CALCULATING MICROBIAL CARBON AND NITROGEN TRANSFORMATIONS IN ACID FOREST LITTER WITH N-15 ENRICHMENT AND DYNAMIC SIMULATION MODELING

Microbial carbon and nitrogen dynamics were determined in litter from a Scots pine and a Douglas fir forest, using a combination of N-15 enr ichment experiments and a dynamic simulation model. In these two fores ts N deposition in throughfall had been manipulated for 4 years. Sampl ing 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 n atural background levels (< 2 kg N ha(-1) y(-1)). The model, containin g three pools of C and organic N (labile organic matter, refractory or ganic matter and microbial biomass) and two pools of inorganic nitroge n (NH4+ and NO3-), was used to calculate gross N transformations and m icrobial metabolic parameters. Chemical characterization of the litter s indicated that differences in total element concentrations and KCl a nd H2O extractable nutrients between the high and low deposition plots were minimal. The model was able to calculate the gross transformatio n 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 pin e site than of the Douglas fir site. In addition, microbial C-to-N rat io and C use efficiency were lower and microbial turnover time was lon ger 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 bo th sites reduced N input resulted in increases of microbial biomass, N O3- immobilization, and nitrification via NH4+ and via organic N, wher eas NH4+ immobilization per amount of microbial biomass decreased. Cop yright (C) 1996 Elsevier Science Ltd.