Estimating carbon source-sink transition during secondary succession in a Virginia valley.

Background and aims: A quantitative understanding of how carbon budgets shift throughout old-field succession is lacking and we provided estimates of annual net ecosystem production (NEP) and net primary production (NPP) for six successional sites. Methods: We utilized 4 years of field measurements of herbaceous and shrub biomass, 2 years of field measurements of tree diameters, 3 years of field measurements of soil surface CO flux, and allometric relations to estimate NEP (2006) and NPP (2004-2006) for six successional sites ranging from 5 years to 90 years old in the Shenandoah Valley, Virginia. Results: NEP of the two late successional sites (1,366 g C/m and 883 g C/m) was significantly greater than that of the two early successional fields (−209 g C/m and 16 g C/m) in 2006. Net primary production (NPP) also increased from early to late succession. The results follow the proposed conceptual model that successional ecosystems are carbon sources at the beginning of secondary succession and then become carbon sinks, but the progression is not linear. Conclusions: This study suggests that the switching point of secondary succession from releasing carbon to sequestering carbon in a temperate, humid climate is approximately between 5 and 19 years following agricultural abandonment. [ABSTRACT FROM AUTHOR]