Effects of pH and initial labile C/NO₃^– ratio on denitrification in a solar greenhouse vegetable soil

ObjectivesDenitrification is one of predominant process for N₂O emission from solar greenhouse vegetable soils. An anoxic incubation experiment was conducted to explore effects of soil pH and initial labile C/NO₃^– ratio on nitrogen gaseous emissions (i.e. N₂O, NO and N₂) and the N₂O product during the denitrification in a solar greenhouse vegetable soil.

MethodsA typical greenhouse vegetable soil in Shouguang City was selected for the study, and soil pH was adjusted to acidic, neutral and alkaline by adding a certain amount of low concentration of acid (0.1 mol/L HCl) or alkaline (0.1 mol/L NaOH) solution. The final soil pH values were 5.63, 6.65 and 7.83 (adjusted), respectively. Sodium glutamate (C₅H₈NO₄Na) was added as a labile organic carbon source, and the ratios of initial labile C to NO₃^– were adjusted to 5∶1, 15∶1 and 30∶1 by adding different amounts of sodium glutamate and no sodium glutamate addition in the control (CK). Four levels of initial labile C/NO₃^– ratios were set up in the three soils with different pH, and there were three replicates in each level. A robotized incubation system was used to monitor the kinetics of gaseous products (N₂O, NO, N₂, and CO₂) of denitrification under anoxic conditions and to estimate the N₂O product ratio of denitrification by calculating Index N₂O/(N₂O + NO + N₂).

ResultsThe increase in soil pH significantly reduced N₂O and NO production in the soil, and the peak values of N₂O and NO production in the acidic soil were significantly higher than those of the alkaline and neutral soils with different initial C/NO₃^– ratios (P < 0.05). With the increase of initial labile C, the production of N₂O was reduced in the neutral and alkaline soils, but kept unchanged in the acidic soil. The addition of sodium glutamate reduced NO emission in the neutral soil. However, there were no differences in NO production in other pH levels. A significant interaction on N₂O production was observed between soil pH and initial labile C/NO₃^– ratio (P < 0.001). The N₂ production rate significantly increased with sodium glutamate addition in the acidic and neutral soils (P < 0.05). The addition of sodium glutamate could promote the reduction of N₂O to N₂ in denitrification process at different soil pH. The CO₂ production was significantly higher in the alkaline soil than those in the acidic and neutral soils. As compared with CK, the CO₂ production increased significantly with the sodium glutamate addition at different soil pH (P < 0.05). The N₂O product ratio in the acidic soil was significantly higher than that in the alkaline soil under different initial labile C/NO₃^– (P < 0.01), the N₂O product ratio was significantly decreased with the increase of soil pH, and a significant linear negative correlation relationship was observed between soil pH and the Index N₂O (P < 0.01).

ConclusionsHigh N₂O and NO emissions were usually found in greenhouse vegetable soils, primarily due to the decline of soil pH. Furthermore, increase in labile C availability promoted the N loss of denitrification, and decreased N₂O production in neutral and alkaline soils. With the increase of both soil pH and labile C/NO₃^–, the N₂O product ratio reduced, but the denitrification activity increased. Both soil pH and carbon availability were crucial factors for estimation of N loss of denitrification by multiplying N₂O flux and N₂O product ratio.