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Optimizing Nutrient Availability in Decoupled Recirculating Aquaponic Systems for Enhanced Plant Productivity: A Mini Review

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tagsmeta/type/narrative-review, meta/region/global

18 min read

Optimizing Nutrient Availability in Decoupled Recirculating Aquaponic Systems for Enhanced Plant Productivity: A Mini Review

Metadata

  • Cite key: altawahaOptimizingNutrientAvailability2025
  • Item type: Journal Article
  • Authors: A. Al Tawaha, P. Megat Wahab, H. Jaafar
  • Affiliation: National Agricultural Research Center (NARC), Baq’a 19381, Jordan (Al Tawaha); Department of Crop Science, Faculty of Agriculture, Universiti Putra Malaysia (UPM), Serdang 43400, Selangor, Malaysia (all three authors) (p.1)
  • Journal: Nitrogen 6 (2025) 3
  • Date: 01/2025
  • Date added: 2025-01-15
  • DOI: 10.3390/nitrogen6010003
  • Funding: “This research was conducted without external funding, relying solely on the resources and dedication of the authors to achieve the outlined objectives.” (p.14, Funding statement)
  • URL: https://doi.org/10.3390/nitrogen6010003
  • PDF: Al Tawaha et al. - 2025 - Optimizing Nutrient Availability in Decoupled Reci.pdf

Opinion

A genuinely useful orientation piece on DRAPS (decoupled recirculating aquaponics) nutrient management, but it is a narrative synthesis, not a research paper — no Methods section, no site, no original measurement anywhere. Its two tables (macronutrient concentrations by system, Table 1; iron-supplementation practices, Table 2) are compilations of other groups’ reported numbers, each individually citable but none original to this paper. Useful as a reading map into the DRAPS literature (Kloas 2015, Monsees 2017/2019, Suhl 2018, Goddek 2016, the Roosta group’s iron/foliar work, and the authors’ own prior butterhead-lettuce DRAPS studies) and as a clear conceptual explainer of why urea and elevated Fe/Ni become viable in a decoupled system. Would cite for background/motivation, not for any number as “measured here.” One internal citation-table slip found (see Extraction notes) is a caution flag for trusting Table 2’s reference numbers without checking the original source.

Abstract

Nutrient management in coupled aquaponic systems presents significant challenges due to competing requirements between fish and plant production within a single-loop framework. These challenges often result in suboptimal nutrient concentrations, compromised system efficiency, and reduced yields. This critical review examines the Decoupled recirculating aquaponics system (DRAPS) as an innovative solution that separates fish and plant nutrient cycles while maintaining water recirculation benefits. This study provides a comprehensive review of DRAPS, emphasizing how its decoupled structure enhances nutrient management and promotes sustainable production. It specifically evaluates the ability of DRAPS to optimize macronutrient and micronutrient levels, control agronomic factors independently, and improve both nutrient and water use efficiency. Additionally, this review highlights the advantages of using urea as a nitrogen source, which can enhance plant productivity without compromising fish health. The findings indicate that the loops of DRAPS facilitate customized nutrient concentrations, fostering optimal growth conditions for both plants and fish. By safely incorporating urea as a nitrogen source, DRAPS increases plant productivity while reducing the risk of ammonia toxicity for fish. Furthermore, independent control over agronomic factors enhances nutrient uptake, nutrient use efficiency, and water use efficiency. This approach minimizes the risks of cross-toxicity and enables higher levels of essential micronutrients, such as iron and nickel, which are beneficial for plant health but can be toxic in coupled systems. DRAPS signifies a significant advancement in sustainable agriculture, particularly in regions with limited water and land resources. By optimizing nutrient management and supporting the high-density production of plants and fish, DRAPS presents a scalable, resource-efficient model that aligns with sustainable development goals. Its capacity for precise nutrient control with minimal environmental impact positions it as a valuable solution for sustainable, high-yield food production in resource-constrained settings.

