Comparison of simulated forest responses to biosolids applications

Organic matter and N are added to humus poets of the LINKAGES simulator of forest growth and N cycling at a range of application rates to investigate long-term effects of biosolids (sewage sludge) on forest productivity. Two conifer plantations (Douglas-fir [Pseudotsuga menziesii (Mirb.) France var. menziesii], loblolly pine [Pinus taeda LI) and a northern hardwood forest located in contrasting climatic regions are investigated. Single applicatio ns of biosolids are given at 0, 5, 10, 20, and 40 Mg/ha, and multiple appli cations are given on seven occasions st 3-yr intervals at rates of 5 and 10 Mg/ha. Highly significant increases in aboveground phytomass and net prima ry productivity of Douglas-fir plantations are obtained in a 100-yr simulat ion with increasing biosolids application rates. Results for loblolly pine from a 50-yr simulation produced about half the growth response of Douglas- fir, Long-term simulations of northern hardwoods shelved modest growth resp onses and small increases in NPP with added biosolids. The phytomass of one overstory and three understory species in the hardwood forest changed in r esponse to different biosolids applications and varying species sensitivity to N supply. Biosolids are a significant resource for enhancing forest pro ductivity, particularly in conifer plantations. Estimates of N leaching los ses from simulated forest sites combined with a literature review of leachi ng lasses suggest that biosolids applications at 3-yr intervals with rates less than 8.5 Mg/ha (0.4 Mg N/ha) during active forest growth may pose litt le offsite contamination risk to ground water or surface waters.