Effects of groundwater fluctuation on nitrate nitrogen transport after nitrogen application in cropland soil

Understanding the effects of groundwater fluctuations on nitrate nitrogen (NO₃^--N) transport in the soil vertical profile is important for reducing nitrogen (N) leaching and nitrate pollution in croplands with shallow groundwater. This study investigated effects of groundwater fluctuations and nitrogen application rate in a greenhouse cabbage with large soil column experimental devices. Two groundwater fluctuation levels (W0, stable; W1, 20 cm fluctuation per 10 days) and three levels of nitrogen application (N0, 0 kg(N)·hm^-2; N1, 225 kg(N)·hm^-2; N2, 450 kg(N)·hm^-2) were tested to determine water contents and NO₃^--N concentration changes at different soil depths, NO₃^--N concentration in groundwater, and crop yield. The results showed that fluctuating groundwater affected NO₃^--N transport depending on the soil depth and nitrogen application rate. At 0–20 cm depth (part of the vadose zone), excessive N application led to NO₃^--N accumulation in soil, but there was no correlation with groundwater fluctuation. At 20–60 cm depth (groundwater fluctuation zone), increased N fertilizer increased the soil NO₃^--N, and groundwater level changes promoted NO₃^--N migration into deeper soil, increasing the NO₃^--N groundwater pollution risk (especially in soil with already high NO₃^--N concentrations). At 60–80 cm depth (the flooding area), there was less NO₃^--N in soil, mainly due to denitrification. Crop yield did not significantly correlate with groundwater level changes. Groundwater fluctuations affected = NO₃^--N transport and should be considered in agricultural areas with shallow groundwater levels to prevent and control nitrogen pollution.