Denitrification and N2O production in subsoil in wheat-maize rotation field in North China Plain

Yuming Zhang1, Chunsheng Hu*,1, Oene Oenema2, Bingzi Zhao3, Wenxu Dong1, Yuying Wang1 , Xiaoxin Li1

1Key Laboratory of Agricultural Water Resources, Center for Agricultural Resources Research, Institute of Genetic and Developmental Biology, the Chinese Academy of Sciences, 286 Huaizhong Road, Shijiazhuang, Hebei, 050021,China, Email: [email protected]

2Wageningen University and Research, Alterra, Wageningen, The Netherlands

3Institute of Soil Science, Chinese Academy of Sciences, Nanjing , Jiangsu, 210008, China

Abstract

Excessive application of fertilizer nitrogen (N) in crop production systems in North China Plain has resulted in the accumulation of nitrate (NO3-) in subsoil and groundwater. Denitrification is a possible pathway for removal of accumulated nitrate in subsoil, but is often neglected and/or regarded as unimportant because of the difficulty of measuring denitrification in the subsoil.

The aim of the study reported here is to quantify the seasonal variations in denitrification activity in a 190cm deep soil profiled under a wheat-maize double cropping system as a function of N fertilizer application. The study was conducted at the Luancheng agro-ecosystem experimental station, CAS, in 2009-2010. The N fertilizer treatments included 0 (CK), 450 (N1) and 750 (N2) kg N ha-1 year-1, in triplicate. Soil cores were taken by the Geoprobe machine and incubated using acetylene inhibition technique.

Denitrification rates in the 1.9 m deep soil profile showed the commonly observed responses to N fertilizer application, irrigation and rainfall, leading to strong a temporal variability. Nitrogen losses through denitrification were significantly higher (a factor of 2.0 to 2.7) in the maize growing season than in the wheat season, likely because of the more wet and warm weather conditions in the maize growing season. On average, only 26% of total denitrification activity in the 190 cm deep soil profiled occurred in the top soil (0-15 cm); 33% occurred in the 15 to 90 cm soil layer and 41% in the 90 to 190 cm soil layer in CK. The contribution of the top soil (0-15 cm) increased to ~ 45% and that of the subsoil (90-190 cm) decreased to ~ 28%, when the N fertilizer application increased to 750 kg per ha per year. The total amount of N lost via denitrification was 6, 15 and 28 kg N ha-1yr-1 in the CK, N1 and N2 treatments, respectively.

In conclusion, the subsoil (15-190 cm) was a large contributor to N losses via denitrification, although the total N losses via denitrification were only in the range of 1 to 6% of total N fertilizer application. Further studies should try to understand the mechanism and controlling factors of denitrification in the low-carbon subsoil.