No recalcitrant material in a warming world – Loss of plant-derived and pyrogenic polymers in subsoils after 4.5 years of whole-soil warming

Soil organic carbon accounts for 2-3 times the amount of carbon (1500-2400 GtC) as compared to the atmosphere, making it an important component of the global carbon cycle. Global warming will also increase soil temperature over the whole soil profiles. As warming experiments to date often focused on topsoils (0-20 cm depth), it is largely unknown how subsoil organic carbon (OC) will be affected by warming, despite the large share of OC stored in subsoils (50% of total soil OC). Furthermore, it remains contentious how temperature sensitive various functional carbon pools are. For example, plant-derived and pyrogenic polymers sometimes have a longer turn-over time as compared to bulk SOC, but could be more sensitive to decomposition under warming.

The whole-soil warming experiment is located in a temperate forest in the Sierra Nevada, CA, US. Soils were warmed by +4°C to 1 m depth with heating rods, maintaining the natural temperature gradient and seasonality. We analyzed plant-derived hydrolysable lipids and the pyrogenic polymers, benzene polycarboxylic acids (BPCA), to better understand the degradation of soil OC and the mentioned polymers.

Our results after 4.5 years of +4°C whole soil warming highlight the vulnerability of even complex plant-derived and pyrogenic polymers in subsoil to warming. Both plant-derived (-27.7±3.3%) and pyrogenic polymers (-37.2±7.9%) were less abundant in subsoils of warmed as compared to control plots, whereas concentrations in topsoils were not affected by warming. These observations underline that in a warming world, previously stable polymeric carbon might be quickly degraded and released to the atmosphere. At the study site, primarily free particulate organic matter was lost, thus the polymeric carbon was potentially part of this unprotected fraction. Taken together, our results underline the importance of studying the effect of soil warming over whole soil profiles. The loss of plant-derived and pyrogenic polymers from warmed subsoils indicates that these compounds are not inherently stable but also prone to degradation in these carbon-limited subsoils with global warming. Our findings contradict the proposed use of plant-derived polymers and pyrogenic carbon for long-term carbon sequestration.