1. Introduction
Soil is a source of nutrients for plants and, as a substrate, enables plant growth. However, it cannot always play this role, given its degradation mainly caused by human activities. This situation is particularly acute in arid regions and areas of intense mining activity [
1,
2,
3,
4]. To guarantee food security for a growing population, increasing yields with adapted techniques remains a major challenge in developing countries where access to water and suitable soil is not guaranteed and public agricultural policies are sometimes deficient in feeding their populations [
5,
6]. This is particularly true in areas where anthropogenic activities are intense, like mining areas where soils have become less conducive to the production of quality vegetables and fruits in recent decades [
1,
7].
For these areas, hydroponics could be an alternative growing technique to ensure that the grown plants are free from heavy metal contamination. Hydroponics is a soil-less cultivation technique in which plant roots are immersed in a nutrient-enriched solution and eventually maintained by a preferably inert substrate [
8]. This method, therefore, has the advantage of dissociating vegetable production and polluted soils. Three important qualities of hydroponics are (i) high yields, (ii) a 40 to 70% reduction in water consumption compared to soil-grown vegetable production, and (iii) its feasibility in areas where access to arable land is limited (due to arid conditions, or simply infertile or polluted soils) [
9,
10,
11,
12,
13,
14].
In most hydroponics systems, plant roots are immersed in nutritive solutions made from chemical fertilizers [
8] derived from petrochemicals. Their mining and manufacturing generate high operating costs and major environmental problems through soil, water, plant, and air pollution [
15,
16]. For some minerals, their purchase prices can rise rapidly, particularly when the cost of the energy required for production is unstable [
17,
18,
19]. Thus, in the face of increasing demand for fertilizers, agriculture will encounter numerous problems of fertilizer scarcity between 2050–2100, as mineral deposits are expected to be depleted [
20,
21,
22,
23,
24]. These problems of food shortage could be exacerbated in developing countries, which are highly dependent on imports of synthetic chemical fertilizers [
25,
26,
27]. This dependence is set to increase over the next few years, given the depletion of the deposits from which these minerals are extracted, on the one hand, and the poverty levels of the populations living in these countries, on the other. It is therefore essential and urgent to think about innovative and sustainable techniques to overcome these global challenges of chemical fertilizer shortages, and bioponics is one such alternative technique. Also known as biological hydroponics, bioponics involves growing plants in an aqueous medium, with the roots immersed in a nutrient solution derived from the partial or total mineralization of animal manure or plant debris [
27,
28,
29].
In developing countries, these organic fertilizers can be acquired at low cost [
30,
31,
32]. As part of the circular economy, it would make it possible to recycle urban waste instead of making it a source of soil and air pollution and diseases (typhoid, malaria, etc.) [
33,
34,
35]. There are several techniques for producing organic fertilizers, such as compost tea [
36,
37,
38,
39,
40] and vermicomposting [
41,
42]. The use of raw materials of animal origin to make compost tea has been shown to have certain advantages, such as the suppression of certain plant diseases [
42,
43,
44,
45,
46,
47,
48].
In recent decades, poultry production has surged in response to the growing demand for meat and eggs from both urban and rural populations. The chicken droppings generated by this industry can serve as a multi-purpose resource for agriculture, offering benefits in terms of fertilization, composting, sustainability, and cost. Additionally, this high demand for chicken meat and eggs has encouraged residents to engage in poultry farming, leading to significant animal waste production. Chicken droppings can enhance crop productivity, soil health, and environmental sustainability through their rational use, while also protecting the environment from various sources of pollution [
49,
50,
51].
This study is part of a more global project, which aims to find sustainable solutions to the various environmental problems facing urban agriculture in Lubumbashi. Firstly, the contamination of soil was characterized, by water and plants in the market gardens of Lubumbashi [
1]. The results showed that the gardens were low, medium, and high in trace metal contamination and that the vegetables were, therefore, highly contaminated. Secondly, organocalcareous soil improvers were applied to clean up the soil. However, vegetables grown from these soils still presented trace metals above the limits imposed by the FAO, demonstrating that the soil improvers did not help in reducing the mobility and bioavailability of heavy metals [
52].
This study aimed to analyze the impact of organic fertilizer produced from chicken droppings on yield and trace metals for lettuce grown in bioponic systems. The chicken droppings were chosen based on their availability in the Lubumbashi region, where intensive and family farming is increasingly popular with the city’s inhabitants, to offset the need for staple foods particularly poultry imported from neighboring countries. This study should help in understanding whether these fertilizers are promising for soilless crops run by populations living in the city of Lubumbashi in the D.R. Congo, which faces heavy soil pollution.
