Increased decomposition of root-derived biomass by warming in a temperate forest soil is depth-dependent

Increased decomposition of root-derived biomass by warming in a temperate forest soil is depth-dependent   Sun B, Zosso, C.U., Wiesenberg GLB, Pegoraro E., Torn MS, Schmidt MWI   The IPCC climate scenario RCP 8.5 suggests temperate regions will warm 4°C by 2100, which could accelerate soil carbon loss, greenhouse gas release, and further promote global warming. Despite low carbon concentrations, subsoils (> 30 cm) store more than half of the total global soil organic carbon stocks. However, it remains largely unknown how this deep soil carbon will respond to warming and how root-derived carbon, a potentially slower cycling part of soil carbon, could contribute to long-term carbon sequestration in soil. After three years of in-situ root-litter incubation, we i) quantified decomposition of root-litter at different depths in a +4°C warming field experiment, ii) assessed whether root-derived polymers degraded differently in warmed and ambient temperature conditions, and iii) identified decomposition products of plant biomass remaining. In a field warming experiment in a temperate forest (Blodgett Forest, Sierra Nevada, CA, USA), 13C-labelled root-litter was incubated at three soil depths (10-14, 45-49, 85-89 cm) in soil cores for one and three years at ambient temperature and +4°C. For bulk soil, we measured carbon and nitrogen concentrations, and δ13C isotope composition. We further quantified and determined the δ13C isotope composition of microbial (PLFA) and root-derived (suberin) molecular marker. The results showed that: 1) In bulk soil, on average there was higher 13C-excess in the control compared to heated plots in topsoil (10-14 cm), meaning more decomposition and loss in the heated plots, but there was no difference in subsoils (45-49, and 85-89 cm). 2) The root-specific molecular marker suberin indicated that warming accelerated the loss of root biomass in topsoil. However, this trend was not found in subsoils and this could be due to scattered hotspots of microbes in subsoil. Nevertheless, 13C-excess of suberin biomarkers was higher than that of bulk soil carbon, which indicates a slower turnover of hydrolysable lipids in root litter compared to bulk root carbon. 3) With warming, the concentrations of hydrolysable lipids (normalized to carbon content) increased at all three depths. This indicates a potential preferential preservation of hydrolysable lipids. But this could also be attributed to faster litter decomposition and incorporation in mineral soil due to warming, especially in the topsoil. In conclusion, warming increased decomposition of root-derived carbon and hydrolysable lipids in topsoil but not in subsoil. On the other hand, warming also increased plant-derived input into topsoil which accelerated the turnover of carbon at this shallower depth. Root-derived hydrolysable lipids in roots are relatively less decomposable than bulk tissues and could be preferentially preserved with warming. Therefore, warming could accelerate the turnover of root-derived carbon, but this is strongly dependent on depth and whether the tissues are available to microorganisms.