be directly ascribed to plant accumulation alone, but are likely associated with the combined influence of sediment heterogeneity, physical redistribution, and geochemical re-equilibration processes operating at the system scale [45,46,51,52].
Beyond total sediment concentrations, BCR fractionation offered additional qualitative insight into the environmental context of metal behaviour within the dredged sediment system. In general, baseline fractionation profiles indicated that metals were dominantly associated with low-mobility fractions, consistent with a sediment matrix characterised by limited immediate metal availability despite elevated pseudo-total concentrations. Following plant growth, shifts among fractions were primarily reflected as moderate internal redistributions between residual, reducible, and oxidisable pools, with the exchangeable fraction remaining largely unchanged. Importantly, the magnitude of these redistributions was overall limited and frequently within the range of analytical uncertainty, indicating that the chemical stability of the sediment was not fundamentally altered over the course of the experiment. Taken together, these patterns characterize a chemically conservative sediment environment, in which plant- and treatment-related responses were expressed under conditions of intrinsically constrained metal mobility [10,21,53,54].
Within this geochemical setting, BAFs serve as dimensionless descriptors of relative sediment-to-plant metal transfer, whose interpretive value is fundamentally system dependent. Accordingly, higher BAFs in roots than in shoots indicated a root-dominated accumulation pattern that is fully aligned with the conservative internal metal handling and limited translocation documented across species and treatments. However, when assessed alongside mass-based uptake metrics, it becomes clear that moderate BAFs do not necessarily correspond to substantial metal removal, particularly in sediment systems characterized by elevated total metal pools. This divergence between relative accumulation and system-scale removal is particularly apparent in the present study, where plant- associated metal pools remained negligible with respect to the total sediment metal burden. These observations underscore the necessity of interpreting BAFs jointly with mass-based metrics when evaluating plant metal behaviour in severely contaminated sediment systems [9,11,14,55].
To relate the present BAF and TF values to previously reported data, selected values for plants grown in contaminated sediments and related contaminated substrates are summarized in Table 5. Because these indices are strongly affected by substrate properties, contamination level, plant species, cultivation duration, and treatment strategy, the comparison should be interpreted descriptively rather than as a direct performance ranking. Nevertheless, several recurring patterns can be identified. Most notably, BAF values are frequently higher in roots than in shoots, while TF values often remain below unity, consistent with preferential metal retention in belowground tissues rather than extensive phytoextraction. These cross-study patterns support the phytostabilization-oriented interpretation of the present results [10,21,22,45,49,51,54].
In a broader context, PGPMs, encompassing both bacterial and fungal inoculants, have been recognized to influence plant–metal interactions through multiple rhizosphere-mediated pathways. Such pathways have been described to include alterations in rhizosphere chemistry (e.g., pH and organic ligand exudation), microbial metal complexation, and indirect effects on root development. However, the extent to which such pathways are expressed is highly context dependent, governed by sediment geochemistry, plant species traits, and system-level constraints, and therefore may not manifest uniformly across experimental settings [52,56,57,58]. Within this framework, the present findings indicate that plant species identity, biomass production, root-dominated metal retention, and sediment constraints were more prominent determinants of metal uptake and allocation than the context-dependent and secondary PGPM-associated responses, supporting a phytostabilization-oriented interpretation for both species under the investigated dredged sediment conditions.
Limitations and Future Research Perspectives
In addition to the considerations outlined in Section 3.2.7, the broader interpretation of the present findings must account for several system-level and methodological constraints. Although the outdoor pot design using real dredged sediment enhanced environmental relevance, it inevitably reduced the degree of experimental control over sediment heterogeneity and fine-scale geochemical variability. Nevertheless, the pot-based setup remains a simplified experimental system that does not fully reproduce field-scale hydrology, rooting conditions, or long-term environmental dynamics. Furthermore, irrigation was managed to avoid excessive wetting and visible leaching; however, substrate moisture was not continuously instrument-monitored, and leachate was not collected or chemically analysed. As a result, potential water-driven changes in metal mobility, redistribution, or leaching losses could not be directly quantified. The relatively short 10-week cultivation period also limits inference on longer-term metal stabilization, exhaustion of sediment metal pools, temporal changes in metal binding, and persistence of sediment–plant–PGPM interactions. Finally, while shifts among BCR fractions and plant-related parameters were documented, the present design does not permit causal attribution of these changes to specific biological or physicochemical processes, constraining extrapolation beyond comparable dredged sediment systems [13,15,21,23,59].
