Utilization of residual nitrogen by plants and its movement in different soil layers

High input of nitrogen fertilizer caused continuous nitrogen surplus in agro-ecosystem. There were large amount of residual NO-3--N in soil profile after crop harvest in irrigation agricultural system of North China. Nevertheless, the fate and supply potential of the residue NO-3--N for succession crops had been unclear. Field micro-plot experiment and technique of external injection of labeled 15NO-3--N (labeled-N) were used. The results showed that the recovery of labeled NO-3--N in 15 cm, 45 cm and 75 cm soil profile by spinach (Spinacia oleracea L.) were 28.2%, 22.3% and 16.3% respectively. Those by wheat (Triticum aestivum L.) Xiaoyan 54 in corresponding positions were 21.8%, 17.4% and 11.5%, while those by Jing 411 were 21.8%, 11.6% and 7.4% respectively, the recovery of labeled NO-3--N in different soil layers by spinach was significantly higher than that by wheat, furthermore, the recovery of labeled NO-3--N by Xiaoyan 54 were significantly higher as compared with Jing 411. The root length density of spinach was lower than that of wheat, and no significant difference of root length density was found in the two wheat varieties in most soil layers. The crop root length density in corresponding soil layers had significant positive correlation with the recovery of labeled nitrate-N; the dry root weight in treatments appeared the tendency with Xiaoyan 545＞Jing 411＞spinach in most soil layers. Labeled NO-3--N moved downwards at the end of growing season. The accumulated apices were appeared in soil layer with 60-80 cm、60-80 cm、80-100 cm respectively in threelabled experiments. But the%Ndff values of spinach at 15 cm position in the whole profile were all low and the lablednitrogen had scarcely no accumulated apices at the profile. At the same time, the residue nitrogen in different soil layers was all upward migration with 15 cm、45 cm and 35 cm respectively. The deeper of labeled residue nitrogen layer, the weaker of redisbution ability in soil profile and the residue nitrogen was more concentrated.