Soil soluble organic nitrogen (SON) content is low, but is one of the most active components in the soil nitrogen pool. Litter decomposition and soil nitrogen transformation contribute considerably to SON; however, their effects on soil SON are unclear. To investigate the role of litter in soil nitrogen transformation, after seven days of pre-incubation, an experiment was carried out to study the effects of litter decomposition on soil SON and its relationship with soil nitrogen by adding contrasting fresh litter, Chinese fir, and broad-leaved ¹⁵N-labeled (atom 6.1%) litter to the soil surface at 25℃ for 210 days. Soil moisture was kept at 60% water-holding capacity by adding deionized water every two days to the surface. The concentration of soluble inorganic and organic nitrogen in 0.5 mol/L K₂SO₄ solution extracts of soil and litter were analyzed at each sampling time. In addition, for the treatment with addition of broad-leaved ¹⁵N-labeled litter, the ¹⁵N abundance in ammonium and nitrate nitrogen was analyzed and the SON was calculated, and their proportion in soil or litter was also calculated. The results showed that soil SON did not increase when it decreased significantly in litter under litter addition, independent of litter type, which may be owing to no leaching. Compared to the control, Chinese fir litter addition increased soil inorganic nitrogen significantly, whereas for higher C/N ratios, broad-leaved litter needed more soil ammonium nitrogen from soil at the early stage of its decomposition. Therefore, ammonium nitrogen in soil under broad-leaved litter addition did not increase. However, soil nitrate nitrogen increased with broad-leaved litter addition, similarly to that of Chinese fir litter addition. Results showed that a considerably higher proportion of soil SON was from ¹⁵N labeled broad-leaved litter in 90-210 days of incubation, and was 74.8% on day 210. In addition, the proportion of soil ammonium nitrogen from litter increased in the first 30 days of the experiment, and then increased to 39.8% by day 210. However, soil nitrate nitrogen was high and increased with incubation time, and its proportion from litter was low, evidenced by the calculation of ¹⁵N isotope data, which indicated that soil nitrate nitrogen was mainly from soil nitrogen nitrification rather than litter source. These results suggested that in the early stages of incubation, soil SON mainly comes from the decomposition of soil organic matter from broad-leaved litter, and the SON from litter possibly mineralized to become ammonium nitrogen. However, the nitrogen of the soil source is more prone to nitrification to form nitrate. Although ¹⁵N-labeled Chinese fir litter was not considered in this experiment, compared to the effect of broad-leaved litter on soil, the biggest proportion of soil SON from Chinese fir litter may occur earlier and the proportion of soil ammonium nitrogen may be higher. These results suggested that soil SON is related to the litter decomposition dynamics and the effect of litter on soil.