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Discoveries in Agriculture and Food Sciences - Vol. 11, No. 6
Publication Date: December 25, 2023
DOI:10.14738/dafs.116.15139.
Shimakawa, Y., Kitaya, Y., Shibuya, T., & Endo, R. (2023). Plant Growth and Nitrogen Flow in an Aquaponics System Integrating
Lettuce Hydroponics and Loach Aquaculture. Discoveries in Agriculture and Food Sciences, 11(6). 89-95.
Services for Science and Education – United Kingdom
Plant Growth and Nitrogen Flow in an Aquaponics System
Integrating Lettuce Hydroponics and Loach Aquaculture
Shimakawa, Y.
Osaka Metropolitan University, Gakuencho 1,
Naka-ku, Sakai-shi, Osaka 599-8531, Japan
Kitaya, Y.
Osaka Metropolitan University, Gakuencho 1,
Naka-ku, Sakai-shi, Osaka 599-8531, Japan
Shibuya, T.
Osaka Metropolitan University, Gakuencho 1,
Naka-ku, Sakai-shi, Osaka 599-8531, Japan
Endo, R.
Osaka Metropolitan University, Gakuencho 1,
Naka-ku, Sakai-shi, Osaka 599-8531, Japan
ABSTRACT
There is an urgent need to develop production technology that uses water and other
material resources effectively to create a stable food supply. Aquaponics combining
hydroponics and aquaculture is expected as an efficient system for producing crops
and animal proteins sustaining the reuse of water and balance of nutrient elements
between both cultures, using dissolved elements in fish excrement for plant growth.
To evaluate the possibility of aquaponics combining lettuce hydroponics and loach
aquaculture, we analyzed the growth performances of lettuce and loach and the
balance of nutrient elements, especially for nitrogen between input and output of
the whole system and nitrogen flow in the system. As a result, lettuce grew in
aquaponics with half strength standard solution as well as hydroponics with a
standard solution by the optimal combination of the number of plants and fish. In
nitrogen balance, almost 70% of nitrogen from the feed was used by growing lettuce
and about 25% of nitrogen from the feed was accumulated in loach.
keyword: aquaculture, hydroponics, sustainability
INTRODUCTION
Due to the recent growth in the global population, concerns have arisen over the stable supply
of food and water resources. Therefore, there is an urgent need to develop production
technology that uses water resources effectively to create a stable food supply (Yamada, 2015).
This study examines aquaponics, a biological production system combining hydroponics and
fish farming that has been attracting attention in recent years as a resource-recycling
production system.
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Discoveries in Agriculture and Food Sciences (DAFS) Vol 11, Issue 6, December- 2023
Services for Science and Education – United Kingdom
Aquaponics can make effective use of water and nutrients (Wongkiew et al., 2017). Sharing
water between hydroponics and aquaculture systems allows plants to utilize the fertilizer
components derived from fish excrement for growth. Therefore, food can be produced while
reusing nutrients and converting waste into high-value resources.
To date, the main focus in the field of aquaponics has been the availability, dissolved
concentration, and accumulation of nitrogen compounds (Goddek et al., 2016). This is because
plant growth depends on nitrogen supply (Xu et al., 2012). Since aquaponics is a biological
production system that combines hydroponics and fish farming, a thorough understanding of
the nitrogen balance is required to increase the production of the system.
Most studies dealing with aquaponics have used tilapia or catfish as the fish to be cultured (e.g.,
Nuwansi et al., 2016; Graber and Junge, 2009; Naznin et al.; Islam et al., 2022) because they are
edible and have a high growth rate. However, some parts of these fish are not edible, such as
the head, internal organs, and bones, which is a drawback. In contrast, loach has an edible
portion of 100% and high nutritional value (Japan Ministry of Education, Culture, Sports,
Science, and Technology). It is also a traditional Japanese ingredient. In addition, loaches are
resistant to dissolved oxygen deficiency because they can breathe using their intestines.
Therefore, loach was adopted as the fish to be cultivated in this study. Many studies have been
conducted on aquaponics cultivation with different types of plants. For this study, lettuce was
adopted as the plant to be cultivated, as it is common in plant factory cultivation.
The nitrogen balance has been the main focus of aquaponics research (e.g., Hu et al. 2015) As a
biological production system combining hydroponics and aquaculture, a thorough
understanding of nitrogen balance is necessary to increase system productivity. In this study,
we also evaluated the usefulness of components derived from fish excreta as fertilizers for plant
growth in aquaponics, using a combination of loach farming and lettuce hydroponics. In
particular, we evaluated the flow of nitrogen within the system, as it is an essential element for
plants and is abundant in fish excreta.
