Movement of nitrogen and phosphorus downslope and beneath a manure and organic waste composting site

This observational study was conducted to assess the movement of nitrate and phosphorus into and through the soil profile beneath a compacted gravel compost pad. The accumulation of nitrate and phosphorus in the vegetated filter strip immediately downslope of the pad was also evaluated. Soil samples were taken from the composting site and the immediate surrounding area in two transects each for the Control (outside of compost pad), Old Pad (combined manure stack and compost area), and Extension pad (compost only area). Each transect was divided into three sampling zones: within the pad, in an intermediate area between the pad and the filter strip, and within the filter strip. Compost samples from windrows of different ages and mixes were also taken for laboratory leaching test to determine the potential of the composts as source of nitrogen and phosphorus. The NO3-N concentrations in the soil beneath the compost pad of the Old Pad and Extension transects were higher than the soil NO3-N concentrations at the same depths and locations in the control transect. These results indicate that the compacted gravel pad did not fully prevent the downward movement and accumulation of NO3-N beneath the pad. The NO3-N concentrations in the soil surface of the pad, intermediate between the pad and filter strip, and the filter strip areas of the Control and Old Pad transects were not statistically different; suggesting that there was negligible NO3-N surface movement and transport from the pad area to the filter strip. The average Mehlich3-P concentration at the soil surface in the pad area was less than in the intermediate and the filter strip areas, indicating that there was surface runoff and downslope transport of phosphorus from the compost site to the filter strip. Statistical analysis showed that there was no significant difference in the Mehlich3-P concentration between the filter strip areas of the Old Pad and Control transects; suggesting that the downslope transport of phosphorus from the compost pad to the filter strip had not yet caused significant accumulation of phosphorus in the filter strip relative to the adjacent field. Leaching tests indicated that during the composting process, mature composts pose a greater potential as a source of NO3-N leaching than the freshly-mixed composts. In contrast, the composting process and operation poses a greater potential as a source of PO4-P during the early stages of composting than with older composts.