Gradient distribution of soil N, P, and K along the distance from Picea mongolica seedlings root surface

Six years old seedlings of Picea mongolica were planted in pots with horizontal and vertical root treatments. Soil samples were stratified collected, and their available nitrogen (N), phosphorus (P), and potassium (K) contents were determined. The results showed that the available contents of N, P and K in soils along the distance from Picea mongolica seedlings root surface had a special gradient distribution. Near root surface, there was a depletion zone of soil available N, P, and K, and along the distance from it, the nutrient element contents increased gradually and reached their background values in the bulk soil. In the horizontal root treatment, the depletion zone of soil available N, P, and K existed in different distance from root surface, owing to the absorption and the use intensity of the seedlings to these nutrient elements. The depletion zone of soil available N and K appeared from 1 cm from root surface, and that of available P existed in 0.5 cm from root surface. In the vertical root treatment, the depletion zone of available N and K also appeared from 1 cm from root surface as in the horizontal root treatment, but the available P content near root surface was lower than that far away from root surface. It is proved that the absorption and the utilize intensity of soil available P by Picea mongolica seedlings was intense, which might become a limiting factor to its growth and development. Therefore, Picea mongolica should be planted on soils rich in phosphorus, or P and N fertilizers should be applied to improve the absorption and the use efficiency of available P by Picea mongolica.