Nitrogen, Phosphorus and Carbon Transformation in Deep Row Biosolids Incorporation for Hybrid Poplar Production in Coastal Plain Mine Reclamation Sites in Virginia.

Our research was conducted to quantify the nitrogen and phosphorus transformations and carbon sequestration potential in deep row incorporation of biosolids for the production of hybrid poplar as a potential bioenergy crop.  The study was conducted on a mineral sands mine reclamation site near the Coastal Plain-Piedmont fall line in Dinwiddie County, Virginia.  The experimental design consisted of 5 treatments – two biosolids types each applied in subsurface trenches at two volumetric rates, and an unamended control.  Application rates were 328 and 656 Mg ha^-1 for the lime-stabilized material, and 213 and 426 Mg ha^-1 for the anaerobically digested biosolids.  Each treatment was replicated four times and arranged in a randomized complete block design.  During the years of 2006-2008 following study establishment, subsurface loss of nitrogen and phosphorus was determined via collection of leachate in zero tension lysimeters, nitrous oxide emission was detected by soil surface incubation chambers, and poplar growth was estimated using allometric functions.  Nitrogen, phosphorus and carbon content and forms were detected in the entrenched biosolids in October 2008 in two years of treatment application. The results showed significant mineralization and subsurface loss of total nitrogen (232 – 702 kg ha^-1) from the biosolids within two years after entrenchment, and also significant atmospheric emissions of nitrous oxide (35 mg N₂O m² hr^-1).  However no adverse loss of phosphorus was detected (0.5 kg P ha^-1).  Amounts of carbon and nitrogen sequestered by the hybrid poplars (75 kg N ha^-1 and 3.3 Mg C ha^-1) during 2 years of growth were lower than the corresponding losses to the groundwater and the atmosphere.  The deep row biosolids incorporation technology does not appear to be protective of environmental quality when the practice is applied to coarse-textured soils.