Effect of initial soil moisture content on nitrate transportation in loessial soil under artificially simulated rainfall conditions

In this study，artificial laboratorial rainfall simulation was conducted under a range of initial soil moisture conditions in sandy loessial soils. The aim was to investigate the related processes such as infiltration-runoff, soil erosion, as well as nitrate loss by runoff and leaching. The results showed that there was a turning point between 49.4% and 76.9% of relative water content from the aspect of the effects of initial soil moisture on time to runoff．Commencement of runoff under higher soil water content condition was 15 minutes earlier than that under lower soil water content condition. The magnitude of soil erosion increased with soil water content，with the quantity of sediment under 97.1% of relative water content being 2.8, 2.3, and 1.5 times larger than that under 22.9%, 49.4% and 76.9% of relative water content, respectively. The higher nitrate concentration of initial runoff solution corresponded to the higher initial soil water content, and the nitrate concentration in runoff of the four treatments all dropped remarkably and about 10 minutes was close to the rainfall background nitrate concentration. When initial water content was lower, however, the extent of nitrate leaching was more serious, hence, nitrate migrated deeper in soil. From the experimental result we found that the nitrate loss from flow and sediment was less than that by deep migration. The results showed that nitrate was primarily lost through leaching under rainfall conditions，and the lower the initial soil water content, the more serious the nitrate losses. Precipitation characteristics of the Loess Plateau are low and concentrated, so adopting measures to increase infiltration, moisture accumulation, apply fertilizers under certain moisture content condition is of great significance for increasing nitrogen use efficiency, and decreasing nitrate leaching.