Changes in nitrogen cycling and retention processes in soils under spruce forests along a nitrogen enrichment gradient in Germany

A network of long-term monitoring sites on 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). Our study is the first to our knowledge 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 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 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.