MATERIALS AND METHODS
Lettuce (Lactuca sativa L. 'crunch') and loach (Misgurnus anguillicaudatus) were cultivated and
bred for 21 days. Dechlorinated tap water (12 L) and loach (initial weight 2.9 g/fish) were
placed in a cultivation container, and 21-day-old lettuce seedlings (fresh weight 2.8 ± 0.2
g/plant) were planted in the holes at 5 cm intervals in a grid pattern (Fig. 1). The environmental
conditions for growing lettuce were set at a temperature of 25 °C, relative humidity of 70%, a
CO2 concentration of 1000 μmol mol-1, a photosynthetic effective photon flux density of 200
μmol m-2 s
-1, and a light period of 16 h d-1. A white fluorescent lamp (FHF32EXD HX-S NEC
Corporation) was used as the light source. Continuous aeration (3.5 L min-1) was provided to
the nutrient solution. Loaches were fed a commercially available goldfish feed (Kyorin Co., Ltd.)
once a day, 5 times a week, equivalent to 1% of their body weight (about 0.03 g / fish).
Test Group
In this study, a commercial fertilizer solution (OAT, Ohtsuka Co., Ltd.) at 1/2 the standard
prescription concentration was used as a standard nutrient solution. Separate test plots were
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Shimakawa, Y., Kitaya, Y., Shibuya, T., & Endo, R. (2023). Plant Growth and Nitrogen Flow in an Aquaponics System Integrating Lettuce Hydroponics
and Loach Aquaculture. Discoveries in Agriculture and Food Sciences, 11(6). 89-95.
URL: http://dx.doi.org/10.14738/dafs.116.15139
established with the standard nutrient solution, 0.5-fold nutrient solution, and 0.5-fold nutrient
solution with 20 loaches, respectively. Furthermore, the amount of dissolved nitrogen
discharged from 20 loaches was quantified by establishing a test plot in which only loaches
were bred in dechlorinated tap water.
Nitrogen Flow
The nitrogen content (g) of each element in the system was calculated using the following
procedure.
The amount of nitrogen contained in the nutrient solution of a test plot (“Nutrient Solution”;
Fig. 2) was calculated from the nutrient solution composition specified by the manufacturer.
The amount of nitrogen contained in the feed provided to the fish (“Feed”; Fig. 2) was calculated
by multiplying the nitrogen content (g g-1) contained in the loach feed by the total feed amount
(g). There was no leftover food. The amount of nitrogen contained in the excretion from loaches
("Excretion from loaches"; Fig. 2) is the amount of nitrogen derived from loach emissions
calculated from the total amount (g) of dissolved nitrogen components discharged by 20
loaches in 21 days in the test plot where only loaches were bred. The amount of nitrogen
remaining in the cultivation container at the end of the experiment ("Residual solution"; Fig. 2)
was calculated from the amount of dissolved nitrogen component (g) at the end of the
experiment. The amount of nitrogen accumulated in the lettuce grown during the experiment
("Lettuce"; Fig. 2), calculated using the nitrogen content of lettuce at the end of the experiment
(g g-dry-1) and the increase in the total dry weight of lettuce during the experiment (g-dry). The
amount of nitrogen accumulated in the loach body during the experimental period ("Loaches";
Fig. 2) was calculated from the nitrogen content in the loach body (g g-dry-1) and the increase
in the loach dry matter during the experimental period (g-dry).
Fig. 2: Schematic diagram of the experimental aquaponics combining lettuce hydroponics and
loach aquaculture.
Nutrient
Solution
Lettuce
Loach Pump
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Discoveries in Agriculture and Food Sciences (DAFS) Vol 11, Issue 6, December- 2023
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Fig. 2: Schematic diagram of nitrogen flow in each treatment. Numerical values in parentheses
indicate amounts of nitrogen (g N) supplied and accumulated in the system during the 21-day
experiment.
