Reduction Of Forest Floor Respiration By Fertilization On Both Carbon Dioxide-Enriched And Reference 17-Year-Old Loblolly Pine Stands

Elevated atmospheric carbon dioxide (CO₂^e) increases soil respiration rates in forest, grassland, agricultural and wetland systems as a result of increased growth, root biomass and enhanced biological activity of soil microorganisms. Less is known about how forest floor fluxes respond to the combined effects of elevated CO₂ and nutrient amendments; until now no experiments have been in place with large forest trees to allow even preliminary investigations. We investigated changes in forest floor respiration (S_ff) in a Pinus taeda L. plantation fumigated with CO₂ by using free-air CO₂ enrichment (FACE) technology and given nutrient amendments. The prototype FACE apparatus (FACEp; 707 m²) was constructed in 1993, 10 years after planting, on a moderate fertility site in Duke Forest, North Carolina, USA, enriching the stand to 55 Pa (CO₂^e). A nearby ambient CO₂ (CO₂^a) plot (117 m²) was designated at the inception of the study as a reference (Ref). Both FACEp and Ref plot were divided in half and urea fertilizer was applied to one half at an annual rate of 11.2 g N m⁻² in the spring of 1998, 1999 and 2000. Forest floor respiration was monitored continuously for 220 days – March through November 2000 – by using two Automated Carbon Efflux Systems. Thirty locations (491 cm² each) were sampled in both FACEp and Ref, about half in each fertility treatment. Forest floor respiration was strongly correlated with soil temperature at 5 cm. Rates of S_ff were greater in CO₂^e relative to CO₂^a (an enhancement of ∼178 g C m⁻²) during the measurement period. Application of fertilizer resulted in a statistically significant depression of respiration rates in both the CO₂^a and CO₂^e plots (a reduction of ∼186 g C m⁻²). The results suggest that closed canopy forests on moderate fertility sites cycle back to the atmosphere more assimilated carbon (C) than similar forests on sites of high fertility. We recognize the limitations of this non-replicated study, but its clear results offer strong testable hypotheses for future research in this important area.