[Soil soluble organic nitrogen content in different forest stands in Wanmulin Nature Reserve]

An investigation was made on the soil soluble organic nitrogen (SON) in two natural forests Altingia gracilies (ALG) and Castanopsis carlesii (CAC) and an adjacent 35-year-old Cunninghamia lanceolata (CUL) plantation in Wanmulin Nature Reserve in Jianou, Fujian Province. Among the three forest stands, ALG had a significantly higher content of soil SON, being 95.3, 78.3, and 72.5 mg x kg(-1) in 0-5 cm, 5-10 cm, and 10-20 cm soil layers, respectively (P<0.05), while CAC and CUL had lesser differences in their soil SON content, which was 74.5, 70.1, and 65.6 mg x kg(-1) in the three soil layers for CAC, and 78.6, 68.9, and 69.1 mg x kg(-1) in the three soil layers for CUL, respectively. The proportion of SON to total soluble nitrogen (TSN) in 0-20 cm soil layer was 79.17-80.78% for CAC, 68.64%-74.51% for CUL, and 59.97%-69.66% for ALG. With increasing soil depth, the proportion of soil SON to soil TSN and total nitrogen (TN) for the three forest stands all increased. Soil SON content had a significant positive correlation with soil organic carbon (r=0.982, P<0.01), and also, had definite correlations with soil TSN, TN, NH4(+)-N, and NO3(-)-N. It also correlated with the factors such as forest type, topography, altitude, and tree age.