Contribution of amino compounds to dissolved organic nitrogen in forest soils

Dissolved organic nitrogen (DON) may play an important role in plant nutrition and nitrogen fluxes in forest ecosystems. In spite of the apparent importance of DON, there is a paucity of information concerning its chemical composition. However, it is exactly this chemical characterization that is required to understand the importance of DON in ecosystem processes. The primary objective of this study was to characterize the distribution of free amino acids and hydrolyzable peptides/proteins in the DON fraction of Oa horizon leachates along an extreme edaphic gradient in northern California. In situ soil solutions were extracted by centrifugation from Oa horizons collected beneath Pinus muricata (Bishop pine) and Cupressus pygmaea (pygmy cypress) on slightly acidic/fertile and highly acidic/infertile sites. DON accounted for 77 to 99% of the total dissolved nitrogen in Oa horizon leachates. Nitrogen in free amino acids and alkyl amines ranged from 0.04–0.07 mg N/L on the low fertility site to 0.45–0.49 mg N/L on the high fertility site, and accounted for 1.5 to 10.6% of the DON fraction. Serine, glutamic acid, leucine, ornithine, alanine, aspartic acid and methylamine were generally the most abundant free amino compounds. Combined amino acids released by acid hydrolysis accounted for 48 to 74% of the DON, suggesting that proteins and peptides were the main contributor to DON in Oa horizon leachates. Together, nitrogen from free and combined amino compounds accounted for 59 to 78% of the DON. Most of the DON was found in the hydrophobic fraction, which suggests the presence of protein/peptide-polyphenol complexes or amino compounds associated with humic substances. Because free and combined amino acids can be an important nitrogen source for some plants, soil DON may play an important role in plant nutrition and ecosystem function.