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.