Effects of long-term N application and straw returning on N budget under wheatmaize rotation system

In a 4-year field experiment, effects of the longterm N application and straw returning on crop yield, NUE, residual Nmin and N budget are studied under wheatcorn rotation system in the Guanzhong Plain. The results show that the N application significantly increase the crop yields of winter wheat and summer maize with averages of 64.1% and 48.8% respectively, and the trends are increased at first and then decreased with N application rate increasing. The yields are not decreased in the 4-year successive harvests, though the nitrogen application rates are reduced by 27.3% for winter wheat and 55.6% for summer maize. The crop yields under the straw returning on are increased gradually with planting years. The accumulative NUEs of crops are increased with the increase of cropping rotation time and the NUEs in the four years are from 33.3% to 56.6%, which shows that the N fertilization has a carryover effect. The nitrogen application significantly enhances the residual Nmin in which the residual nitrateN is apparently larger than the residual ammoniumN. The residual nitrateN in 0-100 cm soil layer shows a positive relationship with the annual N fertilizer inputs. The straw returning shows no effects on mineral N residual in this experiment. The results of N budget show that the N accumulative uptakes by the crops are firstly increased and then declined, while the residual Nmin and apparent N loss are increased significantly. The straw returning has strong influences on nitrogen budgets of the rotation system, N accumulative uptakes by the crops, and the NUEs are increased by 13.0% and 26.2%, respectively. Both the apparent N loss and apparent N loss efficiency are decreased by 22.9%, while the apparent fertilizer N is kept in soil (Nmin residual) and its percentage in the straw returning treatment is not affected significantly compared with the treatment without straw returning.