Figure 1:

We hypothesise that direct and indirect effects of topography, vegetation and soil characteristics control litter decomposition in the Arctic tundra. Solid boxes indicate variables with field data. We did not collect data on soil organisms or microbial communities, but instead rely on strong established links between vegetation, soil, and microclimate with soil organisms and, ultimately, decomposition.
Plant functional traits as well as soil characteristics directly influence soil organisms, particularly microbes, and hence affect local decomposition rates and carbon cycling (Cornwell et al. 2008, Eskelinen et al. 2009, Sundqvist et al. 2011, Freschet et al. 2012). Decomposability of leaf litter, associated with traits such as leaf dry matter content (LDMC) or leaf carbon:nitrogen ratio (leaf C/N; e.g., Freschet et al. 2012), can differ widely between plant functional types (Cornelissen et al. 2007). Thus, traits of the local plant community can directly influence soil microbial communities and decomposition rates (Strickland et al. 2009, Keiser et al. 2011, DeMarco et al. 2014, Veen et al. 2015). Soil pH and nutrient concentrations also structure microbial communities in the ground and can therefore be expected to affect decomposition (Figure 1; Fierer and Jackson 2006, Eskelinen et al. 2009). However, we do not yet understand to what extent these vegetation and soil characteristics influence decomposition rates directly, relative to the indirect influences of terrain and vegetation via their impact on microclimate. Disentangling these relationships is required for predicting future decomposition rates in changing tundra environments.
Here, we address this knowledge gap by adapting the Tea Bag Index (TBI; (Keuskamp et al. 2013) protocol for a network of stratified randomly placed microclimate monitoring plots. We buried pairs of standard tea bags with litter of contrasting quality along gradients of topography, vegetation productivity and water availability across an Arctic tundra landscape at Qeqertarsuaq (Disko Island), Western Greenland (Figure 2), with the following research questions in mind: (1) How important is micro-environmental variation in determining variation in decomposition rates across a tundra landscape?; (2) Does the relative importance of environmental drivers of mass loss differ between labile (Green Tea) and recalcitrant (Rooibos Tea) litter?; and (3) Within the hypothesised system of topography, vegetation, soil and decomposition (Figure 1), are direct (traits) or indirect (microclimate) effects of variation in vegetation more influential for determining decomposition?
Our study improves our understanding of the factors driving variation in litter decomposition across a tundra landscape and will help to inform predictions of future decomposition and carbon cycling dynamics in the changing Arctic.