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Nutrient Cycling in an Agricultural Watershed: II. Streamflow and Artificial Drainage¹

ABSTRACT

Streamflow is an important means of nutrient export off most watersheds. In agricultural areas, effluent from artificial drainage can be an important contribution to streamflow nutrient loads. The objective of this study was to examine seasonal variations of nutrients in streamflow and to quantify the effects of artificial drainage effluent on streamflow nutrients on an agricultural watershed. Concentrations and loads of NO₃-N, NH₄-N, organic N, dissolved molybdate-reactive P, total P, Ca, Mg, K, Cl, and SO₄-S were measured in streamflow and artificial drainflow on a 1568-ha agricultural watershed near Tifton, Ga. All 10 streamflow sampling sites had maximum NO₃-N and SO₄-S concentrations in winter and most reached minima during summer. Concentrations of NO₃-N, Ca, Mg, K, and Cl were generally much higher in drainage water than streamflow. Load of NO₃-N from drained row-cropped fields was about 40.4 kg ha^–1 y^–1, 60 times the per-hectare NO₃-N load from the mixed cover watershed. Loads of organic N, Ca, Mg, K, SO⁴-S, and Cl from the mixed cover watershed were 2.8, 9.3, 4.5, 6.5, 17.0, and 30.0 kg ha^–1 y^–1, respectively. Loads (per hectare) of organic N were lower from the drained fields, but loads of Ca, Mg, K, SO₄-S, and Cl were lower from the mixed cover watershed. In-stream and riparian zone processes apparently converted inorganic N to organic forms and removed N through denitrification. Annual streamflow N load was about 29% of the precipitation input of 12.2 kg ha^–1 y^–1. Sediment-associated N and P loads in streamflow were 0.35 and 0.09 kg ha^–1 y^–1, respectively, accounting for 9.0 and 8.5% of total N and P, respectively. The two highest flow events for the year carried 19% of the total annual flow and 19% of the annual sediment load, but had 30 and 27% of the total annual sediment N and P loads, respectively. This enrichment of sediment-associated N and P indicates that part of this sediment load moved directly from upland areas.

Key Words: subsurface runoff • nitrogen • calcium • magnesium • potassium • phosphorus • sulfate • riparian ecosystems

¹ Research supported by Natl. Sci. Foundation grants DEB 78-10841 and DEB 82-07210 to the Univ. of Georgia. Additional support from USDA-ARS, Southeast Watershed Res. Lab., Tifton, GA 31793; and Inst. of Ecology and Dep. of Agron., Univ. of Georgia, Athens, GA 30602.

² Former Research Associate, Inst. of Ecology, presently Ecologist, USDA-ARS, Southeast Watershed Res. Lab., former Associate Professor of Agronomy and Associate Ecologist, Univ. of Georgia, presently Professor of Microbiology, South Dakota State Univ., Brookings, SD 57007, and Laboratory Director, USDA-ARS, Southeast Watershed Res. Lab., respectively.

Received for publication February 14, 1983.