Summary

This mini-review makes the conceptual case for decoupled recirculating aquaponic systems (DRAPS) over conventional single-loop coupled aquaponics (CRAPS), arguing that splitting the fish/biofilter loop from the plant loop resolves the core problem of coupled systems: fish, nitrifying bacteria, and plants each wanting a different pH, temperature, and nutrient composition from the same water. Drawing entirely on previously published studies (none original to this paper), the authors walk through soilless culture types used in DRAPS (NFT, DWC, substrate), compile two tables of literature-reported nutrient concentrations and iron-supplementation regimes, and build a case for two DRAPS-enabled practices: supplementing urea as a secondary nitrogen source (safe in DRAPS because the fish loop is insulated from the resulting NH4-N/NH3 spike) and running higher iron (and nickel, as a urease cofactor for urea hydrolysis) concentrations than would be tolerable for fish in a coupled system. It closes by arguing DRAPS also allows independent optimization of pH, root-zone temperature, and loop flow rates for whichever crop is grown. The paper is a useful map of the DRAPS literature and a clear explainer of the urea/Ni/Fe mechanism, but it reports no original data, no search methodology, and repeatedly extrapolates “ideal” concentration values from single hydroponics textbooks (Resh, Jones) without reconciling the several different pH/EC ranges it quotes for the same crop.

Review scope

  • Region / system covered: No specific site or geographic region — a technology-level synthesis of decoupled recirculating aquaponic systems (DRAPS) generally, covering leafy crops (lettuce, basil, chicory, spinach, parsley) and fruiting crops (tomato, cucumber, pepper, strawberry, eggplant) reported across the cited literature. Authors are based in Jordan and Malaysia, but neither country’s conditions are specifically analysed.
  • Argument: DRAPS’s two independent loops let fish, nitrifying bacteria, and plants each be held at their own optimal water chemistry (pH, temperature, EC) instead of the single compromise pH/temperature CRAPS forces on all three; this decoupling in turn makes it safe to (a) add urea as a supplementary N source without ammonia risk to the fish, and (b) run elevated Fe and Ni concentrations that would otherwise be toxic to fish, together increasing nutrient use efficiency, water use efficiency, and yield.
  • Evidence base: narrative synthesis of secondary sources — no search string, database list, or inclusion/exclusion criteria stated anywhere in the paper; 137 references, the large majority secondary citations of others’ experimental or review work, plus two of the authors’ own prior primary DRAPS studies on butterhead lettuce (refs 36–37).
  • Typologies or frameworks introduced: Table 1 (p.5) — macronutrient (NO3, P, K, Ca, Mg, S) concentrations produced from fish waste across seven literature-reported aquaponic systems, by plant/fish species, stocking density, feeding rate, and coupled/decoupled/UVI type; Table 2 (pp.8-9) — iron-supplementation sources, forms, and concentrations across eleven literature-reported fish/plant/hydroponic-unit combinations; Figure 1 (p.3) — schematic of the two DRAPS loops (rearing tank/mechanical filter/biofilter/sump 1 vs. sump 2/hydroponic unit); Figure 2 (p.11) — schematic of the nitrification cascade (NH3 -> AOB -> NO2 -> NOB -> NO3 -> plant uptake).
  • Key figures cited: NO3 in aquaponics “ranged from 32.4 to 187 mg/L” [secondary, Table 1, cites refs 13,25,33,42,43] (p.5); ideal N concentration for leaf crops “200 mg/L,” composed of 75:25 NO3:NH4 [secondary, cites Resh 2022 and Jones 2005] (p.5); urea N content “46%” and fish-toxicity threshold “3 mg/L or higher” NH4-N [secondary, uncited specific source within that sentence] (p.5-6); ideal Fe concentration for plants “2 to 5 mg/L” against a typical aquaponic Fe range of “0.01 to 0.03 mg/L” [secondary, cites refs 36-38, 92-94] (p.8); ideal P “50 mg/L” and K “210 mg/L” for lettuce [secondary, cites Resh 2022] (p.7); optimal root-zone temperature “20-25 degC” for leafy crops and “22-27 degC” for fruiting crops [secondary, cites He et al. and related RZT studies, refs 127-133] (p.12-13); optimized first-loop flow rate “9.2 m3/day or 6.4 L/min” [secondary, cites Endut et al. 2010] and second-loop NFT flow rate “1 to 2 L/min” [secondary, cites Resh] (p.13).

DRAPS vs. CRAPS (system comparison)

This paper: Describes CRAPS as a single loop shared by fish, nitrifying bacteria, and plants, each with different pH optima (fish 6.5-8.0, plants 5.5-6.5, nitrifying bacteria 7.0-8.0) and different temperature optima (warm-water fish 24-30 degC, plants cooler) (p.2). DRAPS instead runs two loops — fish+biofilter in loop 1, plants in loop 2 — allowing independent optimization and greater resilience to a failure in either subsystem, at the cost of higher infrastructure/operational complexity (p.2-3, Figure 1).

