Insights into production and consumption processes of nitrous oxide emitted from soilless culture systems by dual isotopocule plot and functional genes

Microbial mechanism of biochar addition to reduce N2O emissions from soilless substrate systems

The soilless peat-based substrate partially solves the global soil problem in greenhouse vegetable production. However, it still produces serious N2O emissions due to the application of nutrient solutions. The pyrolysis biochar is regarded as an effective measure to reduce soil N2O emissions. However, the effect and mechanism of biochar on N2O emissions from the soilless substrate remain unknown. Therefore, this study set up six treatments by adjusting the ratio of biochar addition of peat-based substrate: 0% (0BC), 2% (2BC), 4% (4BC), 6% (6BC), 8% (8BC) and 10% (10BC) (v/v). The results showed that compared to the control treatment, N2O emissions reduced by 81%, 71%, 51%, 61%, and 75% in the 2BC, 4BC, 6BC, 8BC and 10BC treatments, respectively. In addition, lettuce yield increased by 10% and 7% in the 2BC and 4BC treatments and decreased by 0.5%, 4% and 6% in the 6BC, 8BC and 10BC treatments, respectively. Combining stable isotope technology, qPCR analysis and high-throughput sequencing, five microbial pathways of N2O production, including bacterial and archaea nitrification (BN and AN), denitrification performed by fungi, denitrifier bacteria and nitrifier bacteria (FD, DD and ND), were roughly distinguished. In addition, the extent of N2O reduction was obtained by δ18O vs.δ15NSP map. For all treatments, overall, the DD process (over 50%) was the main process of N2O production and reduction, while ND and AN processes were almost negligible (less 5%). In detail, the decrease of N2O emissions was caused by decreasing the contribution of FD in the 6BC, 8BC and 10BC treatments and reducing the contribution of BN in the 0BC and 2BC treatments. In addition, biochar addition increased the extent of N2O reduction to N2. In summary, the 2% biochar addition presented the greatest extent of N2O reduction to N2 (83%) and the lowest N2O emissions as well as the highest lettuce yields and nitrogen utilization efficiency. Therefore, 2% biochar is deemed the most optimal addition to the peat-based substrate.

Comparing the Nutritional Needs of Two Solanaceae and One Cucurbitaceae Species Grown Hydroponically under the Same Cropping Conditions

Switching over to closed-loop soilless culture systems, thus preventing pollution of water resources by nitrates and saving water and fertilizers, requires accurate estimations of the mean nutrient-to-water uptake ratios. To contribute to this objective, three fruit vegetable species (tomato, eggplant, cucumber) were grown hydroponically in a floating system under identical cropping conditions to quantify species differences in nutrient uptake. The composition of the nutrient solution used to feed the crops was identical for all species. The total water consumption and the concentrations of most nutrients (K, Ca, Mg, N, P, Fe, Mn, Zn, Cu, B) in the nutrient solution and the plant tissues were measured at crop establishment and at two different crop developmental stages. The obtained data were used to determine the uptake concentrations (UCs) using two mass balance models, one based on nutrient removal from the nutrient solution and a second based on nutrient recovery in the plant tissues. The experiment was conducted in the spring-summer season. The results revealed that the nutrient uptake concentrations were substantially different between species for all nutrients except for N, while there were also significant interactions between the two methods used for their estimation of some nutrients. Thus, the UCs of N, P, Ca, and some micronutrients were significantly higher when its estimation was based on the removal of nutrients from the nutrient solution compared to recovery from plant tissues, presumably because with the first method, losses due to denitrification or precipitation could not be separated from those of plant uptake. The comparison of the three greenhouse vegetables revealed a similar UC for nitrogen, while cucumber generally showed significantly lower UCs for P and for the micronutrients Fe, Zn, and Cu at both cropping stages compared to the two Solanaceae species. The obtained results can be used to precisely adjust the nutrient supply in closed-loop soilless cultivations to the plant uptake thus avoiding both depletion and accumulation of nutrients in the root environment.

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 (N₂O). So far, there is little knowledge about the sources of N₂O 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 N₂O production based on the N₂O isotopocule method and the ¹⁵N tracing approach. Gas samples from the headspace of rock wool substrate were collected prior to and after ¹⁵N 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 N₂O emissions, when a typical nutrient solution with a low ammonium concentration (1-6 mg L^-1) was applied. Furthermore, the isotopic shift in ¹⁵N site preference and in δ¹⁸O values indicated that approximately 80-90% of the N₂O produced were already reduced to N₂ by denitrifiers inside the rock wool substrate. Despite higher concentrations of ammonium present during the ¹⁵N labeling (30-60 mg L^-1), results from the ¹⁵N tracing approach showed that N₂O mainly originated from bD. Both, ¹⁵N label supplied in the form of ammonium and ¹⁵N label supplied in the form of nitrate, increased the ¹⁵N enrichment of N₂O. This pointed to the contribution of other processes than bD. Nitrification activity was indicated by the conversion of small amounts of ¹⁵N-labeled ammonium into nitrate.

Discussion/Conclusion

Comparing the results from N₂O isotopocule analyses and the ¹⁵N tracing approach, likely a combination of bD, nD, and coupled nitrification and denitrification (cND) was responsible for the vast part of N₂O emissions observed in this study. Overall, our findings help to better understand the processes underlying N₂O and N₂ emissions from hydroponic tomato cultivation, and thereby facilitate the development of targeted N₂O mitigation measures.