Impacts of nitrogen deposition on litter and soil carbon dynamics in forests

Nitrogen (N) deposition has a major impact on litter and soil carbon (C) dynamics in forests. Averaged across ecosystems, N deposition has negative impacts on the abundance and diversity of saprotrophic microbes (fungi and bacteria) and changes their species composition toward more opportunistic species. Concomitant with these changes in microbial communities, changes in enzyme activity occur, with hydrolytic-enzyme activity increasing and oxidative-enzyme activity decreasing. A reduction of oxidative-enzyme activity is usually accompanied by a decline in manganese availability. Genomics data confirm that N addition downregulates genes for oxidative enzymes. In the initial stages of litter decomposition an enhancement has repeatedly been observed, but this effect is transient. Soil respiration, the flux of CO2 from the soil to the atmosphere, is reduced, with a significant reduction of heterotrophic respiration by saprotrophic microbes. Most effects of N deposition are stronger in experiments of longer duration or where higher N doses have been added. Often effects are stronger in boreal and temperate forest ecosystems than in subtropical and tropical forests. It is unclear, however, whether this difference is driven by climate, or whether climate is a proxy for differences in the dominant mycorrhizal type. Suggestions for further research are provided. Whereas N deposition slows down the C cycle and hence enhances C sequestration, the amounts sequestered seem to be too small to constitute a major contribution to climate change mitigation.