Nitrogen (N) additions to forests commonly result in declines in organic matter decomposition and soil carbon efflux (SCE), ultimately enhancing the soil carbon (C) sink. However, the mechanisms coupling C and N cycles in soils that drive these changes are not well understood. We hypothesized that N-induced shifts in belowground C allocation would alter soil respiration. We used two methods to estimate belowground C inputs over two years: (1) a mass balance method assuming that total belowground C flux (TBCF) equals the difference in SCE and aboveground litter C inputs, and (2) a stable isotope method using one-year in-growth cores with a 13C signal distinct from the forest soil, and a two end-member mixing model to determine root-derived C inputs. We also assessed relative contributions of root and heterotrophic respiration to total SCE through a lab incubation experiment. We measured these processes at the Fernow Experimental Forest (FEF) in West Virginia, the site of over 28 years of whole-watershed nitrogen fertilization (35 kg N ha-1 yr-1). The FEF provides a unique opportunity to study the long-term soil response of an entire watershed to elevated N deposition.
Results/Conclusions
Using the mass balance approach, we found lower TBCF in the N fertilized watershed, which was driven by a ~12% reduction in annual SCE (873 vs. 992 gC m-2 yr-1), as leaf litter inputs did not differ between watersheds. Although standing fine root biomass was greater in the reference watershed, the 13C isotope method revealed ~20% greater fine root production (FRP) in the fertilized watershed and nearly double the amount of root-derived C. These results suggest that under high nitrogen availability, fine root production increases while fine root turnover time decreases. Results from the lab incubation experiment revealed that fine roots from the fertilized watershed had lower respiration rates per gram of tissue, despite greater root N concentration. No detectable difference was found in heterotrophic respiration between watersheds in the 0-10 cm mineral soil measured in lab incubations. Overall, our results indicate that lower root respiration per gram of tissue coupled with a smaller standing pool of fine roots could account for a 50% decrease in autotrophic respiration and most of the reduction in total SCE in the fertilized watershed, suggesting that plants, rather than microbes, drive declines in soil respiration in response to N additions in this forest.