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The functioning of aquaponic systems is based on a dynamic equilibrium of the nutrient cycles (Somerville et al. 2014). It is therefore necessary to understand these cycles in order to optimise the management of the systems. Plants growing hydroponically have specific requirements, which should be met during their various growing stages (Resh 2013). Therefore, nutrient concentrations in the different compartments of the system must be closely monitored, and nutrients should be supplemented to prevent deficiencies (Resh 2013; Seawright et al. 1998) either in the system water or via foliar application (Roosta and Hamidpour 2011).
According to Delaide et al. (2016), in some cases, supplementing an aquaponic solution with mineral nutrients in order to reach the same nutrient concentrations as in hydroponics could lead to higher yields than those achieved in hydroponics. The first step to take towards a balanced system is the correct design and relative sizing of the compartments (Buzby and Lin 2014). If the hydroponic compartment is too small compared to the fish tanks, then the nutrients will accumulate in the water and could reach toxic levels. The feed rate ratio (i.e. the amount of fish feed in the system based on the plant-growing surface and the plant type) is often used for the first sizing of the system (Rakocy et al. 2006; Somerville et al. 2014). However, according to Seawright et al. (1998), it is not possible to reach a plant/ fish ratio which will enable an optimal match of plants' needs if only fish feed is used as an input. To make sure that the system is well balanced and functions properly, monitoring methods are usually based on the nitrogen cycle (Cerozi and Fitzsimmons 2017; Somerville et al. 2014), but to ensure optimal functioning of the system, it is necessary to monitor more closely the balance of the other macronutrients (N, P, K, Ca, Mg, S) and micronutrients (Fe, Zn, B, Mn, Mo, Cu) (Resh 2013; Somerville et al. 2014; Sonneveld and Voogt 2009) as well. Recent studies (Delaide et al. (2017), Schmautz et al. (2015, 2016)) have started tackling this topic. Schmautz et al. (2015, 2016) compared the impact of three different hydroponic layouts (i.e. nutrient film technique (NFT), floating raft and drip irrigation) on the nutrient uptake of aquaponic tomatoes. Drip irrigation was the system which produced slightly better yields with tomatoes. The mineral content of the fruits (P, K, Ca, Mg) was equivalent to the conventional values even though the iron and zinc contents were higher. The leaves however had lower levels of P, K, S, Ca, Mg, Fe, Cu and Zn than in conventional agriculture. Delaide et al. (2016) followed the cycles of macro- and micronutrients in a coupled aquaponic system. They observed that K, P, Fe, Cu, Zn, Mn and Mo were lacking in their aquaponic solution, while N, Ca, B and Na were quickly accumulated. Graber and Junge (2009) noted that their aquaponic solution contained three times less nitrogen and ten times less phosphorus than a hydroponic solution. As for potassium (K), it was 45 times lower compared to hydroponics. Nevertheless, they obtained yields as similar yields even though the quality was poorer due to a lack of potassium (K).
Factors Influencing the Nutrient Cycles
Light intensity, root zone temperature, air temperature, nutrient availability, growth stage and growth rate all influence a plant's nutrient uptake (Buzby and Lin 2014). Experiments conducted by Schmautz et al. (2016) and Lennard and Leonard (2006) showed that the hydroponic method could also play a role in a plant's nutrient uptake capacity, and it is therefore necessary to match the growing system to the type of vegetables being grown. NFT and DWC (deep water culture — raft) are thus suitable for leafy greens, whereas drip irrigation on rockwool slabs is more suitable for fruity vegetables (Resh 2013).
Carbon is provided to the fish via the feed (Timmons and Ebeling 2013) and to the plants via COsub2/sub fixation. Fish can use 22% of the carbon contained in the fish feed for biomass increase and metabolism. The rest of the ingested carbon is either expirated under the form of COsub2/sub (52%) or excreted in a dissolved (0.7—3%) and solid (25%) form (Timmons and Ebeling 2013). The expirated COsub2/sub can be used by plants for their own carbon source as well (Körner et al. 2017). The uneaten part of the feed carbon is left to decompose in the system. The type of carbohydrates found in fish feed (e.g. starch or non-starch polysaccharides) can also influence the digestibility of the feed and the biodegradability of the waste in an aquaculture or aquaponic system (Meriac et al. 2014).
Nitrogen is absorbed by the plants either in the nitrate or ammonium form (Sonneveld and Voogt 2009; Xu et al. 2012) depending on the concentration and plant's physiology (Fink and Feller 1998 cited by Wongkiew et al. 2017). Associations between plants and microorganisms should not be overlooked as plants affect the presence of the microorganisms in aquaponics, and microorganisms can play a significant role in the nitrogen uptake capacity of plants (Wongkiew et al. 2017). The uptake of nitrogen by plants is affected by the ambient carbon dioxide concentration (Zhang et al. 2008 cited by Wongkiew et al. 2017).
