Old-field succession: Land-use legacies and below-ground respiration

Background/Question/Methods

In order to investigate the effects of ~80 years of old-field succession in the SC Calhoun Critical Zone Observatory (CCZO) regarding soil biogeochemistry, chronosequence landuse history plots have been instrumented and studied. Chronosequence plots consist of 3 reference hardwood forest plots, 3 plowed agriculture plots, and 3 old-field secondary pine forest plots. In this framework, reference hardwood soil profiles are minimally degraded in terms of erosion, soil structure, and soil biogeochemistry. Each landuse history comparison plot has four 750ml gas reservoirs buried at 0.5m, 1.5m, 3m, and 5m with sampling ports at the surface for in situ CO₂ and O₂analyses as well as soil gas collection for laboratory analyses. Plowed agricultural plots represent degraded, non-regenerated states, while old-field pine forest plots are expected to be intermediately degraded. This investigation seeks to determine how different landuse histories and above ground ecology among continuously cultivated, old-field secondary forests, and reference hardwood plots are reflected in soil profile metabolic gas concentrations of oxygen and carbon dioxide.

 Results/Conclusions

Results indicate that hardwood forest soils generally have higher summer CO₂ concentrations than agricultural or old-field pine forests during peak growing seasons but undergo rapid declines in shallow soil CO₂ concentrations at 0.5m and 1.5m relative to pine forests during fall and winter. Old-field pine forest soil CO₂ is frequently greater than reference hardwood forest soils during winter months. This seasonal response is likely due to an additive interaction between forest metabolism and soil profile structural characteristics. Root respiration and CO₂ efflux drops dramatically in deciduous communities compared to pine-dominated evergreen communities in the fall and winter. Additionally, landuse history legacies reflected in soil macroporosity are thought to play a role as intact and extensive macropores in hardwood forests may allow for soil gasses to evade more rapidly into the above-ground atmosphere relative to previously deforested and plowed old-field pine forests. Agricultural soils have much lower CO₂ concentrations relative to either forested condition and serve to highlight the degree of regeneration and vertical propagation of respiring old-field pine forest root networks. Oxygen concentrations are reciprocal to CO₂ and serve to confirm and reflect the metabolic aerobic consumption of O₂ belowground.