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Soil nitrogen responses to the conversion of cropland from Medicago sativa grassland in the semi-arid loess area

WANG Jun~(1,2),LIU Wen-zhao~1,LI Feng-min~(1,3),JIA Yu~3 (1.Institute of Soil and Water Conservation,Chinese Academy of Sciences and Ministry of Water Resources,Yangling 712100,China;2.Department of Environmental Science,Northwest University,Xi'an 710069,China;3.Key Lab of Arid and Grass Agricultural Ecology,Lanzhou University,Lanzhou 730000,China)  
The responses of soil nitrogen to the conversion of cropland from perennial M.sativa grassland in the semi-arid loess area was investigated in two years of field experiments.Compared with M.sativa grassland,soil total nitrogen decreased rapidly and soil mineral nitrogen increased in the two years after the conversion.The crop phase in the rotation had a significant effect on the soil nitrogen changes.In the second year after the conversion,planting Zea mays or Solanum tuberosum increased the soil NO_3~--N accumulation,while planting Triticum aestivum increased the soil NH_4~+-N and total mineral nitrogen,while fallow was intermediate between them.Z. mays significantly reduced soil total nitrogen compare with T.aestivum consequently Z.mays should be avoided after the conversion of perennial M. sativa grassland to cropland in the semi-arid loess area.
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