Future research should build on the present system-level assessment by examining the persistence and scalability of phytostabilization-oriented responses under field-relevant conditions. Key aspects to evaluate include whether root-dominated metal retention remains stable across extended or repeated cultivation cycles, temporal changes in sediment metal binding, the influence of water dynamics on metal mobility, and direct characterization of PGPM establishment and rhizosphere-level processes.
From an applied perspective, future studies should assess whether the modest and context-dependent PGPM-associated responses observed here can be enhanced or made more consistent through integrated amendment-based management strategies. These may include biochar-based approaches derived from harvested biomass, as well as other organic amendment strategies, alone or in combination with PGPM inoculation [24,59]. Such work would help determine how the phytostabilization-oriented responses observed in this study can be translated into practical recultivation and management strategies for contaminated dredged sediments.
5. Conclusions
This study evaluated whether bacterial and fungal PGPM inoculation was associated with changes in plant metal uptake and internal allocation in a contaminated dredged sediment system cultivated with Sorghum bicolor L. and Cannabis sativa L. Across the evaluated parameters, PGPM inoculation resulted in overall modest and variable responses, without evidence of consistent or directional enhancement of metal uptake across species or treatments. Instead, intrinsic plant traits shaped metal behaviour, with both species showing conservative internal handling characterized by dominant root retention and limited translocation, compatible with a phytostabilization-type response. At the system scale, absolute metal uptake by plants represented only a minor fraction of the total sediment metal pools, suggesting that phytoremediation outcomes were constrained by the magnitude and geochemical stability of the dredged sediment matrix. Taken together, these results imply that in heterogeneous dredged sediment systems, expectations of PGPM-assisted phytoremediation should be calibrated toward context-dependent modulation and stabilization rather than systematic facilitation of metal extraction.
Acknowledgments
This research was supported by the Science Fund of the Republic of Serbia, #6769, Natural based efficient solution for remediation and revitalization of contaminated locations using energy crops—ReNBES. During the preparation of this work, the authors used ChatGPT-5.1 (OpenAI) to assist with language refinement, structural organization, and improvement of clarity in selected sections of the manuscript. All generated content was carefully reviewed, edited, and verified by the authors to ensure accuracy, consistency with the underlying data, and alignment with the authors’ scientific interpretation. The authors take full responsibility for the content of the published article.
