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TECHNICAL REPORTS

Surface Water Quality

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

^a State Key Laboratory of Soil Erosion and Dryland Farming on Loess Plateau, Institute of Soil and Water Conservation, CAS and MWR, Northwestern Sci-Tech University of Agriculture and Forestry, 26 Xinong Road, Yangling, Shaanxi, 712100 China
^b USDA-ARS National Soil Erosion Research Laboratory, 275 South Russell Street, Purdue University, West Lafayette, IN 47907-2077

^* Corresponding author (flzh{at}ms.iswc.ac.cn)

Received for publication December 22, 2002. 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 NO₃–N and PO₄–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 NO₃–N and 0.11, 0.54, 0.91, and 0.72 mg L^–1 for PO₄–P, respectively. Similar trends were observed for the concentrations of NO₃–N and PO₄–P under the high fertilizer treatment. The total NO₃–N loss under the FD treatment was only 0.01% of the applied nitrogen, while under the Sp and Sp + R treatments, the total NO₃–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.

Abbreviations: C, control fertilizer treatment (no fertilizer input) • L, low fertilizer treatment (40 kg P ha^–1, 100 kg N ha^–1) • H, high fertilizer treatment (80 kg P ha^–1, 200 kg N ha^–1) • FD, free drainage hydraulic gradient • Sa, saturation hydraulic gradient • Sp, artesian seepage without rain hydraulic gradient • Sp + R, artesian seepage with rain hydraulic gradient

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