Management of nitrogen fertilizer application, rather than functional gene abundance, governs nitrous oxide fluxes in hydroponics with rockwool

Aims Nitrous oxide (N₂O) is a strong greenhouse effective gas (GHG); the primary human source of N₂O is agricultural activities. Excessive nitrogen (N) fertilization of agricultural land is now widely recognized as a major contributor. In soil, the microbial processes of nitrification and denitrification are the principal sources of N₂O. However, it remains poorly understood how conventional hydroponics influences GHG emission. Here, we compared GHG fluxes from soil and rockwool used for hydroponics under identical nutrient conditions. Methods Tomato plants (Solarium lycopersicum, momotaro) were grown in soil or by hydroponics using rockwool. In situ emissions of CH₄, CO₂, and N₂O, and the abundance of genes involved in nitrification and denitrification were measured during cultivation. Results Hydroponics with rockwool mitigated CO₂ emission by decreasing the microbial quantity in the rhizosphere. Dilution of the nutrient solution significantly decreased N₂O emission from rockwool. Although proliferation of nitrifiers or denitrifiers in the rhizosphere did not induce N₂O emission, reuse or long-term use of rockwool induced a 3.8-fold increase in N₂O emission. Conclusions Our data suggest that hydroponics has a lower environmental impact and that adequate fertilizer application, rather than bacterial control, governs N₂O fluxes in hydroponic cultivation using rockwool.