Appropriate nitrogen fertilizer amount increases use efficiency of residual nitrate in vegetable field and raises spinach yield

Abstract: Nitrogen fertilizer is often largely used for vegetative production and thus the high amount of nitrate is left in soils. It is important to understand the utilization rate of the residual fertilizer for lowering its negative influence on ecosystem since the nitrate is easy to leach with rainfall and irrigation water. The purpose of this study was to investigate the effects of residual nitrate in topsoil layer on spinach yield and its utilization rate and nitrogen fertilizer use efficiency. The experiment was conducted in the Experimental station of Anhui Agricultural University in 2007. The experiment design included 4 nitrogen fertilizer application rates (no fertilizer N0, traditional fertilizer amount N1, 70% of the traditional fertilizer amount N2, and 40% of traditional fertilizer amount N3) and 3 15N labeled NO3-N treatments (no 15N, 15N in 15 cm depth, and 15N in 30 cm depth) with 3 replicates for each treatment. The labeled NO3-N was injected in 15 cm and 30 cm soil depth of the micro-plots of each nitrogen fertilizaer treatment plot. T Soil samples were collected in 0-60 cm depth every 10 cm at 5 and 10 cm away from the labeled point after spinach harvest. The results showed that compared with the N0, the spinach yield was not significantly different from that in the traditional amount treatment N1 when the nitrogen fertilizer application amount was decreased to 40% traditional amount (p>0.05). Under different nitrogen fertilization treatment, the residual 15N in soil profile moved downward but with the moving depth for the 30 cm labeled 15N treatment higher than that for the 15 cm labeled 15N treatment (p<0.05). The 15N abundance was higher (p<0.05) at the 5 cm horizontal distance away from the labeling points than that at the 10 cm horizontal distance away from the labeling points. As the nitrogen fertilizer amount increased, the residual nitrogen utilization rate decreased. In the 15 cm labeled 15N plots, the residual nitrogen utilization rate in the N3 treatment was as 2.1 and 1.3 times as that in the N1 and N2 treatment. In the 30 cm labeled 15N plots, the residual nitrogen utilization rate in the N3 treatment was as 3.8 and 2.2 times as that in the N1 and N2 treatment. The utilization rate of in-season nitrogen fertilizer decreased with increasing nitrogen fertilizer amounts. When the nitrogen amount was decreased to 40% traditional nitrogen amount, the utilization rate of in-season nitrogen fertilizer could reach 45% (15 cm labeled 15N treatment) and 43.8% (30 cm labeled 15N treatment). This study suggested the nitrogen fertilizer reduced to 40% traditional nitrogen amount can greatly improve the utilization rate of redisual nitrogen in soil and the in-season fertilizer efficiency while keeping the spinach yield. The results can help with reasonable application of nitrogen fertilizer in vegetables.