Sources of nitrous oxide emissions from hydroponic tomato cultivation: Evidence from stable isotope analyses

INTRODUCTION: Hydroponic vegetable cultivation is characterized by high intensity and frequent nitrogen fertilizer application, which is related to greenhouse gas emissions, especially in the form of nitrous oxide (N(2)O). So far, there is little knowledge about the sources of N(2)O emissions from hydroponic systems, with the few studies indicating that denitrification could play a major role. METHODS: Here, we use evidence from an experiment with tomato plants (Solanum lycopersicum) grown in a hydroponic greenhouse setup to further shed light into the process of N(2)O production based on the N(2)O isotopocule method and the (15)N tracing approach. Gas samples from the headspace of rock wool substrate were collected prior to and after (15)N labeling at two occasions using the closed chamber method and analyzed by gas chromatography and stable isotope ratio mass spectrometry. RESULTS: The isotopocule analyses revealed that either heterotrophic bacterial denitrification (bD) or nitrifier denitrification (nD) was the major source of N(2)O emissions, when a typical nutrient solution with a low ammonium concentration (1–6 mg L(−1)) was applied. Furthermore, the isotopic shift in (15)N site preference and in δ(18)O values indicated that approximately 80–90% of the N(2)O produced were already reduced to N(2) by denitrifiers inside the rock wool substrate. Despite higher concentrations of ammonium present during the (15)N labeling (30–60 mg L(−1)), results from the (15)N tracing approach showed that N(2)O mainly originated from bD. Both, (15)N label supplied in the form of ammonium and (15)N label supplied in the form of nitrate, increased the (15)N enrichment of N(2)O. This pointed to the contribution of other processes than bD. Nitrification activity was indicated by the conversion of small amounts of (15)N-labeled ammonium into nitrate. DISCUSSION/CONCLUSION: Comparing the results from N(2)O isotopocule analyses and the (15)N tracing approach, likely a combination of bD, nD, and coupled nitrification and denitrification (cND) was responsible for the vast part of N(2)O emissions observed in this study. Overall, our findings help to better understand the processes underlying N(2)O and N(2) emissions from hydroponic tomato cultivation, and thereby facilitate the development of targeted N(2)O mitigation measures.