Abstract: An exponential fertilization experiment was conducted with six N application levels (0 (CK), 100, 200, 400, 600, 800 mg·seedling^-1) to examine the effects of N application levels on the growth and N accumulation of pecan (Carya illinoensis) seedlings. The objective of this study was to reveal the N demand and determine the optimal N application amount for pecan seedlings. Results showed that the stem biomass, leaf biomass, and total biomass of pecan seedlings increased as fertilization levels increased from 0 to 600 mg·seedling^-1 and then decreased with the levels increased from 600 to 800 mg·seedling^-1. The maximum values of these three variables were 2.30, 4.39 and 9.33 g·seedling^-1, being 2.30, 4.99 and 2.17 times as high as that of the CK respectively. Root biomass decreased with the increases of N supply. For exponential fertilization treatments, the N concentration increased by 79%-640% in roots, 101%-352% in stems, and 43%-103% in leaves with N content increasing 184%-666% in roots, 240%-935% in stems, and 402%-823% in leaves. The N content and total biomass of pecan seedlings increased as the N fertilization increased from 0 to 600 mg·seedling^-1 and then decreased with the fertilization levels  from 600 to 800 mg·seedling^-1. The N concentration of pecan seedlings increased with the increases of N supply. We concluded that 600 mg·seedling^-1 would be the optimal N application level for pecan seedlings in greenhouse.

Key words: species distribution, environmental factor, loess hilly region, niche, Robiniahe pseudoacacia