Content and bioavailability of organic forms of nitrogen in the O horizon of a podzol

It is often thought that the most important source of nitrogen for plants and microorganisms comes from amino acids and amino sugars when they are hydrolysed in acid conditions. We did a microcosm experiment to test the hypothesis. In the experiment spruce seedlings (Picea abies L. Karst) were grown for 145 days in soil taken from a podzol Oa horizon under a long‐term nitrogen fertilization experiment (control and N‐treated soil). Net changes in different pools of organic N were determined using standard fractionation (acid hydrolysis and pyrophosphate extraction). During the experiment the amino acid and amino sugar pools decreased significantly (14% and 15% for the control and 10% and 17% for the N treatment), whereas no significant change was observed in the non‐amino acid plus non‐amino sugar fraction. On a per organic C basis there was even a significant increase in the non‐amino acid plus non‐amino sugar fraction of 11% for the control and 8% for the N treatment. Pyrophosphate extractions suggest that amino acids or amino sugars associated with the humin fraction were more accessible to microbes and plants than those associated with the humic acid, fulvic acid and hydrophilic substances. The long‐term N fertilization (about 73 kg N ha⁻¹ was added annually as NH₄NO₃ during a 24‐year period) resulted in an enrichment of all major fractions of organic N, i.e. amino acids, amino sugars and non‐amino acids plus non‐amino sugars. This enrichment was largely the result of small increases in all of the amino acids rather than large increases in just a few.