Seasonal variations in a Douglas fir stand in total and soluble nitrogen in inner bark and root and in total and mineralizable nitrogen in soil

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Abstract
Monthly sampling of stem inner bark, roots, litter, humus, and mineral soil was carried out in six 0.1-ha plots in a 47-year-old Douglas fir (Pseudotsugamenziesii (Mirb.) Franco) stand for 1 year. Pairs of plots had received urea fertilization at the rates of 0, 224, and 448 kg nitrogen (N)/ha 4 years previously. Inner bark and roots were chemically analysed to determine concentrations of total N, soluble N, ninhydrin-positive compounds, monosubstituted guanidino amino compounds, arginine, and proline. Concentration of total N was determined in litter, humus, and mineral soil samples, and concentration of mineralizable N was also determined in humus and mineral soil.All sets of data showed an effect of season, but analysis of root soluble N, root guanidino compounds, root arginine, and litter total N were the most satisfactory for distinguishing differences in N level due to fertilizer treatments. For this purpose root sampling was best done in June and July, and litter sampling between January and May. Soluble N concentration was higher in inner bark than in roots, but arginine concentration was higher in roots than in inner bark. Root arginine concentration was high in winter and decreased to a low level in October before starting to accumulate again, suggesting it behaves as a N storage compound. Litter total N showed a decrease in concentration in July and August, followed by a recovery.

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