Root respiration of Douglas-fir seedlings: effects of N concentration

Six-month-old Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings were grown at three N concentrations and with controlled root temperatures. Measurements of root respiration were conducted on undisturbed root systems by passing humidified air with 1000 μl l−1 CO2 through root boxes onto an infrared gas analyzer. The objective was to understand the effects of N on soil respiration by examining total root respiration rate per seedling, specific root respiration rate g−1 root dry wt, and root dry wt after N fertilization. Total respiration rates of seedlings grown at 50 mg l−1 N concentration were significantly higher than those grown at 10 mg l−1 or 200 mg l−1 N concentration. Seedlings grown at N concentration of 200 mg l−1 had significantly smaller roots than those grown at the two lower N concentrations. The specific respiration rate increased as N concentration was increased from 10 to 50 mg l−1 N concentration, but remained constant as N was further increased from 50 to 200 mg l−1. The increase of total respiration rate with the increase in N concentration from 10 to 50 mg l−1 was attributed to the increase in specific respiration, whereas the subsequent decrease in total respiration with the increase in N concentration from 50 to 200 mg l−1 was attributed to the decrease in root dry wt. The depression of soil respiration after the addition of N fertilizers to relatively fertile soil may be explained by reduced root and mycorrhizal mycelial growth.