Wastewater and Sludge Nutrient Utilization in Forest Ecosystems
Although forest ecosystems have evolved efficient mechanisms to assimilate and retain modest levels of annual geochemical input, their productivity is frequently limited by low levels of available nutrients. A review of research studies conducted in the major U.S. forest regions indicates that the nutrients and organic matter in wastewater and sludge representa resource of substantial potential benefit to augment site nutrient capital and ameliorate the environment for plant growth. Wastewater irrigation provides phosphorus that is strongly held in upper layers of mineral soil and cations (potassium, calcium, sodium) that accumulate but are subject to loss with leaching anions (sulfate, nitrate, chloride) during periods of groundwater recharge. Applied nitrogen that is not lost to the atmosphere by volatilization or denitification accelerates forest floor decomposition, accumulates in soil in association with organic matter, is taken up by plants, or, following nitrification, is leached as nitrate to groundwater. Nitrogen utilization is greatest in young poplar forests growing in association with understory vegetation. Sludge applications provide phosphorus, potassium, and calcium that largely remain in the forest floor and upper layers of mineral soil. Calcium and potassium are subject to loss by leaching with anions (principally nitrate) during recharge periods. Applied nitrogen that is not lost through volatilization or denitrification is initially stored in the forest floor, where the resulting decrease in C:N ratio accelerates the decomposition of organic matter, and eventually the nitrogen is assimilated by vegetation or leached to groundwater as nitrate. Nitrogen uptake is highest on sites occupied by young poplar (200 to 400 kg/ha-yr) and somewhat less inmiddle-aged stands of Douglas-fir (90 kg/ha.yr) and loblolly pine (105 kg/ha.yr). Nitrogen application rates of 400 to 500 kg/ha have been associated with tree growth increases of up to 40% without producing soil leachateconcentrations of nitrate that exceed the 10mg/l U.S. EPA standard. Repeated sludge applications should provide a cumulative positive effect on forest site quality that could lead to permanent increases in productivity.
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