Surface soil carbon size-density fractions altered by loblolly pine families and forest management intensity for a Spodosol in the southeastern US

The effects of genotypic differences on soil organic carbon (SOC) cycling and their interactions with forest management systems are poorly understood. This study was undertaken to examine the effects of family and family x management interactions on SOC and to evaluate the distribution of SOC across different size-density fractions in a forested Spodosol. The study site consisted of a 6-year-old loblolly pine plantation that was managed under two intensities (high and low level of fertilization and chemical understory control) and included three full-sib families (fast, medium and slow growers, designated a priori based on above ground growth). The fast growing family exhibited 59% higher C (p=0.04) than the slow growing family in the 2,000 to 250-mu m light density fraction. The medium grower exhibited 8% higher aggregate C than the fast grower in the 250 to 150-mu m medium density (p=0.05) and 2,000 to 250-mu m heavy density (p=0.06) fractions. Family and management effects were detected among all three density fractions. The 2,000 to 250-mu m medium density fraction contained the most C. A modified size-density fractionation and sonication procedure proved sensitive for detecting significant family and management effects in as few as 6 years. These results highlight the potential of genotypic deployment as a factor influencing C sequestration and the need to fully understand the long-term effects of genotypic differences and forest management activities on SOC pools.