title: Content and distribution of forms of organic N in soil and particle size fractions after long-term fertilization.
abstract: Most of the N in surface soils occurs in organic forms, and when mineralized it plays a key role in soil fertility and plant nutrition. Our objective was to study the effect of long-term applications of organic manure on the content and distribution of forms of organic N in bulk soil and soil particle size fractions to characterize the inherent soil nitrogen fertility. Five treatments were as follows: (1) CK (no fertilizer and no manure added), (2) mineral fertilizer only, (3) straw + NPK, (4) green manure + NPK and (5) pig manure + NPK. Soil particle size fractions (0-2, 2-10, 10-50 and 50-100 microm) were isolated without chemical pretreatment by ultrasonic dispersion in water followed by sedimentation. The content of total N and forms of organic N in the bulk soil increased after long-term fertilization, and the effect varied with fertilizer type. The plot treated with only mineral fertilizer gave the highest NH3-N and the lowest amino sugar-N content in all treatments. The highest content of amino sugar-N and amino acid-N was found in the treatment of pig manure + NPK. The content (g kg(-1) fraction) of hydrolysable N within size fractions was in the order 0-2 > 2-10 > 50-100 > 10-50 microm, but the contribution of different size fraction to hydrolysable N decreased in the sequence 10-50 > 0-2 > 2-10 > 50-100 microm. Most of the applied mineral fertilizer N that remained in soils was distributed in the particle size fraction < 2 microm while most of the remaining N from manure applied with NPK was transferred into amino sugar-N in each size fraction, and amino acid-N in the size fractions > 2 microm during the process of humification.
authors.affiliation: College of Natural Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, China.
authors.forename: Y C
authors.initials: YC
authors.lastname: Xu
authors.forename: Q R
authors.initials: QR
authors.lastname: Shen
authors.forename: W
authors.initials: W
authors.lastname: Ran
mesh_terms: D005308:Fertilizers / Q000032:analysis*; D008954:Models, Biological; D009584:Nitrogen / Q000378:metabolism*; D017672:Nitrogen Compounds / Q000378:metabolism*; D010316:Particle Size; D012987:Soil / Q000032:analysis*; D013997:Time Factors
chemical_list: D005308:Fertilizers; D017672:Nitrogen Compounds; D012987:Soil; D009584:Nitrogen