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Utilizing High Application Rates of Biosolids and Hybrid Poplar Trees on Surface and Deep-Row Applications: Case Studies from in Maryland and British Columbia

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The development of beneficial reuse systems that utilize biosolid applications to grow short-rotation hybrid poplar plantations is becoming operational in many regions. The use of this technique on mine spoils or other low fertility sites has the environmental benefit of obtaining needed land reclamation and providing environmental benefits. Most biosolid – hybrid poplar systems utilize systematic applications of biosolids every few years at agricultural rates to provide a regular supply of nutrients throughout the rotation. This type of regular surface application provides logistical challenges in application and soil disturbance, can be cost inefficient with repeated stand entries, and may generate potential concerns with odor. Higher rate, single applications, on low fertility soils and reclamations sites are possible, provided they are environmentally safe and do not result in adverse environmental impacts. An understanding the factors affecting the limits of application will allow for the development of new application technologies and approaches.

This presentation will discuss two case studies that will help to illuminate the use of knowledge in the factors that regulate the rate of release of nutrients from biosolid applications to hybrid poplar plantations in two specific circumstances. The first is in Vancouver, British Columbia, and involves applications of up to 4000 kg N/ha to an established plantation on a sand island in the middle of the salmon-bearing Fraser River. Poorly developed sandy soils and a water table within a few meters of the surface present concerns regarding this site to receive such high rates of application. Monitoring nitrogen leaching, denitrification, ammonia volatilization, soil immobilization and mineralization rates indicate that even at this high rate, nitrogen leaching was all but absent except for an initial spike within the first few months after application – similar in magnitude to application rates considerably less.

The second case study is from Maryland and utilizes deep-row application of biosolids, followed by the planting of hybrid poplar trees to utilize the buried biosolids on a six-year rotation. The result has been the successful reclamation of a 93-acre gravel spoil in Prince George's County, Maryland, evidenced by the production of forest products, improvement of wildlife habitat, and no degradation of water quality. The deep row technique involves placing biosolids (approximately 20 percent solids) at a rate of at least 1000 kg N/ha in deeprows that are 76 centimeters deep and 107 centimeters wide, and dug 2.4 meters apart. The deep row is filled with biosolids, covered with about 30 centimeters of overburden, and then planted with hybrid poplar cuttings. The roots quickly colonize around the deep rows and appear to uptake nitrogen and other nutrients as they are mineralized, eliminating off-site movement. When mineralization and foliar leaf samples are at appropriate levels, (after about six years) the trees are harvested, biosolids are reapplied, and trees are replanted. Since 1983, there has been no impairment of water quality from the site.

Matching the application rate and capability of hybrid poplar to utilize nitrogen is a great opportunity on a dwindling land base near large biosolid-producing metropolitan areas. These case studies will address some of the factors and variables that will allow a better understanding of how higher application rates can be utilized in biosolid-hybrid poplar production systems.
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Document Type: Research Article

Publication date: 01 January 2001

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