The effects of irrigation and nitrogen application rates on yield of spring wheat (longfu-920), and water use efficiency and nitrate nitrogen accumulation in soil

In the middle reaches of Heihe River Basin, the environmental deterioration caused by excessive nitrogen (N) and irrigation application in agricultural ecosystems has received much attention in recent years. A combination of careful irrigation and N management is needed to improve N uptake efficiency and to minimize potential nitrate (NO3--N) leaching. A field experiment was designed to investigate the effects of different irrigation and N application rates on grain yield, water use efficiency (WUE) and soil nitrate-N (NO3--N) accumulation on a recently reclaimed sandy farmland at the margin of the Gobi Desert in 2006 and 2007. The experiment followed a completely randomized split-plot design, taking the various irrigation treatments (0.6, 0.8 and 1.0 of the estimated evapotranspiration-ET) in the main-plots and N supply treatments as split-plot (with five levels of 0, 79, 140, 221 and 300 kg N ha-1). The results indicated a wide annual variability in grain yield, kernel numbers and straw yield, mainly due to greater rainfall and irrigation rates in the second year. A significant irrigation effect was observed on grain yield, kernel numbers and straw yield. The highest levels were achieved with a high irrigation supply, although WUE generally decreased linearly with increasing seasonal irrigation rates in 2 years. The low irrigation treatment (0.6 ET) produced significantly lower grain yield (20.7 %), kernels number (9.3 %) and straw yield (12.2 %) than high irrigation treatment (1.0 ET). The low irrigation treatment had a higher WUE (4.25 kg ha-1 mm-1) than that of 3.25 kg ha-1 mm-1 with high irrigation over the 2 years. On contrary to the irrigation, the N application rate of 221 kg ha-1 had the highest values of grain yield, kernel numbers, straw yield and WUE under the 3 irrigation regimes. The average grain yield of 221 kg N ha-1 were found to be 99.1, 45.1, 20.0 and 7.4 % higher than those of 0, 79, 140 and 300 kg N ha-1, respectively over the 2 years. The 221 kg N ha-1 had the highest WUE (4.75 kg ha-1 mm-) among all N treatments. No more NO3 --N accumulation was found in the 200 cm soil profiles under the medium (0.8 ET) and low irrigation, when the N application was below 221 kg ha-1, at harvesting stage and there was a little potential for NO3--N leaching. High irrigation led to a high NO3--N leaching and a high plant N-uptake, resulting in lower NO3--N accumulation at the harvest stage compare to the sowing or soil preparation stage. This was within the 200 cm soil profiles and over the 2 wheat growing seasons. The N application rate of 221 kg N ha-1 produced a high grain yield of spring wheat, but the N application rate was lower than that of 221 kg N ha-1 as concerned NO3--N accumulation at harvesting stage under deficit irrigation. The optimum economic N rates ranged from 174 to 226 kg ha-1.