To do this, two experiments were set up. The first experiment was implemented to understand the impact of varying amounts of fresh chicken droppings on the preparation of bioponic nutrient solutions and, subsequently, its impact on lettuce yield. The second experiment consisted of determining the optimum nitrogen concentration of the bioponic nutrient solution to optimize bioponic vegetable production. Both yield and quantification of heavy metals were studied in this second experiment to better understand whether this solution is a viable alternative to grown vegetables.
2. Materials and Methods
For both trials performed, a bioponic system was used. The sequence of steps taking place during a bioponic production is described in (
Figure 1). These steps were identical for both performed trials.
The difference between the trials resides in the preparation of nutrient solutions used. In the first trial, varying amounts of dry matter of chicken droppings were used during the preparation of the nutrient stock solution. In the second test, whilst the dry matter of chicken dropping was kept constant, the total nitrogen of each solution used was varied.
2.1. Plant Material and Growth Conditions
The trials were carried out in a shadehouse located in the Biofortification, Defence, and Crop Valorization (BIODEV) research unit of the Faculty of Agronomic Sciences at the University of Lubumbashi, D.R. Congo.
The lettuce seeds (
Lactuca sativa var. Lucrecia rz) were obtained from the Laboratory of Integrated and Urban Plant Pathology at Gembloux Agro-BioTech, Université de Liège, Belgium. These lettuce seeds were sown in 36 × 36 × 40 mm rockwool cubes, Grodan, Roermond, Netherlands. Lettuce plants were grown under ambient light conditions and at an average temperature of 20 °C (
Figure A2). Eight days after germination, vigorous seedlings with 2–3 true leaves were transplanted onto 2 × 1 m floating rafts, at a rate of 36 plants per floating raft, in a 5 cm diameter hydroponic basket. Lettuces were harvested 42 days after being planted in the rafts (on Day 63 according to
Figure 1). All rafts were made of recycled wood, covered with polyethylene bags containing 600 L of nutrient solution, and homogenized by a 950 L/h submersible pump in continuous operation (Sicce, Pozzoleone, Italy) (
Figure 2).
2.2. Biofilter Preparation
Two weeks before the anaerobic digestion of the chicken manure, bio-balls composed primarily of clay pellets, plastic caps, and biomedia (small plastic cylinders) were prepared in a 100 L capacity tank to ensure the proper development of nitrifying bacteria responsible for the nitrification process of the manure in the rafts. A mixture of 2.5 kg of well-decomposed fresh manure and 2.5 kg of mature compost was combined in 60 L of water, in which 25 L of bio-balls enclosed in a mosquito net were placed. This mixture was maintained under aerobic conditions with an airflow rate of 1.5 L/min into the tank containing the bio-balls for two weeks. The prepared 25 L of bio-balls were then divided into twelve parts, corresponding to the number of rafts (
Figure 3).
2.3. Production of Stock Solution from Chicken Droppings
Thus, chicken droppings were chosen as the organic material for these experiments because it was readily available from both small and large poultry farmers in the city, and it was cheaper than mineral fertilizers [
53]. The chicken droppings were purchased from an industrial poultry farm located about 15 km from the Faculty of Agricultural and Environmental Sciences at the University of Lubumbashi. For the production of nutrient solutions, all treatments (0.35%; 3.5%, and 7% dry matter D.M.) were repeated three times, i.e., three cans per treatment leading to a total of nine cans for trial 1 (
Table A2). For trial 2, six repetitions (six cans) of the single modality (2.5% dry matter—D.M.) were produced (
Table A3). To determine the dry weight of chicken droppings, ten 100 g samples of fresh chicken droppings were taken, weighed, and placed on aluminum plates in an oven at 40 °C for 48 h and then at 105 °C for 24 h (
Table A1).
Once the stock solutions were prepared, the mineralization and/or anaerobic digestion of chicken droppings was carried out over a period of 7 days in 200 L canisters.
Following this anaerobic digestion, the nutrient solutions were filtered to remove large particles using a 500 µm mesh sieve, followed by a screening cloth. Only after this filtration process were the nutrient solutions supplied to the rafts.
