In the southeastern United States, fertilization and weed control treatments, along with the deployment of genetically improved planting stock, are routinely used to increase aboveground productivity.  This project examined the effects of intensive management and genetic selection of loblolly pine (Pinus taeda L.) on soil CO₂ efflux and carbon cycling. In Florida, two field installations of two families of loblolly pine, one “fast” grower and one “slow” grower, were studied in a replicated, family block design with two levels of nitrogen and phosphorus fertilization (high and low culture). Measurements of root biomass and repeated measurements of forest growth, soil CO₂ efflux, and litterfall were used to determine C allocation patterns.  Soil CO₂ efflux and litterfall measurements were used to estimate Total Belowground Carbon Flux (TBCF), giving an estimate of C allocation to roots. 

The high culture treatment and family had a significant (p<0.01) effect on loblolly pine mean aboveground biomass increment at both sites, with the fertilization effect being nearly 2x greater than the family effect.  The high culture significantly (p<0.05) decreased soil CO₂ efflux, fine root biomass, and TBCF in three of the four families studied.  Litterfall was significantly increased by greater fertilization and the family effect was significant.   The Family x Fertilization effect for TBCF was significant at Study B, indicating that one family did not significantly decrease TBCF in response to the heavier fertilization treatment.  Results from these studies suggest that increasing levels of fertilization reduced belowground allocation and likely C inputs to the soil in these managed loblolly pine ecosystems.