Forest floor and soil nutrients five years after urea fertilization in a grand 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), nitrogen (N), phosphorus (P), sulfur (S), and potassium (K) in the forest floor and upper 30 cm of soil. Effect of fertilization and thinning treatments on available N, P, S, and K in the soil was determined by bioassay with barley as a test plant in a growth chamber experiment. Field 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 significantly greater for the NT-F treatment than the other three treatments. This finding supported results of other studies that the forest floor retained a substantial portion of applied N. We did not detect any differences among thinned and fertilized treatments and the control for concentrations 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 fertilization on total or available N after 5 years. The salient feature of the bioassay trial was reduced availability of soil S with the NT-F treatment compared to the control and the T-NF treatment. Availability of S was also lower for the TF treatment compared to T-NF treatment. Application of a large quantity of N at this site with extant low levels of soil S further depressed S availability. Nitrogen fertilization should be supplemented with S in sites where soil S levels are comparable to those we observed (0.008%)Tiedemann et al "Forest floor and soil nutrients five years after urea fertilization in a grand Fir forest." Northwest Science. 1998; 72(2): 88-9