Calculating microbial carbon and nitrogen transformations in acid forest litter with 15N enrichment and dynamic simulation modelling

Microbial carbon and nitrogen dynamics were determined in litter from a Scots pine and a Douglas fir forest, using a combination of 15N enrichments and a dynamic simulation model. In these two forests N deposition in throughfall had been manipulated for 4 years. Sampling was from plots with about ambient N deposition (40 kg N ha−1 y−1) and from plots with N-deposition in throughfall being reduced to natural background levels ( < 2 kg N ha−1 y−1). The model, containing three pools of C and organic N (labile organic matter, refractory organic matter and microbial biomass) and two pools of inorganic nitrogen (NH4+ and NO3−), was used to calculate gross N transformations and microbial metabolic parameters. Chemical characterization of the litters indicated that differences in total element concentrations and KCl and H2O extractable nutrients between the high and low deposition plots were minimal. The model was able to calculate the gross transformation rates with a variance of generally less than 10%. The turnover rate of NH4+ was much lower in the ambient deposition plot of the Scots pine site than of the Douglas fir site. In addition, microbial C-to-N ratio and C use efficiency were lower and microbial turnover time was longer in the Scots pine site. The main part of the produced NO3− was due to the oxidation of NH4+. In the high deposition plots of both sites, a negligible part of total inorganic N was immobilized as NO3−. At both sites reduced N input resulted in increases of microbial biomass, NO3− immobilization, and nitrification via NH4+ and via organic N, whereas NH4+ immobilization per amount of microbial biomass decreased.