The presence and transformations of nitrogen (N) in the environment depend on a variety of environmental factors but are also strongly influenced by anthropogenic activities such as modern agriculture. Understanding N transformations within the context of agricultural systems is crucial for efficient use thereof.
The aim of this study was to investigate the changes in concentration of N forms (ammonium, nitrite, nitrate, and organic N) within an aquaponics system, a modern agricultural system, in order to obtain insights into environmental pressures influencing N transformation processes.
By measuring the concentrations of the individual N compounds, complemented by the determination of abiotic parameters and other relevant nutrients within the system water at 13 sampling points, significant differences between compartments that build up an aquaponic system could be demonstrated.
These differences were attributed to individual microenvironments specific to the aerobic loop, anaerobic loop, and radial flow settler as a connection between the two, shaping the microbial processes within the aquaponic system.
Nitrogen (N) is an element occurring in all organisms, including humans. Being one of the most common elements on earth, nitrogen is continuously moved around the biosphere in what we know as the nitrogen cycle. The nitrogen cycle is strongly influenced by anthropogenic activities and is dependent on a variety of environmental factors (Widdison and Burt, 2008).
For instance, modern agriculture systems are highly inefficient in their use of N, with between 50 and 70 % of applied N lost to the environment, instead of being converted into plant biomass. This can result in environmental toxicity and affects climate change (Coskun et al., 2017; Erisman et al., 2011; Fowler et al., 2013; Galloway et al., 2008; Schlesinger, 2009).
Understanding N transformations and the microbial communities involved therein, as well as understanding the potential environmental impact of food production technologies that use N, is therefore crucial (Robertson and Groffman, 2007).
While N transformations in soil and natural aquatic systems are often studied, there is still a noticeable lack of research regarding the N transformations in aquaponics, a newer food production technology combining recirculating aquaculture and hydroponic culture (Wongkiew et al., 2017).
In aquatic food production systems, four of nine N forms (Robertson and Groffman, 2007), organic N (Norg), ammonium (NH4+), nitrite (NO2−), and nitrate (NO3−), require monitoring to avoid them reaching toxic concentrations for the organisms in the system (Dodds and Whiles, 2010; Timmons and Ebeling, 2010).
Where an excess of N waste is present, its removal is necessary. This is particularly important for the more toxic forms NH4+ and NO2− and less critical for NO3− due to its lower relative toxicity (Timmons and Ebeling, 2010).
In aquaponic systems specifically, N is required to fulfill the nutritional requirements of fish and crops. The primary input of N into an aquaponic system is via proteins in fish feed as Norg. These are ingested, metabolized, and transformed by the fish into ammonia (NH3) and primarily released into the aqueous medium via passive gill diffusion (Randall and Wright, 1987).
The remaining Norg present in the fish excreta, non-consumed feed, and decaying biomass is mineralized to NH4+ (Cai et al., 2017). Some of this inorganic N (Ninorg) forms can be further transformed to NO2− and NO3− via nitrification, to nitrogen gas via denitrification and/or anammox, or assimilated into biomass by microbes and plants (Kułek, 2015; Robertson and Groffman, 2007; Widdison and Burt, 2008).
In aquaponic systems, N transformations mainly depend on the presence or absence of oxygen and organic carbon, which creates the correct environmental conditions for particular groups of microbes (Schmautz et al., 2020).
The different compartments constituting the aquaponic systems are designed to steer the environmental conditions to achieve the desired microbial activity in order to ensure that concentrations of Norg, NH4+, NO2− and NO3− are kept below their tolerance range, in turn ensuring fish and plant welfare.
The aim of this study was to compare N concentrations in the different compartments of the aquaponic systems in order to obtain insights into the environmental conditions which could influence the N transformation processes in these systems and to what extent.
By measuring the concentrations of the individual N compounds, complemented by the determination of other relevant nutrients in the system water, conclusions concerning the biochemical performance of the system can be drawn, and enable the metabolic processes in the aquaponic system to be steered in the right direction.
The dataset showed a clear distinction between aerobic loop, FS, and digested sludge (DS and SS), with the RFS as a connection between the aerobic and anaerobic loop, confirming the results of Schmautz et al. (2020) looking into the microbial diversity in different compartments of the aquaponic system.
Aerobic loop samples had high NO3− and oxygen levels, while the RFS had higher NO2− levels with increased influence from the ambient temperature, causing variation in the temperature of the measured samples, and causing FS to have high levels of TN and Norg. In contrast, digested sludge (DS and SS) had high electrical conductivity and NH4+ content.
Measuring the concentrations of individual N compounds within the aquaponic system, in addition to other relevant abiotic parameters, assists in drawing conclusions concerning the performance of the organisms present in the system, that is, that they are able to support in steering the metabolic processes involved.
While large differences in the water parameters between compartments were not to be expected due to the high water circulation rate and low water volume of the system, it could be shown that N concentrations, ratios, and abiotic parameter values varied significantly amongst the compartments.
Thus, each compartment represented a different microenvironment responsible for specific microbial processes within the aquaponic system (Schmautz et al., 2020).
While this is the first paper to describe detailed N transformations within such a system, further research using nutrient-mass-flow analyses and metagenomics to support these findings is necessary in order to better understand the role of microbial communities in these processes and allow the translation of these processes to other managed systems. In doing so, the long-term operation of such systems could be secured by assuring N conservation through its removal from wastewater, overcoming existing environmental challenges.
Source: Schmautz, Z., Espinal, C. A., Smits, T. H., Frossard, E., & Junge, R. (2021). Nitrogen transformations across compartments of an aquaponic system. Aquacultural Engineering, 92, 102145.
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