Appendix Group
Supplementary Materials
The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/jox16030102/s1, Table S1: Cultivar, source, and seed-lot quality parameters of Sorghum bicolor L. and Cannabis sativa L. used in the pot experiment; Table S2: Daily meteorological parameters during the 10-week cultivation period [26]. Table S3: Element-specific AAS conditions and validated method-performance parameters for heavy metal determination.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.
Conflicts of Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
| Parameter | Unit | Value |
|---|---|---|
| pH | / | 7.92 ± 0.18 |
| Electrical conductivity | µS/cm | 211.33 ± 35.87 |
| Organic matter | % | 7.91 ± 1.02 |
| Total nitrogen | % | 0.22 ± 0.01 |
| Total phosphorus | % | 0.060 ± 0.01 |
| Total potassium | % | 0.33 ± 0.04 |
| Cation exchange capacity | cmol(c)/kg | 64.39 ± 26.10 |
| Texture | % Sand (50–2000 µm) | 79.91 ± 1.37 |
| % Silt (2–50 µm) | 6.95 ± 0.27 | |
| % Clay (<2 µm) | 13.14 ± 1.13 |
| Treatment Code | Metal Trend |
|---|---|
| S | Zn > Cr > Cu > Ni > Pb > As > Cd |
| S/B | Zn > Cr > Cu > Ni > As > Cd > Pb |
| S/F | Cu > Ni > As > Cr > Cd > Zn > Pb |
| H | Cu > Zn > Cr > Ni > As > Cd > Pb |
| H/B | Zn > Cr > Cu > Ni > As > Cd > Pb |
| H/F | Ni > Zn > Cu > Cr > As > Pb > Cd |
| Metal | Treatment Code | |||||
|---|---|---|---|---|---|---|
| S | S/B | S/F | H | H/B | H/F | |
| Cr | 0.25 ± 0.08 | 0.12 ± 0.01 | 0.16 ± 0.01 | 0.13 ± 0.02 | 0.10 ± 0.01 | 0.12 ± 0.01 |
| 2.40 ± 0.81 | 2.01 ± 0.37 | 2.03 ± 0.27 | 0.66 ± 0.21 | 0.54 ± 0.08 | 0.99 ± 0.06 | |
| Ni | 0.28 ± 0.06 | 0.23 ± 0.03 | 0.22 ± 0.01 | 0.18 ± 0.03 | 0.15 ± 0.02 | 0.21 ± 0.03 |
| 2.11 ± 0.85 | 1.44 ± 0.28 | 1.37 ± 0.09 | 0.86 ± 0.23 | 0.74 ± 0.11 | 1.23 ± 0.15 | |
| Cu | 0.08 ± 0.03 | 0.09 ± 0.01 | 0.08 ± 0.02 | 0.12 ± 0.04 | 0.09 ± 0.00 | 0.14 ± 0.03 |
| 0.67 ± 0.08 | 0.74 ± 0.14 | 0.91 ± 0.18 | 0.60 ± 0.16 | 0.59 ± 0.16 | 0.68 ± 0.09 | |
| Zn | 0.30 ± 0.16 | 0.30 ± 0.05 | 0.30 ± 0.05 | 0.29 ± 0.04 | 0.36 ± 0.01 | 0.45 ± 0.13 |
| 0.96 ± 0.26 | 0.98 ± 0.26 | 0.96 ± 0.17 | 0.63 ± 0.29 | 0.78 ± 0.15 | 0.86 ± 0.09 | |
| As | 0.02 ± 0.02 | 0.03 ± 0.02 | 0.04 ± 0.01 | 0.04 ± 0.02 | 0.07 ± 0.01 | 0.04 ± 0.03 |
| 2.29 ± 0.39 | 1.99 ± 1.76 | 3.47 ± 1.12 | 2.29 ± 1.04 | 3.31 ± 1.46 | 2.69 ± 1.37 | |
| Cd | 0.22 ± 0.15 | 0.15 ± 0.06 | 0.24 ± 0.05 | 0.02 ± 0.01 | 0.03 ± 0.01 | 0.03 ± 0.01 |
| 1.78 ± 0.42 | 1.76 ± 0.65 | 1.58 ± 0.12 | 1.33 ± 0.21 | 1.56 ± 0.65 | 1.92 ± 0.28 | |
| Pb | 0.27 ± 0.02 | 0.27 ± 0.16 | 0.21 ± 0.06 | 0.31 ± 0.10 | 0.28 ± 0.10 | 0.68 ± 0.25 |
| 0.01 ± 0.01 | 0.01 ± 0.00 | 0.01 ± 0.00 | 0.01 ± 0.01 | 0.01 ± 0.00 | 0.01 ± 0.00 | |
| Metal | Treatment Code | |||||
|---|---|---|---|---|---|---|
| S | S/B | S/F | H | H/B | H/F | |
