The promotion of decomposition of root-derived biomass by warming is depth dependent in a temperate forest

IPCC climate scenarios (RCP 8.5) suggest 4°C warming until 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 deep soil carbon will respond to warming and how root-derived carbon as a potential recalcitrant part of soil carbon could contribute to carbon stabilization in subsoils. After three years of root-litter incubation, we aim to i) quantify decomposition of root-litter at different depths in a +4°C warming field experiment, ii) assess whether specific plant polymers will degrade differently in warmed and control plots, iii) identify decomposition products of plant biomass remaining.

In a field 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. For bulk soil, we measured carbon and nitrogen concentrations, and δ¹³C isotope composition. For individual molecular analysis, we quantified and determined the δ¹³C isotope composition of microbial biomarkers (PLFA), and plant-derived biomarkers (mainly suberin monomers released after base hydrolysis). We also explored suberin monomers as biomarkers for root-derived biomass (here ω-hydroxy fatty acids).

We observed the following:
i) In the plots without additional labelled root-litter, warming led to heavier suberin biomarker δ ¹³C values compared with control plots, especially in the topsoils. This indicates a more advanced degradation due to warming.
ii) In plots with added labelled root-litter, bulk soil δ¹³C values become heavier with soil depth. For individual suberin markers, we find less excess ¹³C with warming especially in topsoils, indicating more advanced decomposition in topsoils with warming. This advanced decomposition was not found in subsoils.

We conclude that the decomposition of root organic matter is depth dependent, and warming promotes the loss of suberin in topsoils, which contradicts the present assumption of suberin as a slowly degrading part of plant-derived organic matter.