Abstract: Based on the data of the second national soil survey in China,coupling characteristics and spatial variation of soil organic carbon(SOC) and total nitrogen(TN) in the plow layers of paddy and upland fields were studied.Results show that SOC and TN contents were higher in paddy fields than in upland fields by 47.8% and 45.5%,respectively,but spatial variation of SOC and TN were found higher in upland than in paddy fields.Soil organic carbon and total nitrogen ratio((C/N)) was about 10.8 in paddy fields,higher than 9.9 in upland fields.The highest C/N ratio value was found in the paddy fields in Northeast China while the lowest in the upland fields in Northwest China and East China.Meanwhile,results also show significant regional variation of C/N ratio in upland fields,but little significant variation in paddy fields.Correlation analysis shows that SOC was significantly and positively correlated with TN,with correlation coefficients being all higher than 0.8 in paddy and upland fields except for in North China.Moreover,the correlation coefficients may decrease along with the length of agricultural land use.The findings above indicate that there exist close coupled relationship between SOC and TN and distinct regional variation in the plow layer of farmland under different land use patterns.To understand these coupled characteristics will greatly contribute to improvement of regional farmland productivity and agro-ecosystem environment,and as reference to the technique selection and policy making for carbon sequestration in farmland soils in China.

Key words: farmland, coupled carbon-nitrogen dynamics, soil organic carbon, total nitrogen, regional difference