Drip Irrigation Reduced Fertilizer Nitrogen Loss from Lettuce Field—A Case Study Based on ¹⁵N Tracing Technique

Nitrogen losses under different irrigation modes have been evaluated by many studies, yet it is not very clear whether the lost N sources are from the soil or fertilizer. In order to quantitatively investigate the effects of different irrigation modes on fertilizer N loss, we used the 15N-labeledurea (15N abundance of 19.6%) as fertilizer and the lettuce (Lactuca sativa var. angustana iris) as the plant material to conduct a field experiment under three different lower limits of drip irrigation, including 75% (DR1), 65% (DR2) and 55% (DR3), accounting for the field water capacity. A furrow irrigation treatment (FI) with the same irrigation regime as DR2 was used as the control. The fate and balance of 15N under these treatments were studied. The results showed that, after the lettuce harvest, 36.9–48.8% of the applied fertilizer 15N remained in 0–80-cm soil, 32.6–39.4% was absorbed by plants, and 18.6–26.3% was lost via pathways such as volatilization or leaching. Under the same irrigation regime, 15N loss caused by FI (26.3%) was significantly (p 15N, mineral 15N and organic 15N in the deeper soil layers (60 cm depth and below), leading to a potential risk of 15N leaching. The soil 15N residue was relatively lower under DR1, while the crop-absorbed 15N or 15N loss was atthe highest level among the three drip irrigation treatments. The correlation analysis results showed that increasing the total irrigation amount or increasing the irrigation frequency might increase the 15N loss. We concluded that using drip irrigation instead of furrow irrigation with controlling the lower irrigation limit at 65% is conducive to improving crop 15N utilization and reducing 15N loss from lettuce fields