Combined application of chemical and organic fertilizers long-term increase physical proteetion of organic nitrogen in yellow paddy soil

【Objectives】 A physical-chemical fractionation method is an effective tool for deeply investigating soil organic carbon fractions due to its linkage of stabilization mechanisms of organic carbon in soil. In this study, the method was used for better understanding the effects of long-term fertilization on yellow paddy soil organic nitrogen soil organic nitrogen(SON) fractions and guide proper fertilization. 【Methods】 The soil samples were collected from a 18-years field experiment in yellow paddy soil, the soil organic nitrogen fractions and their proportions in total organic N were investigated using the physic-chemical fractionation method. The treatments included: no fertilizer (CK), chemical fertilizer alone (NPK), organic manure alone (M), the chemical fertilizer combined with manure at a conventional rate (0.5 MNPK) and a greater rate (MNPK). 【Results】 The NPK treatment had no significant effects on total SON and its fraction contents compared with CK. Total SON, unprotected coarse particulate, physically-protected, chemically-protected and biochemically-protected organic nitrogen(ON) contents were significantly increased by 27%-51%, 23%-39%, 128%-274%, 29%-42% and 13%-28% correspondingly under the manure treatments (M, 0.5MNPK, MNPK). Compared with CK, unprotected fine particulate ON content was decreased. Among these fractions, the unprotected ON showed the highest percentage (46%-50%), and the physically protected ON showed the lowest percentage (2%-6%) in total SON. 【Conclusions】 Generally, the total SON and its fractions are significantly correlated with each other. Soil unprotected fine particulate ON is negatively correlated with physically-protected ON. The physically-protected ON proportion are higher under the manure treatments (M, 0.5MNPK, MNPK) than the CK and NPK treatments. Manure application promotes the transformation of free fine particulate ON to physically-protected ON due to possibly increased aggregation, thus decreased soil unprotected fine particulate ON content and increased physically-protected ON proportion (increased contribution of physical protection to SON stabilization). Our data indicated that the organic and inorganic fertilizer combination is the most effective practice for increasing the paddy SON content.