Abstract
Black spruce (Picea mariana) forests represent the dominant vegetation type throughout the North American and Siberian taiga and are generally considered to be pristine, N-limiting environments. The aim of this study was to investigate the fundamental underlying mechanisms which control N availability in these soils with particular 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 to constitute 10–20% of the total DON pool. The microbial mineralization of amino acids was rapid in all surface horizons (L, O and Ah; t1/2=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 times per day. From these results, we suggest that the transformation of protein to amino acids and not amino acids to NH4+ is the major factor limiting N availability in these soils. As DON turnover constitutes a large proportion of the annual N flux in these forests, it warrants inclusion in models of climate change in high-latitude ecosystems.
