Microbial Cycling of C and N in Northern Hardwood Forests Receiving Chronic Atmospheric NO3− Deposition

Sugar maple (Acer saccharum Marsh.)-dominated northern hardwood forests in the upper Lakes States region appear to be particularly sensitive to chronic atmospheric NO3− deposition. Experimental NO3− deposition (3 g NO3− N m−2 y−1) has significantly reduced soil respiration and increased the export of DOC/DON and NO3− across the region. Here, we evaluate the possibility that diminished microbial activity in mineral soil was responsible for these ecosystem-level responses to NO3− deposition. To test this alternative, we measured microbial biomass, respiration, and N transformations in the mineral soil of four northern hardwood stands that have received 9 years of experimental NO3− deposition. Microbial biomass, microbial respiration, and daily rates of gross and net N transformations were not changed by NO3− deposition. We also observed no effect of NO3− deposition on annual rates of net N mineralization. However, NO3− deposition significantly increased (27%) annual net nitrification, a response that resulted from rapid microbial NO3− assimilation, the subsequent turnover of NH4+, and increased substrate availability for this process. Nonetheless, greater rates of net nitrification were insufficient to produce the 10-fold observed increase in NO3− export, suggesting that much of the exported NO3− resulted directly from the NO3− deposition treatment. Results suggest that declines in soil respiration and increases in DOC/DON export cannot be attributed to NO3−-induced physiological changes in mineral soil microbial activity. Given the lack of response we have observed in mineral soil, our results point to the potential importance of microbial communities in forest floor, including both saprotrophs and mycorrhizae, in mediating ecosystem-level responses to chronic NO3− deposition in Lake States northern hardwood forests.