How will belowground plant biomass in deep soil respond to warming in temperate forests?

IPCC climate models (RCP8.5) suggest 4°C warming until 2100, which could potentially accelerate soil carbon loss, greenhouse gas release, and thus promote global warming. Despite low carbon concentrations, subsoils (> 30 cm) store more than half of the total global soil carbon stocks. Retaining this is crucial to mitigate soil carbon greenhouse gas release. However, how deep soil carbon will respond to warming and how increased root-derived carbon could contribute to carbon stabilization in subsoils is under-studied and largely unknown. We aim to i) quantify the decomposition rate of root-litter at different depths with a +4°C warming field experiment, ii) assess whether various plant polymers will degrade differently in heated and control plots, iii) identify decomposition products of plant biomass remaining after three years of incubation.

In a field experiment in a temperate forest, ¹³C labelled root-litter was added at different soil depths (10-14, 45-49, 85-89 cm) in 2016 and retrieved half of the cores in 2017 and the remaining half in 2019. So far, we measured bulk soil carbon concentrations and d¹³C-composition of individual microbial biomarkers (PLFA).

Results confirm that bulk carbon concentrations and d¹³C values follow typical depth trends, except for the three horizons containing ¹³C labelled root-litter incubations. Next, we will quantify above- and below-ground biomarkers (cutin and suberin, respectively) and determine compound-specific ¹³C-composition in each molecular fraction in heated and control plots. We suspect that the presumably difficult to degrade compounds (cutin, suberin, and lignin polymers) will degrade slower than bulk organic matter over the observation period, and likely faster in heated than control plots. However, early results from warming experiments provide circumstantial evidence that also these compounds might degrade in synchrony with the bulk organic matter.