These solutions were diluted to reach the desired TAN (total ammonia nitrogen). In the first trial, each stock solution was attributed to one raft. Although the chicken dropping concentration is variable (dry matter (%) in
Table 1), the TAN (total ammonia nitrogen) concentration was fixed to 150 mg/L per raft before anaerobic digestion for all modalities, which was considered to be the highest concentration of nitrogen that plants can absorb in hydroponics [
8]. On the contrary, for the second trial, the initial chicken dropping concentration was identical between modalities, but TAN concentrations were fixed to three different values before anaerobic digestion (i.e., 60, 90, and 120 mg/L TAN). This information is summarized in the table below (
Table 1).
After dilution of the bioponic solutions in the twelve rafts, the latter received 2 L of biomedia and 16/6 L/min of air. Each raft then ran empty for 14 days to allow aerobic digestion to take place (from day 7 to day 21 according to
Figure 1).
For both trials, lettuce was also grown with a reference nutrient solution of Hoagland to assess the yields and quality of lettuce produced in hydroponic cultures [
8]. Given that this reference solution did not require any aerobic digestion, this one was only implemented in the rafts 14 days after the chicken-dropping prepared solutions. For this reference solution, TAN concentration was fixed at 150 mg/L for the first trial and 120 mg/L for the second trial.
2.4. Aerobic Digestion of Nutrient Solutions in Hydroponic Raft Systems before Crop Transplanting: Empty Circulation Phase
The solution diluted in 600 L of water per raft will continue the mineralization process aerobically for 14 days in rafts covered with an impermeable polyethylene bag. Once the bioponic nutrient solution had been diluted in the rafts, the 25 L of biofilter prepared (see
Section 2.2) was divided equally in each raft. The bio-balls were left in the rafts until the end of the experiment.
2.5. Lettuce Cultivation and Control of Parameters
Before the cultivation phase, the water volume was restored to 600 L (i.e., the initial water level of the raft) with the chicken manure-based nutrient solutions and the reference chemical solution. Water was added here to counter the loss related to evaporation which took place. Lettuce (
L.
sativa Lucrecia rz) seedlings were then transplanted in each raft. The pH was then controlled and corrected if necessary to reach the desired pH of 6: sulfuric acid (H
2SO
4) was diluted to 10% in case of alkaline pH, and sodium hydroxide (NaOH) 3 N was used when in acid pH situations. Electroconductivity (EC) was also measured using a conductivity meter. Every seven days, the desired TAN concentrations (60, 90, 120, and 150 mg/L) in the rafts were adjusted to reach the initially implemented TAN levels (
Table 1) to ensure optimal growth of the lettuce plants until the end of the trials. The TAN adjustment was conducted after analyzing the prepared nutrient solutions with a HANNA brand spectrophotometer to determine the amount of nitrogen absorbed by the plants and the amount evaporated. Electrical conductivity, pH, and TAN concentration were monitored throughout the cultivation period. Every seven days, measurements were taken in the rafts of each of the tested treatments in trials 1 and 2, respectively, until the end of the trials.
2.6. Sample Characterization
2.6.1. Trace Metal Quantification in Chicken Droppings Raw Material
Essential elements (Mg and Ca) and trace metals were determined at the agro-pedological laboratory of the University of Lubumbashi and the laboratory of the Office Congolais de Contrôle (OCC). The extraction consisted of taking 3 g of dried chicken droppings powder and 28 mL of aqua regia. Paper filters were used to filter the extract, which was then diluted with demineralized water and digested for 20 min at 175 °C in a microwave digestion vessel. Characterization of the chicken droppings solution can be found in
Appendix A Table A1. Quantification of trace metals (Cu, Co, Cd, Pb, Zn, Fe) in chicken droppings using Perkin Elmer’s Optima 7000 DV ICP-OES spectrometer (PerkinElmer, Inc., Shelton, CT, USA) was performed according to the method developed by [
54]. The sanitary quality of the chicken manure used in the production of bioponic nutrient solutions was determined. These chicken manures contained trace metal elements; however, these levels did not exceed the limits authorized by the WHO for its use in open-field agriculture. Of all the trace metal elements analyzed, only zinc slightly exceeds the toxicity threshold of 300 mg/kg of Zn permitted for agricultural soil, unlike other trace metal elements such as Cu, Co, Pb, Cd, and Fe, which are below toxicity thresholds. Ultimately, the chicken manures used pose no risk of contamination to the nutrient solutions on the one hand and the bioponic lettuces on the other.
