In natural ecosystems, soil organic carbon is derived almost exclusively from residues of previous vegetation or from contributions of growing plants. Roots have long been suspected to be an important source of soil organic matter. After grain harvest, the decomposition of root residues which remained in the field will contribute to soil fertility and crop productivity. Different fertilizations will affect the quantity and chemical properties of crop roots to varying degrees, such as root biomass, carbon and nitrogen nutrient contents in crop root. Recently, more attention has been paid to the effects of root residue decomposition on crop yield and soil physical and chemical characteristics, for instance, crop roots remained in field play a role in sustaining soil moisture and increasing soil organic carbon in microbes. In this study, the relationship between decomposition of crop root residues and soil carbon and nitrogen transformation under different nitrogen fertilizations was discussed. We collected maize roots from a 7-year long term different cultivation and nitrogen fertilizer experimental field located at the south edge of the Loess Plateau (108°04'07"E, 34°17'56"N) in December 2010 after maize harvest. Maize roots in 0-20 cm soil depth were gathered from field plots in 0, 120 and 240 kg N/hm² treatments and marked R₀, R₁₂₀ and R₂₄₀, respectively. At the same time, soil samples were collected in 15-20 cm and 45-50 cm along the maize protection belt. Air-dried soils of 100 g in the two layers were mixed with three maize roots in proportion of 2% dry weight respectively and incubated at 25 ℃ for 105 days, soil moisture was kept at 70% of field water holding capacity. During the incubation period, soil CO₂ release, microbial biomass carbon, dissolved organic carbon and mineral nitrogen contents in soils were determined regularly. Also, we applied a kinetic equation to fit the cumulative carbon mineralization amount of maize root with the incubation time. The results showed that C/N ratios of the three different nitrogen fertilized maize roots were R₀(130:1)>R₂₄₀(74:1)>R₁₂₀(65:1). It was found that carbon mineralization rates of maize root ranked in the order that R₁₂₀>R₂₄₀>R₀ in two soil layers. At the end of the incubation, there performed a significantly negative correlation between C/N ratio of the three maize roots and their cumulative CO₂ release amounts. Root carbon mineralization ratio, potential carbon mineralization amount, soil microbial biomass carbon and dissolved organic carbon contents were all highest in the R₁₂₀ addition treatment compared to the R₀ and R₂₄₀ addition treatments. Soil mineral nitrogen content significantly increased at the beginning of incubation in R₁₂₀ and R₂₄₀ addition treatments. At the two soil layers, carbon mineralization ratios of R₀, R₁₂₀ and R₂₄₀ were 51. 7%, 26.4% and 27.8% greater in the 15-20 cm soil layer than that in the 45-50 cm soil layer, respectively. The findings demonstrate that decomposition characteristics of crop root under different fertilizations should be taken into account when we evaluate the effects of crop residues returned to soil on carbon and nitrogen nutrient cycling in farmland ecosystems.