Nitrogen(N) is a common limiting nutrient in crop production.The N content of soil has been used as an important soil fertility index.Organic N is the major form of N in soil.In most agricultural surface soils,more than 90% of total N occurs in organic forms.Therefore,understanding the compositional characteristics of soil organic N functional groups can provide the scientific basis for formulating the reasonable farmland management strategies.Synchrotron radiation soft X-ray absorption near-edge structure(N K-edge XANES) spectroscopy is the most powerful tool to characterize in situ organic N functional groups compositions in soil.However,to our most knowledge,no studies have been conducted to examine the organic N functional groups compositions of soil using N K-edge XANES spectroscopy under long-term fertilization practices.Based on a long-term field experiment(started in 1990) in a black soil(Gongzhuling,Northeast China),we investigated the differences in organic N functional groups compositions in bulk soil and clay-size soil fraction among fertilization patterns using synchrotron-based N K-edge XANES spectroscopy.Composite soil samples(0～20 cm) were collected in 2008.The present study included six treatments:farmland fallow(FALL),no-fertilization control(CK),chemical nitrogen,phosphorus,and potassium fertilization(NPK),NPK in combination with organic manure(NPKM),1.5 times of NPKM(1.5 NPKM),and NPK in combination with maize straw(NPKS).The results showed that N K-edge XANES spectra of all the treatments under study exhibited characteristic absorption peaks in the ranges of 401.2～401.6 and 402.7～403.1 eV,which were assigned as amides/amine-N and pyrrole-N,respectively.These characteristic absorption peaks were more obvious in clay-size soil fraction than in bulk soil.The results obtained from the semi-quantitative analysis of N K-edge XANES spectra indicated that the relative proportion of amides/amine-N was the highest in both bulk soil and clay-size soil fraction,and it was the most major forms in soil organic nitrogen functional groups.Compared with the FALL treatment,the relative proportion of amide/amine-N was lower whereas that of Pyrrole-N was higher in the CK treatment.In the treatments with combined chemical fertilizers and organic manure,the relative proportion of amide/amine-N decreased with increasing application rates of organic manure,while that of Pyrrole-N had an opposite trend.In bulk soil,the relative proportion of amide/amine-N was the highest for the NPKS treatment than for the other treatments.On the other hand,the relative proportion of nitrile/aromatic-N was the highest for the Fallow treatment than for the other treatments in clay-size soil fraction.It is feasible to use N K-edge XANES spectroscopy for characterizing in situ the changes of organic N functional groups in soil under different fertilization practices.