Summer litter decomposition is moderated by scale-dependent microenvironmental variation in tundra ecosystems

Tundra soils are one of the world’s largest organic carbon stores, yet
this carbon is vulnerable to accelerated decomposition as climate warming
progresses. The landscape-scale controls of litter decomposition are
poorly understood in tundra ecosystems, which hinders our understanding of
the global carbon cycle. We examined the extent to which the thermal sum
of surface air temperature, soil moisture and permafrost thaw depth
influenced litter mass loss and decomposition rates (k), and at which
spatial thresholds an environmental variable becomes a reliable predictor
of decomposition, using the Tea Bag Index protocol across a heterogeneous
tundra landscape on Qikiqtaruk - Herschel Island, Yukon, Canada. We found
greater green tea litter mass loss and faster decomposition rates (k) in
wetter areas within the landscape, and to a lesser extent in areas with
deeper permafrost active layer thickness and higher surface thermal sums.
We also found higher decomposition rates (k) on north-facing relative to
south-facing aspects at microsites that were wetter rather than warmer.
Spatially heterogeneous belowground conditions (soil moisture and active
layer depth) explained variation in decomposition metrics at local scales
(< 50 m2) better than thermal sum. Surprisingly, there was no
strong control of elevation or slope on litter decomposition. Our results
reveal that there is considerable scale dependency in the environmental
controls of tundra litter decomposition, with moisture playing a greater
role than the thermal sum at < 50 m2 scales. Our findings highlight
the importance and complexity of microenvironmental controls on litter
decomposition in estimates of carbon cycling in a rapidly warming tundra
biome.