Phosphorus is one of the essential elements for plant growth and can be absorbed under its ionic orthophosphate form (Hsub2/subPOsub4/subsup-/sup, HPOsub4/subsup2-/sup, POsub4/subsup3-/sup) (Prabhu et al. 2007; Resh 2013). Little is known about the dynamics of phosphorus in aquaponics. The main input of phosphorus in the system is the fish feed (Cerozi and Fitzsimmons 2017; Delaide et al. 2017; Schmautz et al. 2015), and in un-supplemented systems (Chap. 7), phosphorus tends to be limiting and thus can impede plant growth (Graber and Junge 2009; Seawright et al. 1998). According to Rafiee and Saad (2005), fish can use up to 15% of the phosphorus contained in the feed. In a system growing lettuce, Cerozi and Fitzsimmons (2017) noticed that the amount of phosphorus provided by the fish feed can be sufficient or insufficient depending on the growth stage. Up to 100% of phosphorus present in the fish water can be recycled in the plant biomass, depending on the design of the system. Graber and Junge (2009) observed a 50% recycling, while Schmautz et al. (2015) reported that 32% of the phosphorus could be found in the fruit and 28% in the leaves. The solubility of phosphorus depends on the pH, and a higher pH will foster the precipitation of phosphorus, thus rendering it unavailable for the plants (Yildiz et al. 2017). Phosphorus can precipitate as struvite (magnesium ammonium phosphate) (Le Corre et al. 2005) and/or hydroxyapatite (Cerozi and Fitzsimmons 2017; Goddek et al. 2015). These insoluble complexes are removed via solid fish sludge from the system. Schneider et al. (2004) reported that 30—65% of the phosphorus contained in the fish feed remains unavailable to plants as it is fixed in the solid excretions which are then removed through mechanical filtration. Yogev et al. (2016) estimated that this loss can be up to 85%. One option to prevent this massive loss of P via solid sludge is to add a digestion compartment to the aquaponic system. During aerobic or anaerobic digestion, the P is released into the digestate and could be re-introduced into the circulating water (Goddek et al. 2016).
Delaide et al. (2017) found that the major source of K in their system was the fish feed. Fish can use up to 7% of the K contained in the fish feed (Rafiee and Saad 2005). However, potassium is not necessary for fish which leads to a low potassium composition of the fish feed and to even lower levels of potassium available for the plants (Graber and Junge 2009; Seawright et al. 1998; Suhl et al. 2016). To supply potassium, a KOH pH buffer is often used as the pH often decreases in aquaponics due to nitrification (Graber and Junge 2009). In an aquaponic system planted with tomatoes, potassium accumulated mainly in the fruits (Schmautz et al. 2016).
Magnesium (Mg), Calcium (Ca) and Sulphur (S)
The main source for Mg, Ca and S is tap water which facilitates the absorption by the plants as the nutrients are already available (Delaide et al. 2017). Calcium is however present in insufficient levels in aquaponics (Schmautz et al. 2015; Seawright et al. 1998) and is added under the form of calcium hydroxide Ca(OH)sub2/sub (Timmons and Ebeling 2013). According to Rafiee and Saad (2005), fish can use on average 26.8% of the calcium and 20.3% of the magnesium present in the feed. Sulphur is often at low levels in aquaponic systems (Graber and Junge 2009; Seawright et al. 1998).
Iron (Fe), manganese (Mn) and zinc (Zn) derive mainly from the fish feed, while boron (B) and copper (Cu) derive from the tap water (Delaide et al. 2017). In aquaponics, key micronutrients are often present but at too low levels (Delaide et al. 2017), and supplementation from external sources of nutrients is then necessary (Chap. 8). Iron deficiencies occur very often in aquaponics (Schmautz et al. 2015; Seawright et al. 1998; Fitzsimmons and Posadas; 1997 cited by Licamele 2009), mostly because of the non-availability of the ferric ion form. This deficiency can be solved by the use of bacterial siderophore (i.e. organic iron-chelating compounds) produced by genera such as Bacillus or Pseudomonas (Bartelme et al. 2018) or by iron supplementation with chemical chelated iron to avoid iron precipitation.
Reducing nutrient loss is a constant challenge facing aquaponics practitioners. Nutrient loss occurs in several ways, e.g. the settlement of the sludge (37% of faeces and 18% of uneaten feed) (Neto and Ostrensky 2015), water losses, denitrification, ammonia volatilisation, etc. (Wongkiew et al. 2017). As an example, Rafiee and Saad (2005) note that 24% of the iron, 86% of the manganese, 47% of the zinc, 22% of the copper, 16% of the calcium, 89% of the magnesium, 6% of the nitrogen, 6% of the potassium and 18% of the phosphorus contained in the fish feed were contained in the sludge. The sludge can hold up to 40% of the nutrients present in the feed input (Yogev et al. 2016).
