COS 99-2
Who is behind the wheel? The drivers of soil N availability in high-elevation red spruce (Picea rubens Sarg.) forests along a gradient of atmospheric N deposition

Thursday, August 8, 2013: 1:50 PM
101H, Minneapolis Convention Center
Kenneth R. Smith, Biology, West Virginia University, Morgantown, WV
Justin M. Mathias, Department of Biology, West Virginia University, Morgantown, WV
Brenden E. McNeil, Department of Geology and Geography, West Virginia University, Morgantown, WV
William T. Peterjohn, Biology, West Virginia University, Morgantown, WV
Richard B. Thomas, Department of Biology, West Virginia University, Morgantown, WV

Atmospheric deposition of reactive nitrogen (N) can cause considerable changes to nutrient cycling in forest ecosystems including accelerated N losses and changes in N retention and availability. Though this has been observed in a variety of forests, ecosystem responses can be highly variable due to confounding environmental factors. To test the extent to which rates of N deposition drive current soil N availability, we established seven 200-m sample plots in red spruce (Picea rubens Sarg.) forests along a modeled gradient of atmospheric N deposition. We used a variety of field and laboratory techniques to estimate soil N availability at each site along the spatial gradient. During June 2011 we measured C, N, and d15N content of soil and red spruce leaf samples. We also deployed PRSTM-probes at each sample plot to measure soil nutrient supply rates. In July 2012 we resampled soils to conduct a short-term lab incubation to measure specific rates of N transformations. To determine the influence of other environmental factors on N availability, we also measured relative abundance of hardwoods and soil pH at each stand. Additional variables including climate and stand history were also obtained.


We compared each index of N availability to several continuous predictor variables (e.g. atmospheric N deposition, community composition, soil pH, annual precipitation and temperature, and stand age) in a series of regression models. Each regression was compared using Akaike's Information Criterion (AIC) to determine the likelihood of that particular model. In a comparison of all indices of N availability, we determined that mineral soil C:N was most strongly correlated to specific rates of soil N transformations (nitrification, R2 = 0.67, p = 0.02; mineralization, R2 = 0.83, p = 0.004); therefore, soil C:N was used as a metric for N availability for all subsequent analyses. Our results show that soil C:N is most strongly related to relative abundance of hardwoods (R2 = 0.92, p = 0.0006) and soil pH (R2 = 0.64, p = 0.03); whereas, all other relationships (including N deposition) were not significant. These findings do not necessarily invalidate our modeled values of historical deposition; rather they suggest that current rates of deposition may be too minimal to detect a significant response. Instead differences in species composition between sites appear to be the strongest drivers of soil N availability along this spatial gradient.