Microbial priming of ancient soil carbon in an upland forest soil

Background/Question/Methods

Forest soil carbon pools, which store about one-third of the total terrestrial organic carbon, are thought to be stable, but little is known about how global change might affect these pools. Microbial priming might reduce soil organic carbon (SOC) pools as a consequence of global change. We incubated three carbon rich horizons of a soil profile under ideal conditions (A horizon and two Bh horizons at 1 and 2.5 m deep) in a sandy spodosol in an area immediately adjacent to a long-term atmospheric CO₂ enrichment site in central Florida. The Bh horizons had a SOC mean residence time as measured by radio carbon dating of about 500 years at 1 meter and over 5,000 years at 2.5 meters. Isotopically labelled surface litter, glucose, and alanine were added to the soil to simulate potential plant inputs. Previous studies at the site showed that plant roots proliferated throughout the soil profile and at depth in response to elevated CO₂. CO₂ evolution and isotopic values were measured at intervals over a 60 day incubation period. The recalcitrant soil carbon-degrading enzymes phenol oxidase and phenol peroxidase were measured initially and at the end of the incubation.

Results/Conclusions

A priming effect (PE) was detected in all portions of the soil profile in response to substrate additions, but the strength of the effect varied with depth and by substrate. PE decreased the mean residence time of SOC at the A-horizon from about 22 years to 18 and 16 years for glucose and alanine, respectively. At the shallow Bh horizon (1 meter), PE decreased the mean residence time of SOC from about 68 years to 16 years with alanine. Glucose and litter only decreased the mean residence to about 50 years. In the deepest Bh horizon (2.5 meters), the PE reduced the mean residence time from 62 years to 14 and 32 years for alanine and litter, respectively. Conversely, glucose additions increased mean residence time to about 432 years. Soil enzymes increased significantly for most substrate additions at the two Bh horizons, but not at the surface. Different portions of the soil profile can behave quite differently to plant inputs based on soil attributes such as carbon quality, the availability and source of nitrogen, microbial community composition, and other factors. The results also suggest that some forest SOC pools might be vulnerable to certain global change scenarios, such as elevated atmospheric CO₂.