LONG-TERM EFFECTS OF SEWAGE-SLUDGE AND INORGANIC FERTILIZERS ON NUTRIENT TURNOVER IN LITTER IN A COASTAL DOUGLAS-FIR FOREST

Rates of litter input, decomposition, net N mineralization, and N and P supply on the forest floor were measured in a 70-year-old stand of P seudotsuga menziesii (Mirb.) Franco on a poor site in western Washingt on, approximately 10 years after applications of sewage sludge and ino rganic fertilizers. Sewage sludge was applied to three plots at the ra te of 6000 kg N ha-1, and nitrogenous fertilizers were applied to two plots at rates of 1082 and 1568 kg N ha-1. The rate of each process in each treated plot was compared with that in an adjacent control plot. Amounts of N returned in needle litter during 1 year were greater on sludged and fertilized plots relative to adjacent control plots. Decom position of a standard needle litter was the same on all plots after a 2-year in situ incubation. Rates of net N mineralization during aerob ic incubations of forest floor material were similar in all plots in f ield incubations, but smaller rates were measured in material from slu dged plots in laboratory incubations. Greenhouse bioassays with Dougla s fir and Sitka spruce (Picea sitchensis (Bong.) Carr.) seedlings show ed greater P supply in forest floor material from sludged plots, but n o differences in N supply between any treatments. Turnover of N in lit ter was not different 10 years after fertilization of this forest with sewage sludge or N fertilizer. There was evidence of long-term enhanc ement of P turnover in litter in plots treated with sewage sludge.