Interannual variability of soil respiration is linked to soil N availability in high-elevation red spruce (Picea rubens) forests in Central Appalachia
The annual release of CO2 from terrestrial soils represents a major flux in the global C cycle and accounts for ~30-80% of respiration for forest ecosystems. Many factors influence interannual variation in soil respiration including climate, plant productivity, litter quality, microbial community, and substrate availability. In addition, atmospheric inputs of reactive N may affect soil respiration through several mechanisms such as increased stabilization of organic matter, shifts in microbial enzyme synthesis, and enhanced plant productivity. To test whether N deposition is a driver of annual soil CO2 efflux, we measured soil respiration in seven high-elevation red spruce (Picea rubens) forests in the central Appalachian Mountains along an atmospheric deposition gradient. Soil respiration was measured between March 2011 and October 2012 using a LI-8100 soil CO2 flux system. To estimate interannual soil respiration for each site, we applied datalogged soil temperature values to relationships between observed soil respiration rates and temperature. We also collected soil and red spruce leaf samples for C, N, and δ15N analyses to assess potential drivers of soil respiration.
Annual soil respiration estimates varied between 720-1060 g C m-2 yr-1 in 2011 and between 860-1430 g m-2 yr-1 in 2012. We attribute this 20-40% increase in soil respiration to variable drought conditions in 2012. Total summer precipitation (June through September) for 2012 was 22% less than summer precipitation in 2011. Additionally, the number of rewetting events was 17% less in 2012. Contrary to our original hypothesis, no significant relationships between annual respiration and modeled N deposition were observed; however, we saw evidence that the variation in soil respiration was due to differences in N availability. We observed the strongest relationships between respiration and the δ15N content of red spruce leaves and soil, especially in the organic soil horizon (R2 = 0.71 and 0.91 for 2011 and 2012, respectively). We found that the slope of the relationship between annual soil respiration and δ15N for organic soil was significantly greater in 2012 compared to 2011 (p = 0.039), suggesting that drought-induced increases in soil respiration are tightly coupled to the activity of nitrifying bacteria. Our data shows that N availability and precipitation interactively affect soil respiration. We hypothesize that seasonal water stress may increase both soil respiration and nitrification potential.