Differences Between Aspen and Conifer Soil Organic Carbon Pools Explained By Carbon Fluxes.

Aspen, the most widespread hardwood species in the Intermountain West, have been found to have significantly higher mineral SOC pools compared to neighboring conifer stands. The proximity of stands leads to suggest that the difference is due to the effect of vegetation rather than climate or soil properties. However, the exact mechanisms and causes for this divergence are not yet fully understood. This, in turn, makes it difficult to predict whether and how SOC pools will change under ongoing aspen decline that is accompanied by conifer encroachment.

In this work we quantify pools and fluxes of C in adjacent aspen and conifer stands in Utah. While aspen have higher aboveground litterfall and understory cover, the transfer of this aboveground C into soil with snowmelt water is lower or similar than under conifer overstories. Furthermore, the retention of C within the top 40 cm soil column is also similar. Snowmelt water transports 42% of the annual conifer litterfall C into soil, but only 13% of aspen litterfall C. This indicates that roots are a more important source of SOC under aspen overstories. However, conifer soils have twice the fine root biomass as aspen. Even though rhizotron image analysis indicates that under both overstories roots turn over once during the growing season, the diameter cutoff of 2-mm was most likely inapt for this comparison as aspen had overall thinner roots than conifers. The rhizotron analysis also suggests higher root growth under aspen. If this pattern prevails after the analysis is complete, it will indicate that root C input is a more important flux for aspen SOC than aboveground litterfall. This, in turn, would mean that a shift from aspen to conifer will change the source of SOC, which might decrease the annual C input fluxes to soils currently occupied by aspen forests.