Denitrification can lead to a loss of 25—60% of the nitrogen (Hu et al. 2015; Zou et al. 2016). Denitrification is also linked to anoxic conditions (Madigan and Martinko 2007; van Lier et al. 2008) and low carbon levels and is responsible for the transformation of nitrate into nitrite, nitric oxide (NO), nitrous oxide (Nsub2/subO) and eventually nitrogen gas (Nsub2/sub) with flows into the atmosphere (ibid.). Denitrification is conducted by several facultative heterotrophic bacteria such as Achromobacter, Aerobacter, Acinetobacter, Bacillus, Brevibacterium, Flavobacterium, Pseudomonas, Proteus and Micrococcus sp. (Gentile et al. 2007; Michaud et al. 2006; Wongkiew et al. 2017). Some bacteria can perform both nitrification and denitrification if dissolved oxygen levels are below 0.3 mg/L (Fitzgerald et al. 2015; Wongkiew et al. 2017). The loss of nitrogen can also occur via anaerobic ammonium oxidation (ANAMMOX), i.e. the oxidation of ammonium into dinitrogen gas in the presence of nitrite (Hu et al. 2011).
Another important loss of nitrogen which should be available for the plants is the consumption of the nitrogen by the heterotrophic aerobic bacteria present in the aquaponic systems. Indeed, the nitrogen used by these bacteria is lost to nitrifying bacteria, and nitrification is thus impeded (Blancheton et al. 2013). These bacteria are particularly present when the C/N ratio increases as they are more competitive and more able to colonise the media than the autotrophic nitrifying bacteria (Blancheton et al. 2013; Wongkiew et al. 2017).
The nutrient concentration of the two major subsystems in an aquaponic system, i.e. the fish tanks (aquaculture) and the hydroponic solution, needs to be balanced for each of their needs. In closed aquaponic systems, nutrients are transported from fish to plants more or less directly through filters (usually a drum filter or a settler and then a biofilter) for nitrification. However, the nutrient needs of crops and the supplied nutrients from the aquaponic subsystem are not in balance. In multi-loop and decoupled systems (Chap. 8), it is easier to provide optimal conditions for both, fish and plant sections. Through modelling of the systems (Chap. 11), the optimal size of the hydroponic area to the fish tanks, biofilters and other equipment can be calculated (Goddek and Körner 2019). This is particularly important with decoupled multi-loop systems which comprise various types of equipment. For example, UASB (upflow anaerobic sludge blanket) (Goddek et al. 2018) or desalination units (Goddek and Keesman 2018) need to be sized carefully as discussed in Chap. 8. The basic mismatch of nutrients supplied by the fish environment and the needs of the crops needs to be rectified and balanced. For the purpose of up-concentrating the nutrients, Goddek and Keesman (2018) have described an appropriate desalination unit (Chap. 8). This approach, however, only solves part of the problem, as the perfectly balanced system is driven by a non-dynamic evaporation rate achievable only in closed chambers and perfectly working plants. The reality, however, is that the evapotranspiration of the crop (ETsubc/sub) in greenhousebased aquaponics systems is highly dependent on multiple factors such as physical climate and biological variables. ETsubc/sub is calculated per area of the ground surface covered by the crop and is calculated for different levels in the canopy (z) by integrating irradiative net fluxes, boundary layer resistance, stomata resistance and the vapour pressure deficitsub,/sub in the canopy (Körner et al. 2007) using the Penman— Monteith equation. This equation, nevertheless, only calculates the water flux through the crop. Nutrient uptake can either be calculated simply by assuming that all diluted nutrients in the water are taken up by the crop. In reality though, uptake of nutrients is a highly complicated matter. Different nutrients have different states, changing with parameters such as pH. Meanwhile nutrient availability to the plants strongly depends on pH and the relationships of nutrients to each other (e.g. K/Ca availability). In addition, the microbiome in the root zone plays an important role (Orozco-Mosque et al. 2018) which is not yet implemented in models although some models differentiate between phloem and xylem pathways. Thus, the vast amount of nutrients is not modelled in detail for aquaponic nutrient balancing and sizing of systems. The easiest way to estimate nutrient uptake is the assumption that nutrients are taken up/absorbed as dissolved in irrigation water and apply to the aboveexplained ETsubc/sub calculation approach and assuming that no element-specific chemical, biological or physical resistances exist. Consequentially, to maintain equilibrium, all nutrients taken up by the crop as contained in the nutrient solution need to be added back to the hydroponic system.