Production Of N₂ And N₂O In Soil Profiles Of A Maize-Wheat Rotation Cropland In The North China Plain

Excessive nitrate（NO₃^--N）accumulated in soil profile of intensive farmland will lead to groundwater pollution and indirect emission of nitrous oxide（N₂O）.However,the quantitative loss of denitrification nitrogen（N₂）,the NO₃^-removal process in profile soil,is uncertain.Most of the previous studies were based on the measurement under anaerobic conditions,and there is still a lack of scientific knowledge on how changes in soil oxygen（O₂）concentration at different depths of profile affect the production of N₂ in each layer of soil.In this study,different O₂ gradients（mean value,minimum value and anaerobic value）and culture temperature（mean value）were set based on the changes of soil O₂ concentration and temperature observed in different depths in the field during the corn growing season on the experimental platform of summer corn-winter wheat rotation under long-term intensive management.Helium culture-direct determination of N₂ was used to quantify the N₂O and N₂ production rates in soils at different depths from 0 to 2 m profiles.The results showed that the soil N₂ and N₂O production in the surface layer（0.5 m）was higher than that in the deep layer（2 m）in the different depths of the profile under both the control（CK,soil background）and nitrate addition(30N,30 mg N kg^-1)treatments.Regardless of soil depth,the rate of gas production was higher in anaerobic condition than in aerobic condition.Similarly,the N₂O production rates were<0.02（0.5 m）and<0.03（1 m and2 m）μg N h^-1 kg^-1ds when the oxygen concentration was the average value of field observation,which were 4-34 times lower than when the oxygen concentration was the minimum value of field observation.With add 30N substrate,the N₂O production rate in anaerobic condition was 2.38-78.33 times higher than that in aerobic condition.Since in situ soil O₂ concentrations were>2%（0.5 m and 1 m）and>14%（2 m）,it was suggested that observations under anaerobic conditions would overestimate N₂ and N₂O production rates under real soil conditions.Moreover,oxygen deprivation significantly increased the N₂O/（N₂O+N₂）ratio in the deep soil,which may be due to the lack of carbon in the deep soil,which was not conducive to the further reduction of N₂O.In addition,based on the observed N₂ and N₂O production rates under aerobic conditions,the denitrified N₂+N₂O loss of 0～2 m soil in the corn growing season（calculated at 120 days）was estimated to be 219（CK）～2826（30N）kg N ha^-1 season^-1,indicating that the soil had a strong ability to remove nitrate nitrogen accumulated in its profile.This greatly reduces the risk of further migration of nitrate nitrogen from vadose zone to groundwater.