Runoff and water quality from inorganic fertilizer and erosion control compost treatments on roadway sideslopes

Dairy manure compost mixed with wood chips (designated as erosion control compost, ECC) was examined as an erosion control material for stabilization of steep slopes on highway construction sites. Eight 0.9 x 1.8 x 0.2 m plots were prepared with a clay loam soil on a custom-made steel bed. Paired plots were randomly assigned one of two treatments: ECC or inorganic fertilizer (IF). For ECC plots, the blend of dairy manure compost and woodchips was applied on top of the soil as a 50 mm layer (according to TxDOT specs) at a rate of 283 mt ha(-1), which resulted in nutrient application rates of 2002 kg N ha(-1), 1426 kg P2O5 ha(-1), and 3002 kg K2O ha(-1). For the IF plots, granular fertilizer was hand broadcast at TxDOT and Texas AgriLife Extension specified rates of 112 kg N ha(-1) as ammonium nitrate, 112 kg P2O5 ha(-1) as triple superphosphate, and 112 kg K2O ha(-1) as potassium chloride. Treatments were established 6 days prior to the first rainfall event, and all plots were seeded on the same day. The steel bed was set at a 3:1 slope, and treatments were subjected to simulated rainfall (88.9 mm h(-1)) under non-vegetated and vegetated conditions from start of the rainfall until a period of 30 min after initiation of runoff. Results showed that time to initiate runoff was greater for ECC-treated plots than IF plots under both non-vegetated and vegetated conditions. Prior to establishment of vegetation, the ECC treatment substantially reduced loss of solids and provided greater erosion control during simulated runoff. However, ECC produced 4 to 10 fold greater TKN, ortho P, and total P and K mass losses under both vegetated treatments due to longer rainfall exposure time and greater total volume of runoff compared to IF. High levels of nutrients in the runoff from organic materials used for erosion control may potentially impact water resources and thus counteract erosion control benefits.