Warming accelerates the decomposition of root biomass in a temperate forest only in topsoil but not in subsoil

Abstract

Abstract. Global warming could potentially increase the decomposition rate of soil organic matter (SOM), not only in the topsoil (< 20 cm) but also in the subsoil (> 20 cm). Despite its low carbon content, subsoil holds on average nearly as much SOM as topsoil across various ecosystems. However, significant uncertainties remain regarding the impact of warming on SOM decomposition in subsoil, particularly root-derived carbon, which serves as the primary organic input at these horizons. In the Blodgett Forest warming experiment (California, USA), we investigated whether warming accelerates the decomposition of root-litter at three depths (10–14, 45–49, and 85–89 cm) by using molecular markers and in-situ incubation of ¹³C-labelled root-litter at each depth. Our results reveal that the decomposition of added root-litter was only accelerated in the topsoil (10–14 cm) but not in the subsoil (45–49 and 85–89 cm) with warming. In subsoil, although the decomposition rate of root-litter derived carbon did not differ significantly between ambient and warmed plots, the underlying reasons for this similarity are distinct. With molecular marker analysis, we found higher microbial activity, indicated by higher concentration of certain fatty acid monomers that could be originally microbial-derived such as octadecanoic acid (C_18:0 fatty acids), octadecenoic acid (C_18:1 fatty acids), and hexadecanoic acid (C_16:0 fatty acids) than those originally derived from roots in ambient subsoil. With warming, the higher concentration of long-chain (C number > 20) 𝜔-hydroxy acids and diacids left after 3 years of root incubation suggested a lower turnover rate and this could be due to lower microbial abundance and lower soil moisture induced by warming. Our study demonstrates that the impact of warming on the decomposition of root-litter in a temperate forest is depth-dependent. The slower turnover rate of long-chain 𝜔-hydroxy acids and diacids shows that they are more persistent compared to bulk root mass and could be preserved in subsoil for longer time as long as the environmental conditions are unfavorable for decomposition with warming.