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Keywords:

  • abiotic N immobilization;
  • acid forest soil;
  • gross N mineralization;
  • gross nitrification;
  • N enrichment continuum;
  • N oxide emissions;
  • 15N pool dilution;
  • NH4+ immobilization;
  • NO3 immobilization;
  • NO3 leaching

Abstract

A network of long-term monitoring sites on nitrogen (N) input and output of forests across Germany showed that a number of Germany's forests are subject to or are experiencing N saturation and that spruce (Picea abies) stands have high risk. Our study was aimed at (1) quantifying the changes in gross rates of microbial N cycling and retention processes in forest soils along an N enrichment gradient and (2) relating the changes in soil N dynamics to N losses. We selected spruce sites representing an N enrichment gradient (indicated by leaching : throughfall N ratios) ranging from 0.04–0.13 (low N),≤0.26 (intermediate N enrichment) to≥0.42 (highly N enriched). To our knowledge, our study is the first to report on mechanistic changes in gross rates of soil N cycling and abiotic NO3 retention under ambient N enrichment gradient. Gross N mineralization, NH4+ immobilization, gross nitrification, and NO3 immobilization rates increased up to intermediate N enrichment level and somewhat decreased at highly N-enriched condition. The turnover rates of NH4+ and microbial N pools increased while the turnover rates of the NO3 pool decreased across the N enrichment gradient. Abiotic immobilization of NH4+ did not differ across sites and was lower than that of NO3. Abiotic NO3 immobilization decreased across the N enrichment gradient. Microbial assimilation and turnover appeared to contribute largely to the retention of NH4+. The increasing NO3 deposition and decreasing turnover rates of the NO3 pool, combined with decreasing abiotic NO3 retention, possibly contributed to increasing NO3 leaching and gaseous emissions across the N enrichment gradient. The empirical relationships of changes in microbial N cycling across the N enrichment gradient may be integrated in models used to predict responses of forest ecosystems (e.g. spruce) to increasing N deposition.