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.