The relationships between SOM quality and decomposition across a gradient of increasing shrub abundance in the Alaskan Arctic

Warming arctic temperatures are shifting dominant vegetation from graminoids to deciduous shrubs whose functional traits can alter the quality of soil organic matter (SOM) via litter inputs and root exudates.  Understanding how SOM quality changes across plant communities which vary in their deciduous shrub cover is essential to understanding and predicting how this community shift will influence carbon (C) storage and nutrient release since SOM quality has been shown to be a strong control over C and N mineralization rates.  Our objectives were to understand how SOM quality, C and N mineralization differ across plant communities that vary in deciduous shrub abundance and to determine which soil variable most strongly correlates with soil C release.  In the summer of 2008 and 2009, 14 and 18 50 meter transects were established near Toolik Lake at the Arctic Long Term Ecological Research Station, located in the foothills region of the Brooks Range, on the north slope of Alaska.  At all sites species composition, shrub height, and shrub foliar nutrients were measured along a 50 m transect.  Three replicates of organic soil were extracted down to permafrost or mineral soil for each transect and separated into two depths (0-10 cm and 10+ cm). Soils were analyzed for moisture content, % C and N, NH₄⁺ and NO₃^- concentrations, and pH. In addition, a 100 day soil lab incubation was conducted to measure rates of C flux.  Data were analyzed using two approaches; first, a two-way Analysis of Variance (ANOVA) with vegetation type and soil depth as the main effects with the transects classified into two or three levels of shrub abundance and second, using linear regressions to determine which initial SOM quality indices best predicted C mineralization rates.  Preliminary results from organic soils from transects collected in 2008 show that transects with greater shrub abundance had both higher % N (p = 0.07) and concentrations of NH₄⁺ compared to transects with lower shrub abundance.  Despite significant differences in initial soil quality no significant differences were seen across shrub abundance levels in cumulative C mineralized over the 100 day incubation.  Organic soils collected from transects collected in 2009 also had higher % N (p < 0.01) and NH₄⁺ concentrations however these soils had significantly higher soil pH (p < 0.01) and lower bulk density (p < 0.001).  There were no differences in cumulative C mineralized across the shrub abundance levels however the shallow soil significantly mineralized more C than the deep soil during the 100 day incubation.  For both years, cumulative C mineralized for after 28 and 100 days was negatively correlated with initial soil bulk density (r² = 0.23-0.50) but positively correlated with initial soil % C (r² = 0.24-0.44).  Carbon mineralized from transects from 2008 were also negatively correlated with initial soil pH (r² = 0.30-0.38) while C-mineralized from transects from 2009 were positively correlated with initial soil % N and NH₄⁺ concentrations but only for the 28 day incubation period.  Based on our results an increase in shrub abundance within the arctic region may not lead to increases in soil C release but any future changes in bulk density, % C, %N, and/or soil pH is likely to alter soil C fluxes.