Microbial community response to nitrogen deposition in northern forest ecosystems

The productivity of temperate forests is often limited by soil N availability, suggesting that elevated atmospheric N deposition could increase ecosystem C storage. However, the magnitude of this increase is dependent on rates of soil organic matter formation as well as rates of plant production. Nonetheless, we have a limited understanding of the potential for atmospheric N deposition to alter microbial activity in soil, and hence rates of soil organic matter formation. Because high levels of inorganic N suppress lignin oxidation by white rot basidiomycetes and generally enhance cellulose hydrolysis, we hypothesized that atmospheric N deposition would alter microbial decomposition in a manner that was consistent with changes in enzyme activity and shift decomposition from fungi to less efficient bacteria. To test our idea, we experimentally manipulated atmospheric N deposition (0, 30 and 80 kg NO3−-N) in three northern temperate forests (black oak/white oak (BOWO), sugar maple/red oak (SMRO), and sugar maple/basswood (SMBW)). After one year, we measured the activity of ligninolytic and cellulolytic soil enzymes, and traced the fate of lignin and cellulose breakdown products (13C-vanillin, catechol and cellobiose). BOWO ecosystem, the highest level of N deposition tended to reduce phenol oxidase activity (131±13 versus 104±5 μmol h−1 g−1) and peroxidase activity (210±26 versus 190±21 μmol h−1 g−1) and it reduced 13C-vanillin and 13C-catechol degradation and the incorporation of 13C into fungal phospholipids (p<0.05). Conversely, in the SMRO and SMBW ecosystems, N deposition tended to increase phenol oxidase and peroxidase activities and increased vanillin and catechol degradation and the incorporation of isotope into fungal phospholipids (p<0.05). We observed no effect of experimental N deposition on the degradation of 13C-cellulose, although cellulase activity showed a small and marginally significant increase (p<0.10). The ecosystem-specific response of microbial activity and soil C cycling to experimental N addition indicates that accurate prediction of soil C storage requires a better understanding of the physiological response of microbial communities to atmospheric N deposition.