TRANSFORMATION OF PHOSPHORUS AND NITROGEN IN DEEP ROW BIOSOLIDS INCORPORATION TECHNOLOGY IN COASTAL PLAIN MINING SITES IN VIRGINIA1

Deep row incorporation of biosolids is a unique alternative land application method that will prevent odor problems and may permit the application of considerably higher than currently permitted biosolids rates. The goal of our research is to assess environmental consequences of employing deep row incorporation of biosolids to restore productivity of mined land for the production of hybrid poplar as a potential bioenergy crop. Our objectives are to quantify the transformations of nitrogen and phosphorus applied to the soil as entrenched biosolids. The study is being conducted on a mineral sands mine reclamation site near the Coastal Plain-Piedmont fall line in Dinwiddie County, Virginia. The experimental design consists of 5 treatments - two biosolids types each applied in subsurface trenches at two rates and an unamended control. Application rates were 328 and 656 Mg ha-1 for the lime-stabilized biosolids 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. The site has been instrumented with suction and zero-tension lysimeters for collection and analysis of leachate from which were determined subsurface loss of nitrogen and phosphorus. Gas chambers have been used to collect soil air samples for accounting of denitrification rates from the entrenched biosolids. Redox potential was determined in the incorporated biosolids to describe the biosolids environment influencing nitrogen and phosphorus chemical transformations. During the first 10-14 months following biosolids application, initial nitrogen loss occurred largely as ammonium and organic N and, after 7-10 months, mostly as nitrate N. There was no significant leaching of phosphorus. Low redox potential in the biosolids' seam validated the occurrence of anaerobic conditions. Higher nitrous oxide emissions occurred from the anaerobically digested biosolids than from the lime stabilized biosolids. The deep row biosolids incorporation technology, when applied to coarse-textured soils, does not appear to be environmentally viable with respect to potential nitrogen loss to groundwater.