Effects of Near‐Surface Hydraulic Gradients on Nitrate and Phosphorus Losses in Surface Runoff

ABSTRACT Phosphorous (P) and nitrogen (N) in runoff from agricultural fields are key components of nonpoint‐source pollution and can accelerate eutrophication of surface waters. A laboratory study was designed to evaluate effects of near‐surface hydraulic gradients on P and N losses in surface runoff from soil pans at 5% slope under simulated rainfall. Experimental treatments included three rates of fertilizer input (control [no fertilizer input], low [40 kg P ha −1 , 100 kg N ha −1 ], and high [80 kg P ha −1 , 200 kg N ha −1 ]) and four near‐surface hydraulic gradients (free drainage [FD], saturation [Sa], artesian seepage without rain [Sp], and artesian seepage with rain [Sp + R]). Simulated rainfall of 50 mm h −1 was applied for 90 min. The results showed that near‐surface hydraulic gradients have dramatic effects on NO 3 –N and PO 4 –P losses and runoff water quality. Under the low fertilizer treatment, the average concentrations in surface runoff from FD, Sa, Sp, and Sp + R were 0.08, 2.20, 529.5, and 71.8 mg L −1 for NO 3 –N and 0.11, 0.54, 0.91, and 0.72 mg L −1 for PO 4 –P, respectively. Similar trends were observed for the concentrations of NO 3 –N and PO 4 –P under the high fertilizer treatment. The total NO 3 –N loss under the FD treatment was only 0.01% of the applied nitrogen, while under the Sp and Sp + R treatments, the total NO 3 –N loss was 11 to 16% of the applied nitrogen. These results show that artesian seepage could make a significant contribution to water quality problems.