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

Abstract

Sugar maple (Acer saccharum Marsh.)-dominated northern hardwood forests in the upper Lakes States region appear to be particularly sensitive to chronic atmospheric NO 3 − deposition. Experimental NO 3 − deposition (3 g NO 3 − N m−2 y−1) has significantly reduced soil respiration and increased the export of DOC/DON and NO 3 − across the region. Here, we evaluate the possibility that diminished microbial activity in mineral soil was responsible for these ecosystem-level responses to NO 3 − 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 NO 3 − deposition. Microbial biomass, microbial respiration, and daily rates of gross and net N transformations were not changed by NO 3 − deposition. We also observed no effect of NO 3 − deposition on annual rates of net N mineralization. However, NO 3 − deposition significantly increased (27%) annual net nitrification, a response that resulted from rapid microbial NO 3 − assimilation, the subsequent turnover of NH 4 + , and increased substrate availability for this process. Nonetheless, greater rates of net nitrification were insufficient to produce the 10-fold observed increase in NO 3 − export, suggesting that much of the exported NO 3 − resulted directly from the NO 3 − deposition treatment. Results suggest that declines in soil respiration and increases in DOC/DON export cannot be attributed to NO 3 − -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 NO 3 − deposition in Lake States northern hardwood forests.