Characteristics of nitrous oxide emissions and the affecting factors from vegetable fields on the North China Plain.

Nitrous oxide (N2O) is one of the most important greenhouse gases emitted from fertilized agricultural soils. Vegetable fields, mostly managed under intensive mode with higher rate nitrogen application, frequent irrigation, and multiple planting-harvest cycles, does contribute to national GHG inventory greatly due to the increasing planting area in China. N2O emissions from four different fields - a maize field (maize), a newly established open-ground vegetable field converted from a maize field four years earlier (OV4), an established open-ground vegetable field converted from a maize field more than 20 years ago (OV20), and an established sunlight heated greenhouse vegetable field converted from a maize field more than 20 years ago (GV20) with four different fertilization treatments for the OV4 field were measured using the closed chamber method between March 15th, 2012 and March 14th, 2013 in suburban area of Beijing, North China Plain. Results showed that the annual N2O emissions from vegetable fields were 3.1-4.6 times higher than the typical maize field. All the N2O emission peaks were occurred after fertilization and the fertilization associated emissions accounted for 81.1% (ranging from 77.0% to 87.2%) of the annual N2O emission with 22.2% time duration in the whole year for vegetable fields. Both the occurrence data and duration of N2O emission peaks were associated with N input type (chemical or manure) and the application rate. The N2O emission peaks appeared earlier (on the 3rd day after application) and lasted shorter when only chemical N was applied; while they appeared later (on the 7th to 10th day after application) and lasted longer when the combination of manure and chemical N were applied. The magnitudes of N2O emission peaks increased when the N application rate was higher. Dicyandiamide (DCD) decreased N2O emissions by 30.1% and 21.1% in the spring cucumber and autumn cabbage seasons respectively (averaged of 24.7% over the whole year). Calculations showed that it is critical to estimate the emission factor (EF) by N type in order to decrease the uncertainty of regional N2O emissions when using EF as calculation method. EFs were 0.20% and 0.42% for manure N in the cucumber and cabbage seasons respectively; and were 0.55-1.30% and 0.8-1.59% for chemical N in the cucumber and cabbage seasons respectively.