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 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 N03 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.