Biomass production of hybrid poplar (Populus sp.) grown on deep-trenched municipal biosolids

Environmentally sustainable options for recycling municipal biosolids generated from wastewater treatment plants are needed. We measured the wood biomass production of a 50 ha hybrid poplar (Populus spp.) tree farm where biosolids were applied at a rate of 380 metric tonnes per hectare in deep trench rows (46 cm deep, 107 cm wide, and 20 cm below the surface) of a clay spoil at a former sand and gravel mine located near Washington, DC. Our aim was to quantify how much wood biomass was produced within a 6-year rotation and to develop allometric models useful for easily estimating wood biomass for this type of novel plantation. We randomly sampled trees from stands 2 to 6 years old to measure total tree height, diameter at breast height (DBH), and total tree weight. The 6-year-old trees had a mean height of 9.7 m and a mean biomass of 20.5 kg. DBH was the best allometric predictor of biomass (r2 = 0.98, P < 0.001), especially for trees with diameters greater than 4 cm. Tree height was a significant, but less precise predictor of biomass (r2 = 0.87, P < 0.001). Standing wood biomass after 6 years was 22,100 kg/ha. Ecological recycling of municipal biosolids to tree plantations can be an environmentally sustainable and energy conscientious means for producing energy, while restoring degraded habitat.