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

Soilless culture systems (SCS) play an increasing role in greenhouse vegetable production. In the SCS, soilless substrates serve as the major substitute for soil, supplying nutrients to plants but releasing greenhouse gases into the atmosphere. Remarkably, there is a serious problem of N₂O emission due to excessive input of N fertilizer. However, the microbial processes of N₂O production and consumption in soilless substrates have been rarely studied resulting in difficultly interpreting for its global warming potential. Therefore, these pathways from two classic soilless substrates under two irrigation patterns were investigated by stable isotope technology combined with qPCR analysis in present study. The results according to the dual isotopocule plot of δ¹⁵N^SP vs. δ¹⁸O showed that the mean contribution of denitrification and the mean extent of N₂O reduction of case i (Reduction-Mixing) were 26.2 and 81.2 % for the treatment of peat based substrate under drip irrigation (PD), 47.7 and 70.3 % for the treatment of coir substrate under drip irrigation (CD), 29.0 and 80.8 % for the treatment of peat based substrate under tidal irrigation (PT), and 50.8 and 47.4 % for the treatment of coir substrate under tidal irrigation (CT). These results were also further confirmed by the abundance of major functional genes including AOA amoA, nirK and nosZ. Altogether, N₂O emission and its microbial processes are determined by substrate types instead of irrigation patterns. For detail, denitrification dominated in the peat based substrate and nitrification dominated in the coir substrate. Compared to the coir substrate, the peat based substrate had higher abundance of functional genes and stronger denitrification and thus generated more N₂O. For the two soilless substrates, moreover, the microbiome replaced the mineral N content as the limiting factor for N₂O emission. In the SCS, in summary, the two soilless substrates play an important role in tomato growth, but might suffer from inorganic nutrient surplus and microbial shortage. More importantly, the combined analysis of N₂O isotopocule deltas and functional genes is a robust tool and provides reliable conclusions for clarifying the microbial processes of N₂O production and consumption, thus it is also recommended for use in environments other than soilless substrates.