Abstract: A small-scaled in-situ field experiment was carried out to study dynamics of nitrogen in farmland originating draninage water retained retained in draninage dithches and in sediments and plants in the ditches as well.Results show that the longer the water is retained in the ditch,the more the nitrogen in the water is sequestrated by the ditch,if the water is retained in the drainage ditch for 8 d or so as recommended.The sequestration rate of NH₄⁺-N was higher than that TN and NO₃^--N.When the water was retained in the ditches for 11 d, NH₄⁺-Nsequestration rate reached 100% in all the four ditches with drainage water different in N and P concentration,i.e.N_HP_L(high N concentration and low P concentration),N_HP_H(high N concentration and high P concentration),N_LP_L(low N concentration and low P concentration)and N_LP_H(low N concentration and high P concentration),whereas NO₃^--N sequestration rate was 84.63%,84.35%,75.67% and 76.14%,and TN sequestration rate 88.02%,89.89%,90.88% and 88.53%,respectively.At the end of the experiment,it was found that N content in the surface layre(0-15 cm)of sediments in the dithches decreased and N accumulation in the plants growing in the ditches was much higher than TN in incoming water,suggesting that the plants absorbed N from the waterbody,soil and sediments at the same time for growth.It is,therefore,recommended that the plants in the ditches should be harvested duly in fall to avoid secondary pollution of N from plant decomposition.

Key words: drainage ditch, nitrogen, agricultural nonpoint source pollution, the Sanjiang Plain