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Nitrogen Cycle in Floating-Raft Aquaponic Systems.

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Title:Nitrogen Cycle in Floating-Raft Aquaponic Systems.
Authors:Wongkiew, Sumeth
Contributors:Molecular Biosciences & Bioeng (department)
Nitrogen recovery
Nitrogen cycle
Organic farming
Soilless agriculture
Date Issued:Aug 2018
Publisher:University of Hawaiʻi at Mānoa
Abstract:Aquaponics recycles nitrogen and produces organic crops (fish and vegetables/fruits) with simultaneous treatment of nitrogen-rich aquaculture wastewater. The nitrogen cycle in aquaponics relies on the symbiotic relationships among bacteria, fish, and plants. However, there is lack of clear understanding of microbial ecology and nitrogen transformations in aquaponics which limits a widespread adoption of aquaponic systems by farmers. Such science-based information is critically important to achieve high food productivity and nitrogen use efficiency (NUE) in aquaponic systems. This study elucidated the nitrogen transformations in floating-raft aquaponic systems using several techniques, such as nitrogen mass balance, natural abundance and enriched nitrogen stable isotope ratios, quantitative polymerase chain reaction (qPCR), and next-generation sequencing of 16S rRNA gene. The nitrogen transformations in the aquaponic systems were studied using tilapia (Oreochromis spp.) and four different plant species, namely pak choi (Brassica rapa L. Chinensis), lettuce (Lactuca sativa longifolia cv. Jericho), chive (Allium schoenoprasum L.), and tomato (Lycopersicum esculentum).
Hydraulic loading rate (HLR), dissolved oxygen (DO), and pH were found to be the critical operating parameters to maintain efficient nitrification, resulting in excellent water quality for fish, plants, and bacteria. DO levels were associated with HLR and found to positively affect nitrite oxidation rate. Nitrite concentration increased in recirculating water under low DO levels. Nitrite concentrations in the aquaponic systems at a steady state significantly increased by 1.8-2.1 times when HLRs decreased from 1.5 to 0.25 m3/m2-day, and total ammonia nitrogen (TAN) concentrations at a steady state significantly increased by 2.1 times when HLRs decreased from 1.5 to 0.10 m3/m2-day. Low pH levels (5.2-6.0) were a major factor that shifted the microbial communities and reduced the relative abundance of nitrifiers in aquaponic components (plant root and biofilter), leading to total ammonia nitrogen accumulation in recirculating water. Interestingly, in plant roots, the abundances of essential nitrifier, Nitrospira spp., did not decrease at low pH levels (pH 5.2), suggesting the benefit of plants in aquaponics for improving nitrogen recovery.
Nitrification and denitrification occurred simultaneously in the aquaponics, resulting in nitrogen loss (10.3-40.4% of nitrogen input, depending on feeding rate). Based on the isotope studies, nitrate was a major source of nitrogen assimilated by plants. However, nitrite and nitrate were the major sources of nitrogen loss via denitrification. Denitrification via direct nitrite reduction (33.7-53.4%), which was enhanced by low DO levels, was found to occur simultaneously with complete denitrification from nitrate. Nitrogen loss via denitrification was reduced by 36.9% and 74.5% when the fish feed feeding rates were decreased by 30% and 70%, respectively. Moreover, with nitrogen loss, the aquaponic systems also emitted nitrous oxide (N2O) gas, a potent greenhouse gas, accounting up to 0.72-1.03 % of the nitrogen input. Aquaponics without the balance between fish feed and plants decreased the nitrogen recovery efficiency and contributed high nitrogen loss via denitrification (under an anoxic condition) and N2O emissions.
The better understanding of nitrogen cycle linking with the microbial community and operating parameters was helpful in developing guidelines for aquaponic growers to improve water quality and archive high productivity from aquaponic systems while reducing environmental problems and operating cost.
Description:Ph.D. Thesis. University of Hawaiʻi at Mānoa 2018.
Rights:All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Appears in Collections: Ph.D. - Molecular Biosciences and Bioengineering

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