Abstract: Abstract: Soil soluble organic nitrogen (SSON) is labile and plays an important role in the nitrogen transformation. Along-term experiment started in Shouguang, Shandong Province in 2004.Since 2011 we have begun the study about soil soluble organic nitrogen. The optional solar greenhouse built in 1999hadfive years continuous tomato planting prior to the trial, with double tomato cropping system (Winter-spring: February-June, Autumn-Winter: August-January next year). Tomato varieties were chosen by farmers with a planting density about 35000 plants/hm2. There four treatment groups for the test: none nitrogen (NN), organic manure nitrogen (MN), traditional nitrogen (CN), traditional nitrogen + wheat straw (CN+S), optimization nitrogen (SN), optimization nitrogen + wheat straw (SN+S).Alkaline persulfate oxidation - UV spectrophotometric method was selected to determine the soluble organic nitrogen in the soil and conventional method to soil inorganic nitrogen and otherindicators.The0-30 cm soil samples were taken at the end of each month and 0-180 cm of soil samples were taken at the end of every season after harvest. The results show that the surface soil soluble organic nitrogen ranges from 8.2 to 24.0 mg/kg, 15.0 to 55.3 mg/kg, 20.0 to 86.5 mg/kg, 40.7 to 101.9 mg/kg for NN, MN, CN+S and SN+S, respectively. The average soluble nitrogen levels for the four test groups are 16.4, 34.2, 47.4, 62.7 mg/kg The soluble nitrogen level in no nitrogen treatment group was significantly lower than the other three treatments. Compare to the no fertilization treatment, application of manure alone or combined with mineral nitrogen fertilizer significantly increased soil soluble organic nitrogen content, and a corresponding with the increase of mineral nitrogen fertilizer inputs. Organic fertilizer treatment increased SSON in the 0-180 cm by 1132 kg/hm2 than non-fertilizer treatments. The 0-180 cm soil layer SSON contents are in a order of CN>SN >MN > NN. With the traditional nitrogen treatments, the 0-180 cm soil layer SSON contents 69.0 mg/kg, 44% higher than NN. The 0-180 cm soluble organic nitrogen content in the range of 29.1 to 88.9 mg/kg comprises 27% to 50% of the total soluble total nitrogen. Compared with no fertilizer, organic fertilizer and nitrogen fertilizer significantly increased soil soluble organic nitrogen content and a corresponding with the increase in soluble organic nitrogen content of fertilizer nitrogen inputs. The accumulated SSON in the CN treatment significantly increased by 1505 kg/hm2. There is no significant difference for soil inorganic nitrogen accumulation from 0-90 cm and 90-180 cm. The cumulative amount SSON in90-180 cm soil layer reached 371, 1062, 1031 and 1121 kg/hm2 for the four test group accounting for 44%, 42%, 45% and 43% of SSON in 0-180 cm soil layer. Straw application significantly reduced the cumulative amount of soil inorganic nitrogen, but has no effect on soluble organic nitrogen. In general, soluble organic nitrogen content ranged between 7.8 to 88.9 mg/kg and for fertilization treatments in 90-180 cm soil layer SSON was up to 962.8 kg/hm2. Thus the soluble organic nitrogen is a major path of nitrogen loss in greenhouse. More attention is needed in the study of nitrogen fate.