Effect of Different Nitrogen Fertilizer Levels on Vertical Distribution of NH_4~+-N and NO_3~--N in Rubber Plantation Soil

Urea was applied as nitrogen (N) fertilizer at different rates to rubber plantations to observe vertical distribution of the soil NH4+-N and NO3--N under the rubber plantations. Application of N fertilizer increased significantly the NH4+-N and NO3--N content in the soil 0～40 cm deep. However, the N fertilizer failed to increase the NH4+-N and NO3--N content of the 0～40 cm soil when the application rate was higher than 0.6 kg/tree. The higher the N fertilizer application rate the higher the amount of the NH4+-N and NO3--N that was leached into the soil 80-100 cm deep.