Amino acid, peptide and protein mineralization dynamics in a taiga forest soil

The availability of inorganic N has been shown to be one of the major factors limiting primary productivity in high latitude ecosystems. The factors regulating the rate of transformation of organic N to nitrate and ammonium, however, remain poorly understood. The aim of this study was to investigate the nature of the soluble N pool in forest soils and to determine the relative rate of inorganic N production from high and low molecular weight (MW) dissolved organic nitrogen (DON) compounds in black spruce forest soils. DON was found to be the dominant N form in soil solution, however, most of this DON was of high MW of which >75% remained unidentified. Free amino acids constituted less than 5% of the total DON pool. The concentration of NO3 − and NH4 + was low in all soils but significantly greater than the concentration of free amino acids. Incubations of low MW DON with soil indicated a rapid processing of amino acids, di- and tri-peptides to NH4 + followed by a slower transformation of the NH4 + pool to NO3 −. The rate of protein transformation to NH4 + was slower than for amino acids and peptides suggesting that the block in N mineralization in taiga forest soils is the transformation of high MW DON to low MW DON and not low MW DON to NH4 + or NH4 + to NO3 −. Calculated turnover rates of amino acid-derived C and N immobilized in the soil microbial biomass were similar with a half-life of approximately 30 d indicating congruent C and N mineralization.