Nitrogen cycle and content in plowable soil layer under different land-use types in Dongting Lake region

By high density sampling and farming household investigation,total N content,microbial biomass N(MB-N) content, total N and MB-N relationship,and N cycle characteristics and N balance of plowable soil layer under different land-use types were studied in Dongting Lake region.Results show 3.00±0.48g/kg total N content and 101.4±49.2mg/kg MB-N content in farmlands respectively.Total N contents for double cropping paddy fields,single cropping paddy fields,dry farming paddy fields and dry lands are 3.12±0.40g/kg,3.03±0.39g/kg,2.79±0.43g/kg and 2.10±0.46g/kg;and MB-N contents are 124.0±56.6mg/kg,96.4±39.2mg/kg,108.0±48.6mg/kg and 75.2±30.5mg/kg respectively. Except for dry farming in paddy soils ,MB-N and total N are significantly passively correlative (P<0 .01)for all other land-use types.The order of N surplus for the different land-use types is:double cropping paddy field(105.0 kg/hm²·a)>single cropping paddy field (75.1kg/hm²·a)>dry farming paddy soil (64.5kg/hm²·a)>dry land(51.9kg/hm²·a).