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

Abstract

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 NO 3 − retention under ambient N enrichment gradient. Gross N mineralization, NH 4 + immobilization, gross nitrification, and NO 3 − immobilization rates increased up to intermediate N enrichment level and somewhat decreased at highly N‐enriched condition. The turnover rates of NH 4 + and microbial N pools increased while the turnover rates of the NO 3 − pool decreased across the N enrichment gradient. Abiotic immobilization of NH 4 + did not differ across sites and was lower than that of NO 3 − . Abiotic NO 3 − immobilization decreased across the N enrichment gradient. Microbial assimilation and turnover appeared to contribute largely to the retention of NH 4 + . The increasing NO 3 − deposition and decreasing turnover rates of the NO 3 − pool, combined with decreasing abiotic NO 3 − retention, possibly contributed to increasing NO 3 − 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.