Environmental Effects of Biosolids Trenching for Reclaiming Mined Land for Hybrid Poplar Production

Entrenching of biosolids in mine reclamation sites for the production of energy crops offers a land application alternative. Researchers in Maryland have entrenched biosolids in sand & gravel mines at loading rates ≥ 384 Mg/ha for the production of hybrid poplar with no local groundwater impairment. The lack of nitrate-N leaching has been attributed to (1) the maintenance of nitrification rate-limiting, low redox conditions in the biosolids and (2) the presence of a fine-textured, semi-confining stratum immediately below the trench. We initiated a similar study in Virginia to determine whether we can use hybrid poplars to assimilate high concentrations of biosolids-applied nutrients with no detrimental impact of nitrogen, phosphorus, and heavy metal leaching during the reclamation of coarse-textured soils. In this report, we compare the effects of two rates each of an anaerobically digested and a lime-stabilized biosolids with an unfertilized control at the Iluka heavy mineral mine reclamation site in Dinwiddie and Sussex Counties. The site was prepared during the late spring of 2006 by digging trenches 45 cm wide × 75 cm deep and 90 cm wide × 75 cm deep with a backhoe to provide two volumetric rates for each biosolids treatment type. We instrumented all treatments with zero tension lysimeters below and suction lysimeters adjacent to the trenches. Each treatment consists of two rows (trenches) with a row spacing of 2.1 m or 2.55 m wide × 15 m long. Each trench was filled with either of the two types of biosolids in early summer 2006 and covered with approximately 30 cm of mine soil. Biosolids were sampled at time of application for analysis of its chemical composition and to calculate the actual constituent loading rates. Hybrid poplar cuttings were planted directly over the trench at a spacing of 3 m between plants for a total of 10 trees/plot in March 2007. Water is being sampled bi-monthly from lysimeters and analyzed for chemical constituents. Denitrification potential above each trench and redox potential within the trenches are being monitored monthly. Based on the initial sampling data following biosolids application, entrenchment of biosolids in very coarse-textured soils containing little organic matter poses environmental risks due to nitrogen leaching. Nitrogen loss occurred initially as ammonium N, but we expect nitrate N to increase as nitrification proceeds. The magnitude of the loss will depend on the capability of the poplar trees to assimilate nitrogen. Considerable amounts of N are also being lost vie denitrification. Phosphorus leaching was low, likely due to the high concentrations of P-binding iron, aluminum, and manganese constituents in the biosolids. Transport of heavy metals from the biosolids was no greater than from the control or low for most of the metals studied. Where detected, metals were largely transported in particulate phase.