Changes in dead plant carbon over 50 years of old-field forest development

Background/Question/Methods

This study examined the evolution of a forest ecosystem carbon budget over fifty years of old-field forest development at the Calhoun Long Term Soil-Ecosystem Experiment (LTSE) in South Carolina, USA. At this site, biogeochemical and ecosystem changes associated with old-field forest development had been documented through repeated tree measurements and deep soil sampling and archiving. We most recently compiled the 50-year C budget of the Calhoun forest, quantifying the increasing importance of detrital C to the ecosystem over time. In addition to compiling allometric estimates of live tree biomass, we quantified the accumulation of woody detritus on the surface of the soil as well as in the soil profile over fifty years and estimated the mean residence times of that detrital carbon storage. We then investigated how organic matter has been incorporated into mineral soils by examining concentrations of dissolved organic carbon and other constituents in soil solutions throughout the ecosystem profile. We quantified changes in solid state soil carbon quantity and quality, both in bulk soils and in soil fractions that have different C sources, stabilities, and residence times. 

Results/Conclusions

The long-term Calhoun ecosystem study demonstrates that 50 years of pine forest development on a degraded cotton field have not increased mineral soil carbon storage in the upper 60-cm of mineral soil. Tree biomass accumulated rapidly over the first 30 years of forest development, followed by an accumulation of leaf litter and woody detritus. Large quantities of dissolved organic carbon leached from the O-horizons into mineral soils. The response of mineral soil C stocks to this flood of C inputs varied by depth.  The most surficial soil (0-7.5cm) saw a large, but lagged, increase in concentrations of soil organic carbon (SOC) over time, an accumulation almost entirely due to an increase of light fraction, particulate organic matter. Yet in the deepest soils sampled (35-60cm), soil carbon content declined over time, and in fact the loss of SOC in deep soils was sufficient to negate all of the C gains in shallower soils. This deep soil organic matter was apparently lost from a poorly understood pool of SOM with decadal turnover. These surprising results indicate that large accumulations of forest carbon aboveground do not guarantee similar changes throughout the belowground soil profile.