[Nitrous Oxide Production in Response to Oxygen in a Solar Greenhouse Vegetable Soil]

To explore the sources of peak nitrous oxide (N₂O) flushes in solar greenhouse vegetable field, an experiment was conducted with two conventional vegetable soils under different initial volume fractions of oxygen (O₂) (0%, 1%, 3%, 5%, and 10%). A robotized incubation system was employed to analyze the gas kinetics[O₂, N₂O, nitric oxide (NO), nitrogen (N₂), and carbon dioxide (CO₂)] every 6 or 8 h and calculate the N₂O/(NO+N₂O+N₂) index. Sodium chlorate (NaClO₃) was used to inhibit the oxidation of NO₂^- to further explore the relationship between N₂O and nitrite (NO₂^-). A parallel off-line incubation in triplicates was conducted under similar conditions to measure the dynamic changes in inorganic nitrogen content[ammonia (NH₄⁺), nitrate (NO₃^-), and NO₂^-]. The results showed that N₂O production under anaerobic condition was significantly higher than that under aerobic condition. The peak value of N₂O in the soil collected from a straw-added plot (DIS) was significantly higher than that in the soil from non-straw added plot (DI) (P<0.01) when the volume fraction of oxygen was ≤ 1%. Oxygen can directly affect N₂O production by delaying or inhibiting N₂O reduction, with significant increase in N₂O production rate under oxygen-depleted condition. However, the N₂production rate decreased significantly with increase in initial oxygen volume fraction (P<0.01). When the initial volume fraction of oxygen was between 1% and 5%, a continuous accumulation of NO₂^- was observed during the incubation period, resulting in the significantly higher N₂O/(NO+N₂O+N₂) index than that in either anaerobic or 10% of oxygen treatments. Furthermore, a linear correlation was observed between NO₂^- and N₂O at 5% and 10% of oxygen with the addition of NaClO₃ (R² ≥ 0.85). Incomplete denitrification and nitrifier denitrification from NO₂^- induction co-occurred in the range of 1% and 5% volume fractions of oxygen, significantly increasing the soil N₂O production and N₂O/(NO+N₂O+N₂) index. In addition, N₂O production under anaerobic condition was significantly higher than that under aerobic condition (P<0.01).