Douglas-fir soil C and N properties a decade after termination of urea fertilization

Abstract

Chemical and microbial soil properties were assessed in paired unfertilized and urea fertilized (>89 g N·m -2 ) plots in 13 second-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stands distributed throughout western Washington and Oregon. A decade following the termination of fertilization, fertilized plots averaged 28% higher total N in the O layer than unfertilized plots, 24% higher total N in surface (0-5 cm) mineral soil, and up to four times the amount of extractable ammonium and nitrate. Decreased pH (0.2 pH units) caused by fertilization may have been due to nitrification or enhanced cation uptake. In some soil layers, fertilization decreased cellulase activity and soil respira - tion but increased wood decomposition. There was no effect of fertilization on concentrations of light and heavy frac - tions, labile carbohydrates, and phosphatase and xylanase activities. No increase in soil organic C was detected, although variability precluded observing an increase of less than ∼15%. Lack of a regionwide fertilization influence on soil organic C contrasts with several site-specific forest and agricultural studies that have shown C increases resulting from fertilization. Overall, the results indicate a substantial residual influence on soi l N a decade after urea fertilization but much more limited influence on soil C processes and pools. Resume : Les proprietes chimiques et microbiennes du sol ont ete evaluees dans des paires de parcelles non fertilisees