Introduction
Feedbacks from high-latitude carbon cycling are pivotal for global
climate change (Schuur et al. 2015, 2022). Not only is the Arctic
experiencing rates of warming at three to four times the global average
(AMAP 2021, Rantanen et al. 2022), tundra soils also store around half
of terrestrial organic carbon (Tarnocai et al. 2009). Decomposition of
leaf litter represents a key ecosystem process in this context, as it
mediates the transition from living plant biomass to soil organic
matter, thereby representing the primary carbon input to Arctic soils.
Generally, it is assumed that decomposition rates in the Arctic will
increase with warming (Aerts 2006, Xue et al. 2016), depending on
moisture availability (Hicks Pries et al. 2013). However, as tundra
ecosystems are characterised by high spatial heterogeneity in abiotic
and biotic conditions, precise predictions of tundra soil carbon
dynamics require a detailed understanding of how this variation in
fine-scale soil temperature and moisture, hereafter termed ‘soil
microclimate’, translates into litter decomposition dynamics (Bradford
et al. 2016).
Local energy fluxes and hence soil abiotic conditions controlling
decomposition rates are determined by tundra terrain features,
vegetation and soil (Aalto et al. 2018, von Oppen et al. 2022; Figure
1). During the growing-season, topographical features, such as higher
elevation, reduced sunlight exposure, or terrain depressions (Opedal et
al. 2015, Aalto et al. 2018), and dense shrub cover (Blok et al. 2010,
Myers-Smith and Hik 2013, Kemppinen et al. 2021, von Oppen et al. 2022)
can cause lower soil temperatures that slow down litter decomposition
rates by reducing decomposer activity. Similarly, terrain depressions or
very fine soil texture can lead to extremely moist soil conditions that
retard decay processes locally (Bryant et al. 1998, Robinson 2002,
Christiansen et al. 2017, Sarneel et al. 2020, Venn and Thomas 2021). In
contrast, tall shrubs might enhance decomposition rates (Blok et al.
2016) as they trap additional snow and create warmer soils through snow
insulation during winter (Kropp et al. 2021). Generally, microclimatic
variation has been shown to drive soil enzyme and microbial activity, as
well as composition (Zak and Kling 2006, Wallenstein et al. 2009, Feng
et al. 2020, Rijkers et al. 2023), thus causing variation in soil
organic carbon stocks across tundra landscapes (Kemppinen et al. 2021).
However, how the combined effects of terrain, vegetation and soil
characteristics on soil microclimate affect in situ tundra
decomposition rates remains uncertain.