Do increased N inputs influence rates of soil N cycling in high-elevation red spruce (Picea rubens Sarg.) forests along a gradient of atmospheric deposition?

Background/Question/Methods

Elevated rates of atmospheric nitrogen (N) deposition can alleviate plant growth restrictions in N-limited forests by increasing available N. However, sustained high rates of N deposition can induce N saturation whereby N inputs exceed biological demand and lead to elevated N losses. Forests receiving elevated N inputs often exhibit a wide variation in rates of N retention. This suggests that factors other than N deposition can strongly influence rates of N availability and loss in forested ecosystems. In this study, we established seven plots in high-elevation red spruce (Picea rubens Sarg.) forests along a gradient of atmospheric N deposition in order to assess whether variations in N input correspond to N cycling rates. During September 2012, we collected soil samples that were incubated for 28 d and periodically harvested to determine changes in NH₄⁺ and NO₃^- pools. We calculated rates of net nitrification and N mineralization as the change in NO₃^- and total inorganic N content (NH₄⁺ and NO₃^-), respectively, after a given time interval per g of dry soil. A small subsample of soils was destructively harvested to measure total C, N, and d¹⁵N content.

Results/Conclusions

The results of our study indicate that modeled values of atmospheric N deposition did not correspond to rates of either mineralization (R² = 0.02, p = 0.76) or nitrification (R² = 0.01, p = 0.83). These soil N transformations were, however, well-correlated with other measured indices of N availability. Mineralization significantly declined with increased soil C:N in both the organic (R² = 0.70, p = 0.02) and mineral horizons (R² = 0.83, p = 0.01). We observed similar declines in net nitrification as soil C:N increased in the organic (R² = 0.64, p = 0.03) and mineral horizons (R² = 0.67, p = 0.02). This trend was also observed for N transformations and soil d¹⁵N content, but was not as significant. While modeled rates of N deposition were not well-correlated with rates of soil N transformations, the results from our laboratory incubation validate field-based estimates of N availability. This suggests that although N deposition may have been historically high in this region, current N availability is likely being driven by other factors such as the relative abundance of hardwood species.