Environmental and agronomic implications of water table and nitrogen fertilization management

Nitrate (NO⁻₃) pollution of surface and subsurface waters has become a major problem in agricultural ecosystems. Field trials were conducted from 1996 to 1998 at St-Emmanuel, Quebec, Canada, to investigate the combined effects of water table management (WTM) and nitrogen (N) fertilization on soil NO⁻₃ level, denitrification rate, and corn (Zea mays L.) grain yield. Treatments consisted of a combination of two water table treatments: free drainage (FD) with open drains at a 1.0-m depth from the soil surface and subirrigation (SI) with a design water table of 0.6 m below the soil surface, and two N fertilizer (ammonium nitrate) rates: 120 kg N ha⁻¹ (N₁₂₀) and 200 kg N ha⁻¹ (N₂₀₀). Compared with FD, SI reduced NO⁻₃-N concentrations in the soil profile by 37% in spring 1997 and 2% in spring 1998; and by 45% in fall 1997 and 19% in fall 1998 (1 mg NO⁻₃-N equals approximately 4.43 mg NO⁻₃ L⁻¹). The higher rate of N fertilization resulted in greater levels of NO⁻₃-N in the soil solution. Denitrification rates were higher in SI than in FD plots, but were unaffected by N rate. The N₂₀₀ rate produced higher yields than N₁₂₀ in 1996 and 1997, but not 1998. Corn yields in SI plots were 7% higher than FD plots in 1996 and 3% higher in 1997, but 25% lower in 1998 because the SI system was unable to drain the unusually heavy June rains, resulting in waterlogging. These findings suggest that SI can be used as an economical means of reducing NO⁻₃ pollution without compromising crop yields during normal growing seasons