Differential immediate and long-term effects of nitrogen input on denitrification N₂O/(N₂O +N₂) ratio along a 0–5.2 m soil profile

High nitrogen (N) input to soil can cause higher nitrous oxide (N₂O) emissions, that is, a higher N₂O/(N₂O+N₂) ratio, through an inhibition of N₂O reductase activity and/or a decrease in soil pH. We assumed that there were two mechanisms for the effects of N input on N₂O emissions, immediate and long-term effect. The immediate effect (field applied fertilizer N) can be eliminated by decreasing the N input, but not the long-term effect (soil accumulated N caused by long–term fertilization). Therefore, it is important to separate these effects to mitigate N₂O emissions. To this end, soil samples along a 0–5.2 m profile were collected from a long-term N fertilization experiment field with two N application rates, that is, 600 kg N ha^-1 year^-1 (N600) and no fertilizer N input (N0). External N addition was conducted for each subsample in the laboratory incubation study to produce two additional treatments, which were denoted as N600+N and N0+N treatments. The results showed that the combined immediate and long-term effects led to an increase in the N₂O/(N₂O+N₂) ratio by 6.8%. Approximately 32.6% and 67.4% of increase could be explained by the immediate and long-term effects of N input, respectively. Meanwhile, the long-term effects were significantly positively correlated to soil organic carbon (SOC). These results indicate that excessive N fertilizer input to the soil can lead to increased N2O emissions if the soil has a high SOC content. The long-term effect of N input on the N₂O/(N₂O+N₂) ratio should be considered when predicting soil N₂O emissions under global environmental change scenarios.