Nutrient concentrations in runoff from different manure amended fields of the tropics under natural rainfall conditions

Abstract

Excessive nutrient losses from agricultural soils represent a major source of surface water contamination. In this study we quantified concentrations of total Kjeldahl nitrogen (TKN), dissolved and total phosphorus (DP,TP), and dissolved organic carbon (DOC) in runoff from two animal farm operations of Puerto Rico. The farms, one dairy and one poultry, represented typical conditions (i.e., topography, ecological zone, management system) of these production systems.Two fields were selected for the runoff studies on each farm, and two runoff collectors were installed in each field. Runoff samples were collected under natural rainfall conditions. The average phosphorus concentration in runoff from the poultry fields (5.87 mg TP/L, 4.82 mg DP/L) was significantly greater than that observed from the dairy fields (2.29 mg TP/L, 1.79 mg DP/L). Dissolved phosphorus concentrations represented more than 90% of the total P concentrations on both farms, a situation that may exacerbate the impact on receiving water bodies. Average DP concentrations exceeded 1 mg/L, a limit proposed for the regulation of runoff P concentrations from agricultural lands, in 70% of the runoff events at the dairy farm, and 100% of the events at the poultry farm. The magnitude of the nutrient concentrations on both farms was significantly affected by the time lapse between the manure applications and the first precipitation event. Nutrient concentrations in runoff samples were also significantly affected by rainfall depth.