Part of the uncertainty in estimates of the land carbon (C) sink derives from the challenges in representing carbon-nitrogen interactions in land ecosystems, and the cascading effects that nitrogen (N) status has on the fluxes and residence times of ecosystem C pools. Forests in the northeastern U.S. represent a relatively large terrestrial C sink, partly attributed to decades of high atmospheric N deposition. In this study, we use a plant economics approach to examine the C response to 30 years of N additions (35 kg N ha-1 yr-1) in a whole-watershed fertilization study at the Fernow Experimental Forest, WV. C and N budgets were constructed from a variety of data sources, collected over the 30-year experiment, to assess the overall change in C and N cycling due to long-term N additions. Additionally, we leveraged our empirical data to run the plant-microbial interactions model, Fixation and Uptake of Nitrogen and Carbon, Organisms, Rhizosphere, and Protection in the Soil Environment (FUN-CORPSE), for both watersheds to explore the broader implications of our findings on the land-atmosphere exchange of C under future scenarios of altered environmental conditions.
Results/Conclusions
From our budgets, we found an overall enhancement in the forest C pool in the fertilized watershed, mainly attributed to the more than 30% increase in woody biomass C. Additionally, we observed reductions in standing fine root C stock in the mineral horizon (-43% to -17%) and soil C efflux (-13%). Our results indicate that N-induced increases in C storage were driven by a shift in C allocation from belowground components to aboveground biomass production. While we also found that root, foliar, and leaf litter C/N ratios were lower in the fertilized watershed, we surprisingly observed an increase in C/N ratios of mineral soil and wood, indicating potential increases in the residence times of these large C pools and altered mechanisms of soil organic matter formation. Furthermore, our model assessments captured the decline in belowground carbon allocation and showed that an interaction between reductions in rhizosphere C allocation and decrease in C/N ratio of plant inputs drove enhanced soil organic matter storage. Overall, the change in the nature of soil organic matter from long-term N additions in this forest will likely have long-lasting impacts on nutrient cycling and C storage at the local level, and our results may inform how widespread N deposition can influence the terrestrial C sink at larger scales.