GULLY TREATMENT AND WATER-QUALITY IN THE SOUTHERN PLAINS

Erosion of agricultural land and transport of associated fertilizer ch emicals nitrogen (N) and phosphorus (P) in runoff, can be detrimental to both soil productivity and water quality. In the Southern Plains, g ully erosion is of concern due to periodically intense rainfall and a large acreage of erodible soils. As little information is available, w e studied the loss of sediment, N, and P in runoff over 13 yr (1980-19 92) from two adjacent extensively gullied native grass watersheds (3.8 and 5.7 ha of 5% slope and class 4 erosion) in the Little Washita Riv er Basin, OK. In 1983, the gullies on one of the watersheds were treat ed by land shaping, Midland Bermudagrass [Cynodon dactylon (L.) Pers] establishment, and construction of a runoff detention pond. Prior to g ully treatment, greater (p > 0.05) amounts of sediment, N, and P were lost from the subsequently treated than untreated watershed. Following gully treatment, 27,500 kg sediment, 7.1 kg N, and 4.1 kg P ha(-1) yr (-1) were lost from the gullied watershed, while only 4,900 kg sedimen t, 3.1 kg N, and 1.6 kg P ha(-1) yr(-1) were lost from the treated wat ershed. While gully treatment had no effect on nitrate-N and ammonium- N loss, dissolved P and bioavailable P lasses were increased six- and three-fold, respectively. This was attributed to the application of fe rtilizer N and P to the treated watershed only. The loss of N and P in runoff from gullied and treated watersheds was accurately predicted u sing kinetic and enrichment ratio approaches with soil properties refl ecting the main zone of runoff and soil interaction. Subsoil (5-20 nn) properties accurately predicted N and P release and transport in runo ff from the gullied watershed, whereas accurate predictions for kilt t reated watershed were obtained with surface soil (0-5 cm) properties. The cost of gully treatment was $1,098 ha(-1), with a reduced loss of 210 kg sediment, 5 g N, and 3 g P in the ensuing 10 years for every do llar spent on treatment.