2.6.2. Physico-Chemical Characterization of Nutrient Solutions
To determine the quality of the nutrient solutions, physicochemical analyses were carried out every week, from the start of digestion to the end of the trials (harvest), using a HANNA HI83300 multiparameter spectrophotometer (HANNA Instrument, Saint Laurent de Mure, France). More specifically, these chemical analyses concerned the control of NPK in its various forms (NH3-N; NH3−; NH4+; NO3-N; NO3−; PO43; P2O5; P; K; K2O; EC; and pH) and this for all modalities for both trials. During the digestion of chicken manure, samples of highly concentrated nutrient solutions were taken. The collected solution was diluted before performing analyses with a spectrophotometer. During cultivation, the targeted TAN (total ammonia nitrogen) was adjusted weekly by adding concentrated TAN solutions to the rafts.
2.6.3. Heavy Metals Characterization in Harvested Lettuce
The determination of trace metals in the dry matter of lettuces harvested after the trials was carried out using the AOAC (1990) method. From a total of 36 lettuces per raft, a representative sample of each raft (20%) was dried at 105 °C for 72 h. In this way, all replicates of each modality were mixed to form a composite sample for analysis. A one-gram dry matter sample of the composite sample was taken and placed in a 250 mL digestion tube and mixed with 10 mL of concentrated HNO
3. This mixture was then boiled for 30 to 45 min to allow oxidation of all the elements. After cooling, 5 mL of 70% HClO
4 and the mixture were boiled until dense white fumes appeared. Next, 20 mL of distilled water was added and the mixture was brought back to a boiling state to remove the fumes. The heavy metals (Cu, Co, Cd, and Pb) present in the vegetables were determined by acid mineralization (HNO
3 + HClO
4), and measurements were carried out by flame atomic absorption (FAA) [
55,
56].
2.6.4. Evaluation of Lettuce Crop Yields
Forty-two days after lettuce transplantation, all plants from each raft were weighed on a precision scale to determine the lettuce crop yields on a per-modality basis (
Figure A3).
2.7. Statistical Analysis
Yields in both trials were analyzed using one-way ANOVA (fixed factor was % D.M. in the first trial and TAN content in the second). One-way ANOVA was also performed on heavy metals data in the second trial. When the means were significantly different (p < 0.05), a Tukey–Kramer test was performed. One-way ANOVA and a subsequent post-hoc test were elaborated using Minitab 19 (Minitab Inc., State College, PA, USA).
NH3-N; NH3−; NH4+; NO3-N; NO3−; PO43; P2O5; P; K; K2O; EC; and pH, on the other hand, were analyzed using a two-way mixed ANOVA (within-subject factor was time and between-subject factor was % D.M. and TAN content in the first and second trial, respectively). If a significant interaction was observed between the two factors, Bonferroni correction for multiple comparisons within each time group was performed. When this was not the case, but a significant main effect was still obtained, Bonferroni correction was also used. Two-way mixed ANOVA and subsequent post-hoc tests were performed in R (R 4.3.2 software, R Development Core Team, Boston, MA, USA).
5. Conclusions
This study aimed to develop and optimize a new bioponic technique to produce liquid fertilizer from chicken manure and implement it in the organic hydroponic (bioponic) cultivation of lettuce (Lactuca sativa rz). To achieve this, two types of trials were conducted under shade netting in the ambient conditions of Lubumbashi. Overall, the results are particularly promising as they demonstrate that quality vegetables can be produced, with interesting yields, exclusively using animal waste as fertilizing material. Our technique, which involves fermenting chicken droppings for hydroponic lettuce production in an environment contaminated by trace metals in Lubumbashi, has proven to be an ecological, and practically implementable approach. The results suggested that using low percentages of dry matter from chicken manure (0.35% D.M) and low concentrations of total ammoniacal nitrogen (TAN) (60 mg/L of TAN) yielded higher outputs compared to bioponic treatments receiving high concentrations of dry matter and/or TAN, respectively, for trials 1 and 2. Although yields obtained with chemical nutrient solutions remain superior to bioponic treatments in vegetable cultivation, the obtained results still demonstrate that chicken manure presents significant potential in urban agriculture.
Additionally, lettuces grown using bioponics are safe for consumption, as they contain no trace metal levels above the FAO/WHO toxicity threshold for vegetables. However, further studies should investigate nitrogen loss mechanisms in the rafts, the role of nitrifying bacteria in organic matter, and the valorization of methane gas produced during the anaerobic fermentation process of chicken manure. Lastly, additional studies could test the cultivation of local plant species with added value and other types of organic matter in bioponic vegetable cultivation.