Nitrogen Mineralization Following Fertilization of Douglas‐fir Forests with Urea in Western Washington

Nitrogen mineralization following repeated applications of urea fertilizer was determined in the A horizon soil from two stands of Douglas‐fir [ Pseudotsuga menziesii (Mirb.) Franco] in the Cascade Mountains of Washington. Repeated applications of urea at rates ranging from 0 to 600 kg N ha −1 were made at annual and 5‐yr intervals over a 6‐yr period. Nitrogen fertilization increased N mineralization potential in these soils. However, soil N mineralization followed a quadratic relationship with the total amount of N applied in fertilizer over the 6‐yr treatment period, increasing up to total application rates of 450 kg N ha −1 and then declining at higher rates. The decrease in N mineralization rates at the high N fertilization rates may be due to changes in the quality of soil organic matter, which reduced the effectiveness of extracellular enzymes and decreases the rate of decomposition and mineralization. Soil pH dropped following urea fertilization, with greater declines observed in the highest rates of urea fertilizer. Decreases in extractable Ca and Mg levels in the soil accompanied the decline in soil pH. These results suggest that high rates of nitrification occurred and that nitrate leaching was stripping Ca and Mg from the cation‐exchange complex in these soils. It appears that repeated applications of urea fertilizer at low to intermediate rates may increase long‐term N availability and thus improve soil quality. However, annual applications of high rates of urea may decrease soil quality because under these circumstances N mineralization did not increase and there was a loss of cations from the soil.