Results
3.1. Temporal and spatial distribution of soil NO3− N content
NO3− N content in the 0–60 cm soil layer differed at different growth stages (Fig. 1).
It increased significantly as the basal N rate increased at overwintering stage, which showed that NO3− N had leached to the 60 cm soil layer.
As more N fertilizer was applied, more NO3− N leached. In the 0–40 cm soillayer, NO3− N was significantly lower at the regreening stage thanat the overwintering stage; there was an opposite tendency in the 40–60 cm soil layer, reflecting the continuous movement of NO3− N downward and that NO3− N in the upper soil would continue to decrease.
At the jointing stage, soil NO3 − N content in all soil layers decreased, possibly owing to plant uptake and leached NO3
− N.
A topdressing N application in the jointing stage increased NO3− N content in the 0–20 cm soil layer in the treatments
N120 and N210(7/3), in the 0–40 cm soil layer in treatments N210(3/7) and N210(5/5), and in the 0–60 cm soil layer in treatment N300 at booting stage. This further confirmed that as high N rates were applied, NO3− N leaching increased. After booting,the NO3− N content in the 0–20 cm soil layer decreased with plant development stage, whereas that in the 20–60 cm soil layer at maturity did not differ from the anthesis stage, demonstrating that NO3− N in upper soil moved down during the late growth periods.