Effects Of Fertilization On NO₃^--N Distribution And N Balance In Protected Soil

Crops in protected field possess the advantage of high yield, high fertilizer demand and so on. The phenomenon of excessive fertilization which lead to low N use efficiency is serious, and results in high accumulation of available N in actual production process which may cause N leaching easily, also lead to nitrate contamination of groundwater. Nitrogen losses and the pollution of environment due to irrational fertilization in vegetable production system has been becoming more and more concerned to the whole world. Aiming at this problem, a series of field experiments with Sweet Bell Pepper and Broccoli Romanesco were carried out in protected field in Beijing suburb. Four treatments were designed to study on the effects of crop yield, quality, N use efficiency, soil NO3--N distribution and apparent N balance of soil-crop system. These treatments were no fertilizer, manure only, conventional fertilization and optimized fertilization. The results showed as follows:(1) Fertilization increased economic yields of Sweet Bell Pepper and Broccoli Romanesco. Contrasted with no fertilizer, the yields of optimized fertilization and conventional fertilization enhanced significantly by 20.17% and 14.97% in Sweep Bell Pepper growing season, while 32.81% and 48.99% in Broccoli Romanesco growing season. Manure didn't enhance economic yield of Broccoli Romanesco significantly. Contrasted with conventional fertilization, optimized fertilization didn't reduce the yield of Sweet Bell Pepper significantly. In addition, optimized fertilization decreased fruits NO3--N content in both two vegetables to a certain degree. The edible parts NO3--N content in all treatments were much lower than the standard of primary food sanitation standard.(2) The results of effects of fertilization on the nitrogen utilization showed that: Nitrogen apparent use efficiency of optimized fertilization and conventional fertilization were 9.11% and 10.42% respectively and inorganic nitrogen fertilizer use efficiency were 5.59% and 3.02% in Sweep Bell Pepper growing season. Nitrogen apparent use efficiency and inorganic nitrogen fertilizer use efficiency of optimized fertilization were 40.21% and 36.39% which enhanced 20.04% and 19.37% contrasted with conventional fertilization respectively in Broccoli Romanesco growing season.(3) The nitrate distribution characteristics in the depth of 0~200 cm soil profiles were studied on Sweet Bell Pepper-Fallow-Broccoli Romanesco rotation system. Compared with the initial experiment, the soil NO3--N content of no fertilization, manure and optimized fertilization fell obviously, while the soil profiles NO3--N content in the depth of 40~60 cm, 80~100 cm, 120~140 cm of conventional fertilization accumulated the highest NO3--N content at the end of every growing season, the peak value were 77.98 mg/kg, 74.44 mg/kg and 65.51 mg/kg, respectively. At the end of the experiment, relative in initial experiment, soil NO3--N accumulation in conventional fertilization and organic manure treatment were lower at 0~160 cm soil profiles and higher in 160~200 cm soil profiles, the soil NO3--N content of conventional fertilization and manure did not higher than 20 mg/kg and without the peak of accumulated nitrate either. While accumulated nitrate peak in 120~140 cm soil profile was found in optimized fertilization and conventional fertilization in Broccoli Romanesco growing season, the peak value were 65.51 mg/kg and 28.03 mg/kg respectively. Compared with conventional fertilization, after the two crops'harvest, the NO3--N residue of optimized fertilization in the depth of 0~200 cm decreased 631.05kg/hm2 and 613.68 kg/hm2, respectively and both showed significant difference.(4) The results of the apparent nitrogen balance in soil vegetable system indicated: Compared with conventional fertilization, the apparent nitrogen balance of optimized fertilization in Sweet Bell Pepper and Broccoli Romanesco growing season were decreased 9.54 kg/hm2 and 169.69 kg/hm2, respectively. The apparent nitrogen losses of the four treatments varied from 28.26 kg/hm2 to 1261.14 kg/hm2 in the Sweet Bell Pepper-Fallow-Broccoli Romanesco rotation system. The apparent nitrogen balance of no fertilizer and manure were very low, while optimized significantly lower than that of conventional. The apparent nitrogen losses were in an order as follows: conventional fertilization> optimized fertilization>manure>no fertilizer.