Abstract:This paper deals with nitrogen characteristics of two man-made forests in Heshan Inter Disciplinary Experimental Station, Academia Sinica, situated in 23˚N, l18˚E. The results show:l) A. mangium and A. auriculaeformis could improve soil significantly. Surface soil (0～10cm) contained organic matter, over 30g·kg-1, nitrogen, over 1.6g·kg-1 A. mangium had much stronger soil effect than A. auriculaeformis. In soil deeper than 30cm, nitrogen content in A. auriculaeformis had only half of that in A. mangium. 2) Nitrogen content did not change very much, but undulation could be observed. The level of nitrogen content in both live and dead leaves of A. mangium was higher than that of A. auriculaeformis, accounting for the quick improvement of soil by A. mangium. Both species largely reused the nitrogen from senesced leaves. The resorption rate of A. mangium ranged from 24 % to 58 %, while 37 % ～60 % for A. auriculaeformis. 3) Nitrogen mineralization was multiplied as the soil was disturbed. The total mineralized nitrogen of soil core incubation was 15.04 mg·kg-1 for A. mangium after 30 days’incubation, while 20.81 mg·kg-1 for A. auriculaeformis respectively. On the contrary, the total mineralized nitrogen of ground soil incubation was 46.17 mg·kg-1 for A. mangium, 44.53 mg·kg-1 for A. auriculaeformis. The nitrate was the dominant form of mineralized nitrogen. 4) The different organs of both species had rather different nitrogen content, with highest in leaf, lowest in trunk. The leaves of A. mangium contained 33.4g ·kg-1 N, while 31.4 g·kg-1 for A. auriculaeformis. The trunk of A. mangium contained 5.4g·kg-1 N, while 3.9 g·kg-1 for A. auriculaeformis. The stem and root of both species had rather similar pattern of nitrogen content. The higher the diameter class the lower the nitrogen content was the low C/N ratios of the species were beneficial to the soil amelioration.