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Evaluating recycling fertilizers for tomato cultivation in hydroponics, and their impact on greenhouse gas emissions.

Operating within a scheme of circular economies (CE) and the incorporation of nutrient cycling are principle components of future sustainable food systems (Springmann et al., 2018). The socio-political awareness of CE has prompted interest into classifying waste as a resource in economies of the future. Recycling-fertilizers are one such example of closing the loop with regard to nutrient and resource losses. However, the broad field of recycling fertilizers lacks relevant knowledge on the applicability of such practices and their agro-ecological potentials, particularly in hydroponics. Soilless culture systems offer an environmentally friendly and resource efficient alternative to traditional cultivation systems; fitting within the CE scheme (Despommier, 2009). The objective of this experiment was to examine the multiple perspectives of a sustainable integration of recycling fertilizers in hydroponic cultivation-creating a proof-of concept for nutrient cycling in CE. With an interdisciplinary approach, this study focused on: plant nutrition, growth performance, fruit quality, and greenhouse gas emissions. Using the nutrient film technique (NFT), three recycling-based fertilizer variants were tested against mineral fertilizer as control (n=4), with 11 tomato plants (L. cv. Pannovy) per replicate and treatment: Two nitrified urine-based fertilizers differing in ammonium:nitrate ratio (1) Aurin (1:1) and (2) C.R.O.P. (up to 1:8), (3) a mixture of Struvite + Vinasse (S+V), and (4) mineral fertilizer. Nutrient solutions (NS) were supplemented for ideal growth conditions and exchanged weekly. The closed chamber method was adapted for gas measurements (N2O, CH4, CO2) from the root zone. Preliminary results indicate no differences in total shoot biomass (in fresh matter, FM) between recycling fertilizers and the mineral fertilizer. Variances were observed between the fruit biomass (in FM) of the different fertilizer types and decreased in the following order: Mineral>CROP>S+V>Aurin. The adapted Aurin treatment showed a high susceptibility of Blossom-End-Rot, likely due to the higher proportion of ammonium in Aurin. N2O emissions were highest in S+V, which corresponded to our hypothesis that N2O emissions positively correlate with organic C input by the fertilizer amendments. In contrast, the urine-based recycling fertilizers (Aurin and C.R.O.P) did not show higher N2O emissions than the mineral fertilizer. Overall nitrified urine with lower ammonium:nitrate ratio (e.g. C.R.O.P.) showed a high potential as recycling fertilizer in NFT systems for tomato cultivation, whereas alternative cultivation strategies may complement Aurin’s higher ammonium content. The S+V proved to supply sufficient P and K for yield. Further data evaluation still to be completed, pending laboratory results and statistical analysis.

Halbert-Howard A, Häfner F, Karlowsky S, Schwarz D, Krause A (2020) Evaluating recycling fertilizers for tomato cultivation in hydroponics, and their impact on greenhousegas emissions. Environ Sci Poll Res, Recycling, Resource Recovery and Green Chemistry Practices, 1-20.