Microbial turnover limits mineral-associated organic matter accumulation in Eucalyptus woodland under elevated carbon dioxide

Abstract

Atmospheric carbon dioxide (CO2) concentration continues to rise, altering the plant-soil cycles of carbon (C) and nutrients. Plant inputs and soil microbes represent the precursors and drivers of soil organic matter (SOM) formation and its functional fractions (particulate organic matter (POM) and mineral-associated organic matter (MAOM)). However, how elevated CO2 (eCO2) mediates their accumulation remains unclear. Using an in-growth mesh bag technique, we investigated the effects of eCO2 on the accumulation of new POM and MAOM in a phosphorus (P)-limited Eucalyptus woodland. Unexpectedly, eCO2 decreased MAOM-C and POM-C compared to ambient CO2, and had no effect on root biomass and root traits inside the bags. We explain the eCO2-driven decrease in MAOM-C as a shift from a direct sorption pathway towards a less efficient in vivo microbial turnover pathway, whereby C is lost through microbial decomposition. This was supported by a lower C:Porg ratio in MAOM formed under eCO2, suggesting a greater contribution of P-rich microbial necromass. Differently, the decrease in POM-C under eCO2 suggests a simultaneous increase in microbial decomposition of newly produced POM-C under eCO2, possibly due to greater rhizodeposition and microbial activity. These findings question the role of soils as a C sink under eCO2.