Abstract: Root nodulation can directly reflect forest nitrogen-fixing ability, and is an important content in root study. Some profitable suggestions about forest fertilization and management would be offered by studying the forest nodulation on the lands converted form agricultural lands. In this paper, the root nodulation of young Alnus formosana and its relations with soil nutrients on four forestlands converted from agricultural lands in Sichuan Province were studied. The results showed that the root nodules of young A. formosana at the four sites had the same coral-shaped appearance and a compact structure, with the minimum and maximum diameter being 2 and 30 mm, respectively. The biomass of the root nodules varied from 2.59 to 132.14 g per plant. The horizontal distribution of the nodules was less than 50 cm from A. formosana stem, and their vertical distribution was concentrated in 0-20 mm soil layer. 50.45%-92.5% of the nodules were accreted to fine root (d≤2 mm), but large volume root nodules were accreted to root crown. Correlation analysis showed that root nodule biomass was significantly negatively correlated with soil hydrolytic N, but significantly positively correlated with soil available P (P＜0.05).

Key words: Grassland ecosystem, Nitrogen mineralization, Soil, Factors