| Cr | 0.12 ± 0.08 | 0.06 ± 0.01 | 0.08 ± 0.01 | 0.20 ± 0.03 | 0.19 ± 0.02 | 0.12 ± 0.02 |
| Ni | 0.15 ± 0.07 | 0.17 ± 0.04 | 0.16 ± 0.01 | 0.22 ± 0.04 | 0.21 ± 0.04 | 0.17 ± 0.04 |
| Cu | 0.12 ± 0.06 | 0.13 ± 0.04 | 0.09 ± 0.02 | 0.21 ± 0.08 | 0.16 ± 0.05 | 0.21 ± 0.08 |
| Zn | 0.29 ± 0.09 | 0.33 ± 0.10 | 0.32 ± 0.07 | 0.52 ± 0.16 | 0.48 ± 0.11 | 0.53 ± 0.20 |
| As | 0.01 ± 0.01 | 0.03 ± 0.03 | 0.01 ± 0.00 | 0.02 ± 0.00 | 0.03 ± 0.02 | 0.02 ± 0.02 |
| Cd | 0.12 ± 0.05 | 0.09 ± 0.03 | 0.15 ± 0.05 | 0.01 ± 0.01 | 0.02 ± 0.01 | 0.01 ± 0.00 |
| Pb | 0.04 ± 0.02 | 0.04 ± 0.02 | 0.03 ± 0.01 | 0.04 ± 0.01 | 0.03 ± 0.01 | 0.02 ± 0.01 |
| Plant Species | Substrate | Treatment | Metal | BAF (Shoot) | BAF (Root) | TF Range | Ref. |
|---|---|---|---|---|---|---|---|
| Sorghum bicolor | Dredged sediment (Bega Canal) | Organic acids (GA, TA) | Cr | 0.026–0.360 | 0.402–1.652 | 0.024–0.583 | [21] |
| Ni | 0.065–0.302 | 0.228–0.511 | 0.040–0.253 | ||||
| Cu | 0.077–0.176 | 14.97–20.79 | 0.172–0.373 | ||||
| Cd | 0.879–2.804 | 1.298–1.825 | 0.524–1.616 | ||||
| Pb | 0.007–0.025 | 0.157–0.419 | 0.019–0.107 | ||||
| Cannabis sativa | Cr | 0.006–0.048 | 0.031–0.140 | 0.199–0.926 | |||
| Ni | 0.050–0.137 | 0.018–0.336 | 0.349–0.771 | ||||
| Cu | 0.048–0.150 | 0.175–4.321 | 0.602–1.065 | ||||
| Cd | 0.029–0.117 | 0.159–0.519 | 0.120–0.338 | ||||
| Pb | 0.004–0.022 | 0.009–0.096 | 0.237–3.107 | ||||
| Typha angustifolia | Dredged sediment | Aeration, waterlogged | Cr | 0.020–0.032 | 0.088–0.090 | 0.226–0.357 | [10] |
| Pb | 0.026–0.054 | 0.063–0.148 | 0.365–0.539 | ||||
| Cu | 0.181–0.237 | 0.313–0.576 | 0.397–0.578 | ||||
| Cd | n.d. | 0.314–0.327 | n.d. | ||||
| Brassica napus | Dredged sediment (Bega Canal) | Organic acids, N–fertilizers | Cr | 0.011–0.054 | n.a. | 0.319–1.395 | [54] |
| Cu | 0.063–0.212 | n.a. | 0.359–2.007 | ||||
| Cd | 0.271–0.460 | n.a. | 1.023–2.916 | ||||
| Pb | 0.008–0.037 | n.a. | 0.230–1.192 | ||||
| Cannabis sativa | Contaminated soil | PGPR, EDTA | Cr | 0.003–0.004 | 0.07–0.08 | 0.03–0.06 | [22] |
| Ni | 0.03–0.29 | 0.21–0.29 | 0.13–1.07 | ||||
| Cu | 0.08–0.49 | 0.30–0.65 | 0.27–0.77 | ||||
| Sorghum bicolor | Arid soil | MAC levels | Co | 0.976–1.374 | 1.034–1.811 | 0.539–1.012 | [49] |
| Cd | 0.154–0.302 | 0.166–0.533 | 0.566–1.020 | ||||
| Pb | 0.348–0.597 | 0.569–0.937 | 0.372–0.965 | ||||
| Spartium junceum | Dredged marine sediment | Compost, plant combinations | Cu | ~0.25 | ~0.33 | ~0.72 | [45] |
| Cd | ~0.70 | ~0.79 | ~0.87 | ||||
| Ni | ~0.02 | ~0.15 | ~0.23 | ||||
| Zn | ~0.21 | ~0.20 | ~1.01 | ||||
| Pb | ~0.02 | ~0.05 | ~0.32 | ||||
| Tamarix gallica | Cu | ~0.30 | ~0.52 | ~0.55 | |||
| Cd | ~0.62 | ~0.88 | ~0.70 | ||||
| Ni | ~0.05 | ~0.08 | ~0.06 | ||||
| Zn | ~0.15 | ~0.14 | ~1.02 | ||||
| Pb | ~0.02 | ~0.11 | ~0.10 | ||||
| Noccaea caerulescens | Contaminated soil | None | Cd | 102.5 | 15.5 | 6.6 | [51] |
| Zn | 39.3 | 16.1 | 2.5 | ||||
| Pb | 0.1 | 0.2 | 0.4 | ||||
| Cu | 0.3 | 0.7 | 0.4 |