Nitrogen and Phosphorus in Runoff from Cattle Manure Compost Windrows of Different Maturities.

Manure composting has become commonplace in the beef cattle ( L.) feedlot industry in Alberta. However, the nitrogen (N) and phosphorus (P) characteristics of runoff from windrows subjected to heavy rainfall at different compost maturities are unknown. On Days 18, 26, 40, 54, 81, 109, and 224 of composting, a rainfall simulator generated runoff, which was collected in timed 5-L increments, creating the variable "time during runoff event" (TDRE). The volumetric runoff coefficient of windrows increased from 24% of incident rainfall on Day 0 to 69% by Day 90. Ammonium-nitrogen showed a significant maturity × TDRE interaction on Day 18, increasing from 46 mg L for the 0- to 5-L increment to 172 mg L for the 25- to 30-L increment, as did total dissolved phosphorus (TDP), increasing from 36 to 61 mg L. Nitrate-nitrogen had a runoff export coefficient of 19.5 mg m min on Day 224, which was significantly higher than 1.8 to 6.3 mg m min on Days 18 to 54. Across the 224-d composting period, compost NO-N concentration explained 87% of runoff NO-N, whereas compost water-soluble P explained 68% of runoff TDP. The occurrence and duration of rainfall events relative to the compost maturity spectrum has implications for the magnitude of N and P mobility and overall nutrient losses.