Atmospheric inputs of reactive nitrogen can strongly influence carbon and nitrogen cycling in forest ecosystems. While studies have shown that carbon sequestration in soil organic matter can increase due to lower soil respiration, it is yet unclear how fine root respiration contributes to such decreases. To test how autotrophic contributions to total soil respiration vary with N availability, we established four 200-m sample plots 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 August 2011, autotrophic respiration was measured weekly on intact fine roots using a LI-6400 open-flow infrared gas analyzer. Chamber temperatures within the sample cuvette were maintained at 25°C for each measurement. We simultaneously measured total soil respiration using a LI-8100 automated soil CO2 flux system along with instantaneous measurements of soil temperature and moisture. Soil cores were collected from each site to determine root biomass per unit ground area. At each site, we collected soil and red spruce leaf samples for C, N, and d15N analyses.
To compare root and soil respiration in similar units, we used a model of site-specific soil respiration and temperature to calculate soil CO2 efflux at 25°C. We observed strong relationships relating fine root and soil respiration to soil C:N and foliar chemistry suggesting that increased N availability was associated with increased soil respiration. Soil respiration increased as soil C:N declined to values associated with high rates of net nitrification (R2 = 0.94). However, we observed significant decreases in fine root biomass (R2 = 0.62) and respiration (R2 = 0.47) over the same range of C:N values indicating that fine root growth is diminished under high N availability. We observed significant increases in soil respiration at high values of foliar d15N (an integrated index of greater N availability; R2 = 0.98) while fine root respiration decreased (R2 = 0.69). Our results show that the contribution of autotrophic respiration to total soil CO2 efflux is greater under conditions of low N availability. In contrast to the results of several previous studies, our data suggests that the effect of N on the components of soil respiration (microbial & fine root respiration) results in an overall increase of respiratory carbon losses to the atmosphere.