Warming accelerates the decomposition of root-derived hydrolysable lipids in a temperate forest and is depth- and compound class-dependent

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 a whole-soil field warming experiment at Blodgett Forest Research Station (California, USA), we investigated whether warming accelerates the decomposition of root-derived hydrolysable lipids in the top- (10-14 cm) and subsoil (45-49, 85-89 cm) by using molecular markers and in-situ incubation of 13C-labeled root litter at each depth. Our results reveal that at compound-class level, hydrolysable lipids presented compound-dependent responses. Warming consistently reduced fatty acid mass change across soil depths, particularly at 85-89 cm. In subsoil, there was accumulation of fatty acids, which primarily originated from microbial-derived mid-chain fatty acids such as octadecanoic acid (C18:0 fatty acids), octadecenoic acid (C18:1 fatty acids), and hexadecanoic acid (C16:0 fatty acids). Higher temperature attenuated this accumulation, indicating less microbial transformation of root-derived carbon under warming. At monomer level, ω-hydroxy acids and diacids as suberin markers were more resistant to decomposition than bulk root-derived carbon and their resistance increased with chain-length. Moreover, warming accelerated decomposition of individual suberin monomers in the topsoil but suppressed it in the subsoil. The slower decomposition in the subsoil was likely 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-derived hydrolysable lipids in a temperate forest is compound class- and depth-dependent. The persistence of long-chain ω-hydroxy acids and diacids may provide a potential way for long-term carbon stabilization in subsoil under climate change. Nevertheless, due to the substantial heterogeneity of subsoil environment, further studies are required to confirm and generalize this finding.