Abstract: Effect of addition of dissolved organic matter （DOM） on nitrification in fluvo-aquic soil was studied.The soil used in the experiment was collected from Suqian,Jiangsu.The experiment had three treatments different in DOM addition rate,220,440 and 880 mg·L^-1,respectively.Results show that addition of DOM inhibited nitrification to a certain extent.After 16 days of incubation,nearly 100% of NH⁺₄-N in CK had changed into NO^-₃-N.Compared with CK,the three treatments,DOC220,DOC440 and DOC880,decreased by 7.83%,13.60% and 19.12%,respectively in nitrification,and hence the decrease in NH⁺₄-N concentration in the treatments was much slower than in CK.The addition of DOM,however,increased nitrite accumulation significantly.In CK,NO^-₂-N concentration peaked on D 12 in incubation,up to 67.83(mg·L^-1),whereas in Treatment DOC220,DOC440 and DOC880,it did on D 12,14 and 16,and 21.17%,33.91% and 59.90% higher,respectively.The addition of DOM also decreased the production rate of NO^-₃-N.The maximum concentration of NO^-₃-N,143.61 mg·L^-1,in CK was found on D 16,while only 41.97,78.09 and 91.30 mg·L^-1 in Treatment DOC220,DOC440 and DOC880 respectively,much lower than in CK.In order to eliminate the effect of extraneous nitrogen brought in with the addition of DOM,comparison was made of effects of DOM addition and application of（NH₄）₂SO₄ as the sole nitrogen source on nitrification in soils with same initial nitrogen concentration.Results also show increase in NO^-₂-N accumulation and decrease in NO^-₃-N production,which indicate that the organic carbon and low molecular organic compounds in DOM affected the process of nitrification.The findings of the experiment implied a high risk of NO^-₂-N accumulation existed in environments high in organic matter content,which may have some bad effects on the health of the terrestrial and aquatic systems.

Key words: dissolved organic matter, nitrification, nitrite accumulation