Compared with:

Urea as a nitrogen source

This paper: Argues urea (46% N) is unusable as a direct N source in hydroponics/coupled aquaponics because its hydrolysis to NH4-N becomes toxic to fish above “3 mg/L” (p.5-6), but DRAPS’s fish-loop isolation removes that constraint, so urea can supplement the NO3-N produced by nitrification once the ideal N ratio (75:25 NO3:NH4) is set (p.5, citing Jones 2005). Cites Ikeda and Osawa’s substitution of NH4-N for lettuce and Khan et al.’s 20% urea replacement for spinach as precedents from hydroponics, not aquaponics (p.6).

Compared with:

  • todo Ikeda Osawa 1984 — lettuce N-source/temperature substitution experiment, cited as urea-substitution precedent (p.6, ref 47)
  • todo Khan Watanabe 1999 — urea/Ni supplementation effect on hydroponic spinach growth (p.6, ref 48)

Role of nickel in urea hydrolysis

This paper: Explains Ni as a required cofactor of urease for urea assimilation/hydrolysis in plant tissue, with uptake depending on pH, Ca concentration, and plant species (p.6-7). States the specific role of Ni in urea decomposition “in the nutrient solution of aquaponics and its impacts on the yield and leaf NO3-N content of lettuce remains unclear” (p.7) — i.e., the review explicitly flags this as an open research gap rather than a settled finding, consistent with narrative-review (no original data resolves it here).

Compared with:

Iron supplementation

This paper: States Fe is the most limiting aquaponic micronutrient, with a typical aquaponic range of “0.01 to 0.03 mg/L” against an ideal plant requirement of “2 to 5 mg/L” (elsewhere in the same paragraph also given as “0.2 and 2.5 mg/L” for the plant optimum, see Extraction notes) (p.8). Table 2 (pp.8-9) compiles 11 literature examples of Fe form/concentration/application method across coupled, decoupled, and UVI-coupled systems, mostly Fe-EDTA or Fe-EDDHA at 2-3 mg/L added to water, or FeSO4/Fe-EDTA foliar sprays at 0.5 g/L.

Compared with:

pH optimization

This paper: Reports different bacterial-genus pH optima (Nitrobacter 7.3-7.5 or 7.5-7.8 depending on citation, Nitrospira 8.3, Nitrosomonas 6.0-9.0 or 7.8 depending on citation — see Extraction notes) and different literature-quoted “ideal” hydroponic pH ranges for the same crops (e.g., tomato given as 5.8-6.3, 5.5-6.5, 6.0-6.5, and 5.8-6.4 all in the same paragraph, each attributed to a different citation) (p.11-12). Argues DRAPS’s independent loops let each crop’s pH be set to whichever of these ranges is used without needing to compromise with fish/bacterial pH requirements.

Compared with:

Root zone temperature (RZT)

This paper: States RZT optima of 20-25 degC for leafy crops and 22-27 degC for fruiting crops, each achievable independently in DRAPS’s plant-only second loop (p.12-13), citing Li et al.’s finding that lettuce fresh yield drops at RZT 30-35 degC versus 25 degC, while maximum dry mass occurs at 24 degC.

Compared with:

Linked claims

Citations to chase

  • todo Kloas, Groß, Baganz et al. (2015) — original DRAPS concept paper, “A new concept for aquaponic systems to improve sustainability, increase productivity, and reduce environmental impacts” (Aquac. Environ. Interact.)
  • todo Monsees, Kloas, Wuertz (2017) — “Decoupled systems on trial: Eliminating bottlenecks to improve aquaponic processes” (PLoS ONE)
  • todo Suhl, Dannehl, Zechmeister et al. (2018) — “Prospects and challenges of double recirculating aquaponic systems (DRAPS)” (Acta Hortic.)
  • todo Goddek, Espinal, Delaide et al. (2016) — system dynamics design approach for decoupled aquaponics (already flagged in abusinSustainableFoodProduction2020’s note; still not yet extracted into this vault)
  • todo Aslanidou, Elvanidi, Mourantian et al. (2024) — large-scale coupled vs. decoupled aquaponic system productivity/efficiency comparison (Sci. Hortic.)
  • todo Aslanidou, Elvanidi, Mourantian et al. (2023) — “Nutrients Use Efficiency in Coupled and Decoupled Aquaponic Systems” (Horticulturae)
  • todo Rodgers, Won, Timmons, Mattson (2022) — “Complementary nutrients in decoupled aquaponics enhance basil performance” (Horticulturae)
  • todo Al Tawaha, Megat Wahab, Jaafar et al. (2021a) — “Effects of fish stocking density on water quality, growth performance of tilapia and yield of butterhead Lettuce grown in decoupled recirculation aquaponic systems” (J. Ecol. Eng.) — the review’s own prior primary DRAPS study; NOTE the review’s Table 2 (ref [36]) attributes it to a Basil/UVI-coupled/DWC/Iron-chelate data row that does not match this paper’s actual title subject — verify against the original before treating Table 2’s row 4 as evidence for either paper
  • todo Al Tawaha, Megat Wahab, Jaafar et al. (2021b) — “Yield and nutrients leaf content of butterhead lettuce (Lactuca sativa) in response to fish nutrient solution in a small scale of aquaponic systems” (Ecol. Eng. Environ. Technol.) — second of the review authors’ own prior primary DRAPS studies
  • todo Ferrarezi, Bailey (2019) — basil performance evaluation in aquaponics (UVI system), Table 1’s basil/UVI row
  • todo Blanchard, Wells, Pickens, Blersch (2020) — pH effect on cucumber growth/nutrient availability in a decoupled aquaponic system, Table 1’s cucumber/decoupled row
  • todo Yang, Kim (2020) — N and P mass balance comparison for tomato-, basil-, and lettuce-based aquaponic and hydroponic systems, Table 1 source
  • todo Kasozi, Tandlich, Fick, Kaiser, Wilhelmi (2019) — “Iron supplementation and management in aquaponic systems: A review” (Aquac. Rep.) — a closely related prior review worth comparing scope against
  • todo Endut, Jusoh, Ali, Nik, Hassan (2010) — optimal hydraulic loading rate and plant ratios in recirculating aquaponic systems, source of the first-loop flow-rate figure

Extraction notes

Type classification: narrative-review. Confirmed by reading the full paper, not just the title/abstract: there is no Methods section, no site or system operated by the authors, no data collection of any kind, and both tables (1 and 2) are compilations of numbers with per-row literature citations, not this paper’s own measurements. The title’s own subtitle (“A Mini Review”) and the header banner (“Review”) agree. No search string, database list, or inclusion/exclusion criteria are stated anywhere, so per SCHEMA.md decision rule 4 this is narrative-review, not systematic-review, despite compiling 137 references and two structured tables. Per SCHEMA.md decision rule 1, the authors did not collect data themselves, so this cannot be experiment/quasi-experiment/field-trial/exploratory regardless of how much quantitative material the two tables contain — a prior title-only pass’s tentative narrative-review call is confirmed correct on full read.

Contradictions found:

  • WARN-MINOR Iron optimum for plants: p.8 states the “ideal Fe concentration for the plants ranges from 2 to 5 mg/L” in one sentence, then two sentences later in the same paragraph states “the optimal Fe concentration for the plant is between 0.2 and 2.5 mg/L [93,94],” a different (though overlapping-at-the-edge) range attributed to different references than the first figure (which is uncited in that sentence). Does not affect any extracted cell — this is a review with no trials.csv row — but both ranges are recorded here since a reader citing “the ideal Fe range per this paper” could reasonably reach for either. No resolution possible without checking refs [93,94] directly; noted rather than silently picking one.
  • WARN-MINOR Nitrifying-bacteria pH optima stated twice with different values: Section 4.3.1 (p.11) gives Nitrobacter 7.5-7.8, Nitrospira 8.3, Nitrosomonas 7.8 [121-123]; Section 4.2.2 (p.11) gives Nitrobacter’s optimum pH as “between 7.3 and 7.5” [118] and Nitrosomonas as “a pH range of 6.0 to 9.0” [118]. Different reference numbers attach to each instance, so these may be different sources genuinely disagreeing, not a copy error by the review authors — but the review does not flag or reconcile the disagreement itself. Does not affect any extracted cell (no CSV row for this paper type). Recorded for anyone using this paper as a pH-optimum reference.
  • WARN-MINOR Tomato hydroponic pH range quoted four ways in one paragraph (p.12): “5.8-6.3 [4],” “5.5-6.5 [27],” “6.0-6.5 [4]” (note: same reference [4] given for two different ranges within the same sentence), and “5.8-6.4” (uncited). This is llikely either a transcription slip (duplicate use of ref [4] with different numbers) or the source itself gives a range depending on growth stage that the review compressed into a single sentence without saying so. Does not affect any extracted cell. Recorded as-is; do not treat any single one of these four as “the” ideal tomato pH without checking ref [4] (Jones 2005) or [27] (Resh 2022) directly.
  • WARN-MINOR Table 2, row 4 (ref [36]) citation mismatch: Table 2 (p.8) attributes a “Basil / Nile tilapia / UVI coupled aquaponic / DWC / Iron chelate (13% EDTA Fe) / 2 mg/L at 3-week intervals” data row to reference [36], but reference [36] in this paper’s own reference list (p.15) is “Al Tawaha, A.R.; Wahab, P.E.M.; Jaafar, H.B.; Zuan, A.T.K.; Hassan, M.Z. Effects of fish stocking density on water quality, growth performance of tilapia and yield of butterhead Lettuce grown in decoupled recirculation aquaponic systems. J. Ecol. Eng. 2021” — a butterhead-lettuce, decoupled-system paper, not a basil/UVI/DWC paper. The Table 1 row for “Basil / Nile tilapia / UVI / 49.60 [42]” (p.5) correctly matches ref [42] (Ferrarezi and Bailey, “Basil performance evaluation in aquaponics”), which is the more plausible true source for Table 2’s basil/UVI/DWC data — suggesting Table 2’s citation number for that row may simply be a typo for [42] rather than [36]. Does not affect any extracted cell (no CSV row for this paper), but is flagged because it means Table 2’s reference numbers should not be trusted at face value without checking the original source — relevant to anyone chasing ref [36] or [42] from this review. This is an error internal to the review paper’s own tables, not something introduced during this extraction.

