Changes in soil respiration along a nitrogen availability gradient in high-elevation red spruce forests

Background/Question/Methods

Nitrogen deposition can strongly influence carbon and nitrogen cycling in forest ecosystems. Studies using N additions have shown that carbon sequestration in soil organic matter can increase due to lower soil respiration. We tested this hypothesis along an existing N availability gradient in the southern Appalachian Mountains. Four 200-m plots were established in red spruce (Picea rubens) communities along a modeled gradient of N deposition representing low (3.83 kg ha^-1 yr^-1) to high (9.72 kg ha^-1 yr^-1) wet deposition inputs. From June to November 2010, soil respiration was measured weekly using a LI-8100 soil CO₂ flux system along with simultaneous measurements of soil temperature and moisture. At each site, soil samples were collected for C and N analyses. Red spruce leaf samples were also collected during July 2010 for foliar C, N, and d¹⁵N analyses.

Results/Conclusions

Soil respiration was significantly related to both soil temperature (R² = 0.37) and soil moisture (R² = 0.04) across all red spruce stands. Stronger relationships were observed relating soil respiration to soil C:N and foliar characteristics of spruce stands suggesting that increased N availability was associated with increased soil respiration. Mean soil respiration increased as soil C:N declined to low values typically associated with high rates of net nitrification (R² = 0.70 and 0.75 for organic soil and mineral soil, respectively). Mean soil respiration also increased as both foliar d¹⁵N increased (an integrated index of greater N availability; R² = 0.90) and foliar N increased (R² = 0.93). Collectively, these results are in contrast to those of several previous studies and suggest that the net effect of N on the components of soil respiration (microbial & fine root respiration) results in an overall stimulation of carbon losses to the atmosphere. We hypothesize that although higher N availability might decrease the decay of high-lignin litter that is often found in coniferous forests, a greater N concentration in fine roots will stimulate autotrophic respiration to a much greater extent.