Accrual of soil organic carbon (SOC) acts as a potential carbon sink, helping to mitigate anthropogenic C emissions. The cultivation of deep-rooted perennial crops, such as switchgrass (Panicum virgatum L.), potentially provides a long-lasting carbon sequestration, that increases deep soil C stocks via root biomass inputs. But there may be trade-offs associated with this strategy: the preexisting soil organic matter (OM) pool may be displaced if rates of microbially-stimulated OM degradation increase in response to deep C inputs. In surface soils, many studies have shown ‘priming’, i.e. that inputs of fresh root biomass and exudates increases OM accessibility and stimulate microbial activity. However, it is unknown whether deep soil mineral-associated organic C (MOC) may be more vulnerable to microbial mineralization under switchgrass cultivation. Our study aims to address this fundamental uncertainty regarding the effects of establishment and sustainable cultivation of switchgrass on deep soil C stocks and long-term C dynamics in marginal lands. We applied ecosystem-model-based prediction to estimate whether root biomass input into the subsoil layer may impact long-term C dynamics and accelerate the mineral-associated organic carbon (MOC) loss. For this purpose, we improved our MEND (Microbial-ENzyme Decomposition) model to represent multiple soil layers and used integrated datasets (including edaphic properties, climate condition, above- and below-ground biomass, soil respiration in the lab and in the field) to simulate the long-term C dynamics at different soil layers, different soil types and different root systems.
Results/Conclusions
The results based on our ecosystem models showed that a long-term cultivation of deep-rooted switchgrass potentially increases a larger amount of SOC rather than shallow-rooted crops. Our model prediction further suggested that soil C sequestration (increase in MOC) will occur in both topsoil and subsoil at the silt-loam site (i.e., coarse textured soil) while only occurring in the topsoil at the clay-loam site (i.e., fine textured soil). Discrepancies in the accumulated SOC yet depletion of MOC in the subsoil of the clay-loam site implied that this increase in total SOC is largely attributed to the increase in particulate organic C (POC) rather than MOC. Our on-going DNA-SIP-based analyses of taxonomic and functional microbial community structure will further provide the underlying mechanisms in explaining this microbially mediated soil C dynamics along the soil profile at different edaphic conditions.