FOREST FLOOR AND SOIL NUTRIENTS 5 YEARS AFTER UREA FERTILIZATION IN AGRAND FIR FOREST

Five years after thinning and fertilization with 350 kg ha(-1) of N as urea in an interior Pacific Northwest grand fir (Abies grandis (Dougl .) Forbes) forest, we measured concentrations of total carbon (C), nit rogen (N), phosphorus (P), sulfur (S), and potassium (K) in the forest floor and upper 30 cm of soil. Effect of fertilization and thinning t reatments on available N, P, S, and K in the soil was determined by bi oassay with barley as a test plant in a growth chamber experiment. Fie ld treatment combinations were: control; not thinned, fertilized (NT-F ); thinned, not fertilized (T-NF); and thinned, fertilized(T-F). Total N concentration of the forest floor was substantially and significant ly greater for the NT-F treatment than the other three treatments. Thi s finding supported results of other studies that the forest floor ret ained a substantial portion of applied N. We did not detect anp differ ences among thinned and fertilized treatments and the control for conc entrations of soil total C, N, P, K, or S. Also, availabilities of N, P, and K in soil from thinned and fertilized treatments were the same as the control. Thus, there was no detectable effect of N fertilizatio n on total or available N after 5 years. The salient feature of the bi oassay trial was reduced availability of soil S with the NT-F treatmen t compared to the control and the T-NF treatment. Availability of S wa s also lower for the TF treatment compared to T-NF treatment. Applicat ion of a large quantity of N at this site with extant low levels of so il S further depressed S availability. Nitrogen fertilization should b e supplemented with S in sites where soil S levels are comparable to t hose we observed (0.008%).