RESULTS
Lettuce Growth
Table 1 shows the lettuce weight and nitrogen content at the end of the experiment. The total
dry matter weight of lettuce was 1.6 times higher in the "0.5-fold nutrient solution with 20
loaches" group than in the "0.5-fold nutrient solution" group, indicating that the excreted
components of loach contribute to plant growth. There was no significant difference in the total
0.5-fold nutrient solution
Nutrient solution
(0.78 g-N)
Input Output
Others
(0.16 g-N)
Lettuces
(0.61 g-N)
Residual solution
(0.02 g-N)
0.5-fold nutrient solution with 20 loaches
Feed
(0.94 g-N)
Excretion
from loaches
(0.61 g-N)
Input Output
Lettuces
(1.36 g-N)
Residual solution
(0.06 g-N)
Others
(0.06 g-N)
Loaches
(0.24 g-N)
Nutrient solution
(0.78 g-N)
Nutrient solution
Nutrient solution
(1.56 g-N)
Input Output
Lettuces
(1.42 g-N)
Residual solution
(0.08 g-N)
Others
(0.06 g-N)
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Shimakawa, Y., Kitaya, Y., Shibuya, T., & Endo, R. (2023). Plant Growth and Nitrogen Flow in an Aquaponics System Integrating Lettuce Hydroponics
and Loach Aquaculture. Discoveries in Agriculture and Food Sciences, 11(6). 89-95.
URL: http://dx.doi.org/10.14738/dafs.116.15139
dry weight of lettuce between the "0.5-fold nutrient solution with 20 loaches" group and the
"standard nutrient solution" group. There was no significant difference in the dry weight of the
roots in the test section. Lettuce from the "0.5-fold nutrient solution" group had a lower
nitrogen content than lettuce from the other two test groups, and no significant difference was
observed between the "standard nutrient solution" group and the "0.5-fold nutrient solution
with 20 loaches" group.
Loach Growth
During the 21 days of rearing, loaches were fed approximately 0.45 g of feed per individual and
the average loach weight increased from 2.9 g / fish to 3.3 g / fish. The weight gain of the loach
was equivalent to 89% of the food provided.
Nutrient Solution Component Composition
The composition of the nutrient solution was compared between the "0.5-fold nutrient
solution" group and the "0.5-fold nutrient solution with 20 loaches" group. The breeding of 20
loaches resulted in a solution comprising approximately 80% N, Ca, and K. Increased by 200%
and 8% (Table 2).
In other words, loach excreta have a different composition from that of the standard nutrient
solution.
Nitrogen Flow
In the "0.5-fold nutrient solution with 20 loaches" group, almost all the nitrogen supplied by
the loach excreta was used for lettuce growth (Fig. 2). Furthermore, in this test group, the
product of the total dry weight of lettuce and the nitrogen content in the lettuce (Table 1) did
not differ from that in the "standard nutrient solution" group. This indicates that loach excreta
could be used to supplement chemical fertilizer. The amount of nitrogen in the loach excreta
and the amount of nitrogen accumulated in the loach were approximately 65% (0.61 g N) and
26% (0.24 g N) of the feed nitrogen content (0.94 g N), respectively.
Table 1. Growth and nitrogen contents of lettuces in different treatments.
Total Shoot Root (g g−1
DW) (g/plant) × 0.5 6 - 3.31 ± 0.20 b 2.50 ± 0.16 b 0.81 ± 0.06 a 53 ± 4 b 0.031 b 0.105 b × 0.5 6 20 5.26 ± 0.24 a 4.47 ± 0.21 a 0.79 ± 0.04 a 97 ± 4 a 0.046 a 0.244 a × 1 5 - 5.69 ± 0.29 a 4.95 ± 0.27 a 0.74 ± 0.04 a 116 ± 6 a 0.050 a 0.286 a Mean ± standard error (n = 5-6). Different alphabets indicate a significant difference (Tukey-Kramer test, P<0.05) for each parameter.
No. of
loaches Nutrient Dry weight (g/plant)
solution No. of
lettuces Fresh weight of
lettuce shoot (g/plant)
Nitrogen content
Table 2. The amount of nutrient elements supplied in each treatment during the 21-day experiment.
