Characterizing Soil Organic Carbon Recalcitrance in Longleaf Pine (Pinus palustris Mill) Stands

Abstract:

Historically, longleaf pine (LLP) stands in the southeastern US experienced frequent fires. Today managed LLP stands are burned at 2-5 year intervals to reduce fuels and hardwood competition and manage for biodiversity. These are not stand replacing fires, though considerable amounts of biomass are burned and the conversion rate to biochemically stabilized black carbon (BC) is unknown. The primary mechanisms for long-term carbon sequestration in soil are mineral association, biochemical transformation (e.g. pyrogenesis) and physical protection. We quantified the recalcitrance of soil organic carbon (SOC) and its oxidation resistant fraction (SOC_R; defined as residual SOC following H₂O₂ treatment and dilute HNO₃ digestion) using radiocarbon dating (SOC and SOC_R) and benzene polycarboxylic acids (BPCA) as molecular markers for polyaromatic C associated with BC. Mineral stabilized C is largely represented by SOC_R contents and BC by total BPCA contents. Soils were collected by depth (0-10, 10-20, 20-50, 50-100 cm) at 14 managed LLP stands in Louisiana (LA), Georgia (GA) and North Carolina (NC) burned every two to five years. Across all sites, SOC and SOC_R contents declined with soil depth, though SOC_R:SOC increased with depth (0.13, 0.15, 0.22, 0.31). SOC_R was more ¹⁴C depleted than SOC and Δ¹⁴C values became more negative with soil depth (SOC_R: -195, -318, -458, -553 vs. SOC 23, -39, -156, -334), indicating that SOC_R had a much longer mean residence time. The Δ¹⁴C values correspond to mean ages of SOC_R ranging from 1777 to 6969 years and of SOC from 84 to 3319 years. We obtained very low BPCA yield from SOC_R, and it is unclear whether BC was absent or not accessible with the BPCA method. Preliminary analysis of total BPCA (bulk soil) indicates interactions between soil series and depth. Total BPCA concentration of SOC in the upper 10 cm was 136 g kg^-1 C in LA and more than six times the concentration in GA and NC. On deep sands in NC, the highest BPCA concentration of SOC was found at 50-100 cm (68 g kg^-1 C), which was three times higher than in GA or LA at that depth. Variation in bulk soil Δ¹⁴C was largely explained by SOC_R:SOC and soil depth (combined partial R² 0.84), while BPCA:SOC contributed an additional 0.02 partial R². This implies that SOC recalcitrance is largely controlled by mineral association versus pyrogenic transformation.