Nitrogen mineralization following fertilization of Douglas-fir forests with urea in western Washington

• Published in: Soil Science Society of America journal Vol. 68; no. 5; pp. 1720 - 1728
• Main Author: Fox, T.R
• Format: Journal Article
• Language: English
• Published: Madison Soil Science Society 01.09.2004; Soil Science Society of America; American Society of Agronomy
• Subjects: A horizons; Agronomy. Soil science and plant productions; application rate; Biological and medical sciences; Cascades Mountains; Chemical, physicochemical, biochemical and biological properties; coniferous forests; extractable cations; fertilizer application; Fertilizers; forest soils; Forestry; Forests; Fundamental and applied biological sciences. Psychology; General agronomy. Plant production; General forest ecology; Generalities. Production, biomass. Quality of wood and forest products. General forest ecology; mineralization; Minerals; montane forests; mountain soils; mountains; Nitrogen; Nitrogen fertilization; Nitrogen, phosphorus, potassium fertilizations; nutrient availability; Organic matter; Physics, chemistry, biochemistry and biology of agricultural and forest soils; Pseudotsuga menziesii; repeated nitrogen applications; soil chemical properties; soil pH; soil quality; Soil science; Soil-plant relationships. Soil fertility. Fertilization. Amendments; Trees; urea fertilizers; Washington; United States; North America; America; Cascade Mountain; Nitrogen fertilization; Organic matter; Soil quality; Inorganic fertilizer; Forest soil; Inorganic element; Temperate zone; Pseudotsuga menziesii; Mineralization; Softwood forest tree; Gymnospermae; Secondary forest; Soil science; Nitrogen fertilizer; West; Property of soil; Nitrogen; Experimental study; Urea; Biogeochemistry; Regional study; Forestry; Coniferales; Spermatophyta; Nitrogen Organic compounds
• ISSN: 0361-5995; 1435-0661
• DOI: 10.2136/sssaj2004.1720

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.