Soil amino acid turnover dominates the nitrogen flux in permafrost-dominated taiga forest soils

Black spruce (Picea mariana) forests represent the dominant vegetation type throughout the North American and Siberian taiga and are generally considered ro be pristine, N-limiting environments. The aim of this sludy was to investigate the fundamental underlying mechanisms which control N availabil~ty in these soils with particuiar reference to the dynamics of dissolved organic nitrogen (DON) Our results showed that in these highly organic and low pH soils. soluble N is dominated by organic forms with correspondingly low concentrations of ammonium and nitrate. Amino acids, which are known to be directly taken up by plants growing in these soils, were calculated Lo constitute 10-20% of the total DON pool. The microb~al mineralization of amino acids was rapid in all surface horizons (L, 0 and Ah: I,,? = 5 h), conformed well to Michaelis-Menten kinetics and appeared to have a common mineralization pathway across all horizons and soils tested. The results indicated that the amino acid pool of soil solution is extremely transient, turning over approximately 20 Limes per day. From these results, we suggesl that the transformation of protein to amino acids and not amino acids to NH; is the major faclor limiting N availability in these soils. As DON turnover constitutes a large proportion of the annual N flux in these forests, i t warrants inclusion in models of climale change in high-latitude ecosystems. O 2002 Elsevier Science Ltd. All rights reserved Ke~words : Amino acids; Dissolved organic nillogen; Microbial kinetics; N mineralization: N flux: Permafrosl: Taiga ecosystems