Chronic experimental NO3- deposition reduces the retention of leaf litter DOC in a northern hardwood forest soil

In forests of the Great Lakes region, experimental NO3 deposition has suppressed soil respiration and enhanced DOC export. Reasons for these responses are unknown, but they could arise via two alternatives: (i) direct suppression of microbial activity by NO3 or (ii) indirect suppression of the microbial community via changes in litter biochemistry in response to greater N availability. To test the second alternative, we conducted a controlled laboratory experiment to examine how chronic experimental NO3 deposition affects the contributions of fresh leaf litter to microbial respiration and DOC export. The study reported here used manipulations of mineral soil and fresh leaf litter from a previously studied northern hardwood forest stand in northern Lower Michigan that has received 9 years of ambient and experimental (three times ambient) atmospheric NO3 deposition. We found that cumulative microbial respiration over the 6-week incubation was substantially greater in fresh litter plus mineral soil (20.2-13.4 mg C) versus mineral soil alone (4.4-4.1 mg C); however, experimental NO3- deposition had no effect on microbial respiration across the litter-mineral soil manipulations. DOC production (similar to 75%) was primarily associated with leaching from fresh litter. In contrast, mineral soil was a significant sink for litter-derived DOC. Significantly, the mineral soil sink was less pronounced in soil receiving experimental NO3- deposition in which similar to 30% more DOC was leached compared to the ambient NO3- deposition treatment. Furthermore, mineral soil was also both a source and sink for soluble phenolics; however, NO3- deposition suppressed a mineral-soil sink for phenolics derived from fresh leaf litter. These results suggest that increases in DOC export and declines in soil respiration in response to NO3- deposition in the field are not related to obvious changes in litter biochemistry or to the microbial metabolism of this material. Alternatively, these patterns may be linked to decreased abiotic sinks for litter-derived DOC in mineral soil, an unexpected ecosystem consequence of increased anthropogenic (NO3-) deposition. (c) 2005 Elsevier Ltd. All rights reserved.