Na Ca K M g P B Cu Fe M n M o Zn
× 0.5 6 0 780 492 1008 108 156 1.39 0.10 8.10 3.48 0.10 0.28
× 0.5 6 20 1388 1475 1085 280 245 2.93 0.20 8.13 3.65 0.26 0.36
× 1 5 0 1560 984 2016 216 312 2.78 0.19 16.20 6.96 0.19 0.55
tap water 0 20 608 983 77 172 89 1.54 0.10 0.03 0.17 0.16 0.08
Nutrient
solution
No. of
lettuce
No. of
loaches
Macronutrients (mg) Micronutrients (mg)
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DISCUSSION
In the "0.5-fold nutrient solution with 20 loaches" group, the lettuce dry weight increased by
59% compared to that of the "0.5-fold nutrient solution" group, but the amount of N supplied
to the nutrient solution also increased by about 80%. Since plant growth depends on the
amount of nitrogen supplied (Xu et al., 2012), we infer that the growth of lettuce in the “0.5-fold
nutrient solution” group was suppressed by nitrogen deficiency. However, potassium is also an
essential macronutrient, and the potassium content of the nutrient solution increased by only
approximately 8%, suggesting that the supply of K in both groups was sufficient. From the
above, loach excrement has a different nutritional composition from the standard nutrient
solution. Consequently, loach excrement alone will result in insufficient plants due to a shortage
of potassium. However, the test results show that aquaponics can reduce the amount of
chemical fertilizer required. In all test plots, the dissolved nitrogen component (Residual
solution in Fig. 2) at the end of the experiment was less than 5% of the supply, and nitrogen
was fully utilized in the entire system. Since excessive nitrogen supply reduces utilization
efficiency, further investigations are required into how an increased nitrogen supply in an
aquaponics system could affect the nitrogen utilization efficiency of the system.
Almost 70% of nitrogen from the feed was used by growing lettuce and about 25% of nitrogen
from the feed was accumulated in loach. This fact indicates that in the case of aquaculture alone,
much of the N source supplied as feed is discharged into the environment, suggesting the
importance of recovery by plants and water purification in aquaponics.
A wide variety of plant species can be used in aquaponics, so it can be developed according to
the needs of producers. For example, Naznin et al. (2015) reported the hydroponic culture of
garlic plants combined with tilapia fish culture as a new combination in aquaponics to
investigate its potential and medicinal productivity. Garlic is one of the common vegetables
used as a spice and a medical herb in the world. The medicinal activities of garlic are attributed
to ajoene which is a sulfur-rich compound and derived from heated garlic through the
conversion of alliin and allicin. The possibility of hydroponic garlic culture integrated with fish
culture was evaluated to increase food production in a limited space. Three treatments
including garlic culture only (G), garlic culture with tilapia fish culture (GT) and tilapia fish
culture only (T) were applied and growth of plants and fish and water quality were monitored.
Dry weights of bulbs, leaves and roots were 1.4, 1.6 and 1.6 times, respectively, greater in GT
than in G. The medicinal component `Ajoene` contents in newly developed bulbs, leaves, and
roots were 2.6, 2.2, and 2.4 times, respectively, greater in GT than in G. The specific growth rate
of fish in GT were 1.3 times greater than in T. It was confirmed that the fish supplied additional
nutrients including nitrogen compounds to garlic plants and plants kept the water quality
suitable for fish growth. Garlic and fish can be grown well in the integrated culture system with
mutual benefit. It was confirmed that the combined culture of garlic with fish promoted food
production in a limited space. Islam et al. (2022) reported an aquaponic system with
hydroponic culture of sweet potato in a tilapia fish culture pond. In the experiment, styrofoam
boxes with holes on the bottom side containing nylon net containers filled with hydroballs were
used to culture sweet potato plants on the water surface. The result showed that sweet potatoes
can be produced on the water surface of a fish culture pond effectively to enhance food
production in a limited space. Aquaponics with loach fish allows for fish farming in plant culture
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Shimakawa, Y., Kitaya, Y., Shibuya, T., & Endo, R. (2023). Plant Growth and Nitrogen Flow in an Aquaponics System Integrating Lettuce Hydroponics
and Loach Aquaculture. Discoveries in Agriculture and Food Sciences, 11(6). 89-95.
URL: http://dx.doi.org/10.14738/dafs.116.15139
beds just like this experiment. Loaches, which are relatively small fish, can be kept even in
shallow water depths of 5 to 10 cm, and suffer almost no feeding damage or no mechanical
stress to plant roots. Therefore, there are many options for plant species to be combined as well
as the potential for minimizing water usage.
In conclusion, lettuce grew equally even if a part of the supplied chemical fertilizer was replaced
with the excretory component of loach in this study. Therefore, loach excreta can be used to
partially supplement chemical fertilizer. Aquaponics has the potential to reduce the
environmental load of aquaculture, through the simultaneous production of vegetables and
fish, the reduction of chemical fertilizers in lettuce hydroponics, and the purification of
aquaculture water. Future studies should examine not only nitrogen but also other fertilizer
elements to model the optimum combination of loach, lettuce, and feed supply.
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