Soil CO2 flux and its controls during secondary succession

The area of abandoned agricultural lands has substantially increased over the past several decades. These ubiquitous land use changes can be significant contributors to the dynamics of the global carbon budget. However, the effects of these land use dynamics (i.e., secondary succession) on carbon cycling, and particularly on soil CO2 efflux, are rather sparsely addressed in the literature. Variations in soil CO2 flux during secondary succession may explain the hypothesized shift from carbon source to carbon sink as succession proceeds. We investigated soil CO2 flux of early, mid, and late succession ecosystems in northern Virginia from 2003 to 2006. Soil temperature, moisture, total carbon, total nitrogen, and soil C/N were measured along with soil CO2 flux. Despite the wide array of changes in both physical conditions and vegetation composition/structure that occur during successional shifts, our results showed that soil CO2 flux did not vary throughout succession. Our results support previous findings that soil temperature has a major influence on soil CO2 flux, and we additionally found that the overall sensitivity of soil CO2 flux to soil temperature did not change during succession. Soil moisture had a significant influence on soil CO2 flux only at the two driest sites within the successional sequences (one early and one late successional site), where both root and microbial activities are potentially limited by soil water conditions. The monthly soil CO2 fluxes were significantly different among successional stages in 9 out of the 12 months during the year. The soil CO2 fluxes showed strong seasonality and differences among sites within seasons; however, these differences balanced out at the annual scale. The invariance in annual soil CO2 fluxes throughout succession may indicate that soil CO2 flux overall is independent of any shift from early succession carbon source to later succession carbon sink. Results also indicate that annual soil CO2 budgets are likely determined by the climatic conditions under which succession proceeds. While the transient vegetation types of secondary succession alter the seasonal soil CO2 fluxes, they may not affect the annual soil CO2 output.