Soil nitrogen balance in winter wheat and summer maize rotation system on dryland of Loess Plateau

Field Experiments were carried out to study the soil nitrogen balance in winter wheat and summer maize rotation system under conventional fertilization condition on dryland in semiarid area,southern part of Loess Plateau.The selected N rates were 0 and 240 kg/ha for the winter wheat,and 0,120 and 240 kg/ha for the summer maize.The obtained results showed that mineral N(Nmin) residual in soil had significant effects on crop yield and applied N fertilizer efficiency.Residual soil Nmin after winter crop had obvious benefits on the growth of following summer maize,resulting in a lower use efficiency of N fertilizer applied in latter growing season.Although applying 240 kg/ha N during winter wheat growing season could increase crop yield and N uptake,the N use efficiency was only 39.7%,and the great mass of applied N fertilizer was left over in 0200 cm soil layer or lost from the soil.Due to the after effect of residual N,the apparent N recovery of summer maize was quite low being 22.4% and 4.6% for N loading with N 120 and 240 kg/ha,however,the residual N in the 0200cm soil layer accounted for 51.1% and 87.2% of applied N,respectively.Take the entire rotation duration into consideration,the mean N use efficiencies was 52.2%,42.2% and 28.0% at N rates being 240,360 and 480 kg/ha,respectively.As a result, the residual N was magnificently increased,correspondingly being 12.4%,25.3% and 49.8%.These implied that the application of N fertilizer during summer maize growing season mainly increased soil N residue at high N rates,and N loss could account for a great part at relatively low N rates,when the field had a great deal of residual Nmin in the soil profile.