None of the above reach WARN-BLOCK or WARN-MATERIAL tier because this paper produces no trials.csv/plant.csv cells for any value to become UNCLEAR against — the severity apparatus is retained here purely as documentation for future readers of this note. No entries added to REVIEW.md (that file collects BLOCK/CHECK flags tied to specific trial cells; this paper has none).

quality: frontmatter field: not set. Per SCHEMA.md, quality scoring applies only to experiment/quasi-experiment/field-trial/exploratory papers with trials.csv rows; this is a narrative-review.

[not reported] / [unclear] fields:

  • “Date added” taken from zotero-export.csv’s Date Added field (2025-01-15 13:03:41); not present anywhere in the PDF itself.
  • Several “ideal”/“optimal” figures throughout Section 4 are given without any bracketed citation attached to the specific sentence (e.g., the urea “3 mg/L” fish-toxicity threshold on p.5-6, the “40% of feed for growth” and uncited land-area-style framing statements) — source [unclear] in those specific instances even where the surrounding paragraph cites other sources for adjacent claims.

CSV rows: none produced. Per SCHEMA.md and the task instructions, only experiment, quasi-experiment, field-trial, and exploratory papers get trials.csv/plant.csv rows. This is a narrative-review, so both out/altawahaOptimizingNutrientAvailability2025.trials.csv and out/altawahaOptimizingNutrientAvailability2025.plant.csv contain header-only files with zero data rows. All numeric values appearing in Tables 1 and 2 of this paper belong to the primary studies they cite (refs 13, 24, 25, 33, 36, 42, 43, 86, 95-100), not to this review, and are not extracted here — per SCHEMA.md, “Numbers from review papers are secondary,” and entering them into trials.csv would double-count those primary papers if/when they are separately extracted elsewhere in this vault.

Tags: Meta/Type/Narrative-Review (matching the vault’s existing Meta/Type/Meta-analysis, Meta/Type/Policy, Meta/Type/Experiment leaf-naming convention). Meta/Region/Global (matching the precedent set by tadesseComprehensiveComparisonLettuce2023, a meta-analysis with no single-site focus) since no specific country or site is studied — the authors are Jordan/Malaysia-based but analyse no region-specific data. No Meta/Fish/ or Meta/Plant/ tags: tilapia, carp, lettuce, basil, tomato, and other organisms are discussed only as examples drawn from cited literature, never as something this paper itself grew or measured.

Water-quality panel: none present as original data. The paper discusses water-quality parameters (pH, EC, DO, temperature, TAN/NH4-N, NO2-N, NO3-N) extensively but always as either (a) literature-cited “ideal” or “optimal” ranges, or (b) Table 1’s NO3/P/K/Ca/Mg/S values, which are themselves per-study literature citations, not this paper’s own trial means. Nothing was withheld from trials.csv on the “doesn’t reduce to a trial mean” basis described in SCHEMA.md — the more fundamental reason no water-quality data was extracted is that this paper is a narrative-review and produces no CSV rows at all.

Source: Al Tawaha et al. - 2025 - Optimizing Nutrient Availability in Decoupled Reci.pdf

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