LOBLOLLY-PINE PLANT COMMUNITY EFFECTS ON SOIL CARBON AND NITROGEN

Previous studies suggest that shifts in plant community structure may alter soil nutrient concentrations and availability. A study was conducted to determine the influence of species composition in loblolly pine (Pinus taeda L.) plant communities on surface soil nitrogen (N) and carbon (C) concentrations and potential mineralization after 7 years of growth and development. The study site is located in the Lower Coastal Plain physiographic region near Tallassee, AL. The soil is classified as a Cowarts loamy sand (Fine-loamy, siliceous, thermic Typic Kanhapludults). Utilizing herbicides, distinct plant communities were established in 1984 including herbaceous-hardwood-pine (HHwP), herbaceous-pine (HP), hardwood-pine (HwP) and pine only (P). Organic C and N and potential C and N mineralization were determined on surface soil samples (0 to 5-, 5 to 10-, and 10 to 20-cm depth increments) collected in February 1991. After 7 years, soil organic N differed among the plant communities; soil organic N was lower in the P than in communities including herbaceous species (HHwP and HP). These differences in soil organic N resulted from a net gain under HHwP and HP, rather than net loss under the P community. Gains in soil N may have been due to symbiotic N fixation. In laboratory incubations, respiration and N mineralization were lower in soil from the P than the HHwP and HP communities. Substrate quality as indicated by forest floor C/N and as manifested by relative N mineralization and C/N mineralized were also reduced in the P community compared with others. Our results suggest that silvicultural manipulation of loblolly pine communities may affect soil C and N and, in turn, could influence long-term site quality.