The depth-dependent microbial response to root litter input in an experimental whole-soil warming study

Abstract

Warmer soil temperatures influence belowground plant allocation, root turnover, and soil organic carbon decomposition. However, the fate of root-derived carbon with depth and temperature remains uncertain. This study traced ¹³C-labeled Avena fatua root litter incubated at three depths (10–14 cm, 45–49 cm, 85–89 cm) in a temperate conifer forest, where plots have been warmed by 4 °C to 100 cm depth since 2014. We recovered the added root carbon in soil organic matter (SOM) density fractions and microbial phospholipid fatty acids (PLFAs) after 1 and 3 years of incubation in situ. Root carbon recovery was similar across all soil depths and warming treatment, and was increasingly recovered in the soil over time, driven by fivefold increase in the occluded SOM fraction. Root addition increased fungal abundance by 3.7 percentage points across all depths, with the highest increase in fungal and Gram-negative bacterial relative abundance at 85–89 cm. By alleviating subsoil substrate limitation, root litter inputs promoted microbial groups that are typically constrained at depth. Despite this, soil warming reduced total microbial PLFAs at 45–49 cm by 5.4 percentage points and fungal relative abundance by 0.7 percentage points across all depths, likely due to a 32–18% decrease in soil moisture with warming, which can limit microbial activity. These results indicate that added root carbon is highly decomposable in the subsoil when substrate and moisture availability are not limiting, and highlight the essential role that fungi play in root litter decomposition throughout the soil profile.