Oxygen, not phenolic acids, is the primary driver for nitrification in soilless systems

To close the nutrient and water loop in hydroponic cropping systems has been recommended for both environmental and economic reasons. Despite legal restrictions (e.g., EU Water Directive), nutrient-rich irrigation water from intensive greenhouse cropping systems is still drained to the catchment. The spread of root pathogens as well as organic compound accumulation detrimental to plant growth, in the circulated nutrient solution have been claimed as major obstacles. Phenolic compounds have been suggested as inhibitory to plant growth. Early reports claim the absence of nitrification in hydroponic system putting forward the hypothesis that accumulating phenolic acids might be inhibitory to nitrification. We studied this hypothesis in a three-factorial approach with three phenolic compounds (p-hydroxy-benzoic acid, ferulic acid, vanillic acid; factor 1), concentration of the compound (0, 10, 100 µM; factor 2) and oxygen supply (factor 3). Nitrogen was supplied exclusively as ammonium. Phytotrone experiments were conducted with one-week-old tomato seedlings (temperature: 19°C, relative humidity: 75%, day length: 19 h, light intensity: 250 µmol s^‑1) and lasted for two weeks. Nutrient solution samples were collected every three days and analyzed with respect to pH, O₂-content and saturation. Samples for nitrogen dynamics were taken 24 and 48 h after the previous administration of the compound as well as directly before the following one. Our results indicate that aeration (oxygen supply) is the decisive factor driving nitrogen oxidation in the nutrient solution and that the concentration of the tested phenolic compounds as well as the nature of the administered phenolic compound were of importance in non-aerated treatments.