Microbial communities under chronic nitrogen deposition: Effects of decomposition on litter of varying quality

Background/Question/Methods Anthropogenic N fertilization has been demonstrated to both stimulate and suppress decomposition of forest organic horizons depending on leaf litter quality.  Litter with low C:N ratios and lignin (high quality) generally decompose more rapidly following N addition, while litter with high lignin content and C:N ratios (low quality) experience delayed decomposition. Here, we examine litter differing in quality not only by species (maple and oak) but also by leaf component (leaf and petiole) in a two-year litterbag decomposition experiment (serial collections after 5, 8, 17, and 20 months of decomposition). We hypothesized that 1) decomposition would diverge according to litter-quality, and 2) that divergence would be explained by shifts in the microbial community. We expected that microbial specialists on high-quality litter would proliferate in the environment provided by N deposition, while high lignin litter specialist would be outcompeted. Litter types represent a gradient of C:N ratios (maple leaf: 61.8; oak leaf: 78.8; maple petiole: 124.0; oak petiole: 133.0). Here we examine decomposition rates relative to C:N ratios and activity of microbial exo-enzymes. Upcoming results will provide microbial biomasses and genomic insight into the changing microbial community within collected litter using phospholipid fatty acid and molecular (16S, ITS) analysis).

Results/Conclusions Expectations of differential decay were supported in the most extreme litter types, with maple leaves decaying faster with N than without and oak petioles decaying more slowly with N than without. Maple petioles and oak leaves showed less clear patterns of divergence, with maple petioles trending towards slower decay with N addition and oak leaves exhibiting similar decay across treatments. Initial results thus support divergent responses based on initial C:N ratios, with low quality petioles demonstrating inhibited decay, and low quality leaves showing enhanced. After eight months of decay, activity of cellulose degrading enzyme B-Glucosidase showed significantly higher activity in petioles than leaves (t=2.3, p=0.03) but no significant difference by species or treatment. Activity of all cellulose and hemi-cellulose degrading enzymes increased after five and eight months of decomposition. Additional results providing the response of lignin degrading enzymes, microbial biomass, and community data will inform the connection between C:N ratios and mechanisms of decomposition. These preliminary results present strong arguments for examining litter quality by metrics beyond simple litter species, and suggest key differences in enzymatic activity across leaf components.