Soil CO2 Efflux in Two Loblolly Pine Families Is Suppressed by High Levels of Fertilization.

The metabolic activity of heterotrophic microbes and roots can be directly assessed with measurements of soil CO2 efflux. In many experiments, soil CO2 efflux has been suppressed through the addition of N-fertilizer, which if related to microbial activity, could be indicative of increased accumulation of soil organic matter. In contrast, if soil CO2 efflux is suppressed because of decreased allocation to roots, then soil organic matter could decrease because of less belowground C input. Here we report how the soil CO2 efflux in two families of loblolly pine, from seed sources in Florida and Texas, responded to two levels of N+P fertilizer addition. The experiment was located in Austin Cary Memorial Forest (29º44’N, 82º09’30’’W) near Gainesville, Florida on a Spodosol (Pomona series). In 2000, seedlings were planted at a 1.8 x 3.0 m spacing in 30 x 30 m plots across 4 blocks. Efflux measurements began February 23, 2009 and have since been repeated weekly. Because soil CO2 efflux measurements were collected under relatively invariant temperatures, the data were normalized to the y-intercept (assuming a Q10=2.0) so that treatments could be directly compared. The results indicate that the stand of faster growing Florida loblolly family had a marginally greater (p=0.08) soil CO2 efflux than the slower growing Texas family, suggesting either greater allocation to roots or greater microbial activity. In contrast, the fertilizer application suppressed respiration by 19% (p<0.05). The Fert x Fam interaction was not significant (p=0.35). These results suggest that rates of belowground C cycling have been significantly suppressed by fertilization. Ongoing measurements of soil organic matter and root respiration will be used to deduce which component of soil CO2 efflux was suppressed by fertilization.