Assessing Soil Organic Carbon Stocks in Fire-Affected Pinus Palustris Forests

Abstract:

This study aimed to quantify the vertical distribution of soil organic carbon (SOC) and its biochemically resistant fraction (SOC_R; defined as residual SOC following H₂O₂ treatment and dilute HNO₃ digestion) in managed longleaf pine (LLP) stands located at Fort Benning, Georgia, USA (32.38 N., 84.88 W.). Although it is unclear how to increase SOC_R via land management, it is a relatively stable carbon (C) pool that is important for terrestrial C sequestration. SOC concentration declines with soil depth on upland soils without a spodic horizon; however, the portion that is SOC_R and the residence time of this fraction on LLP stands is unknown. Soils were collected by depth at five sites with common land use history, present use for active military training and a three-year prescribed fire return cycle. Soils were treated with H₂O₂ and dilute HNO₃ to isolate SOC_R. In the upper 1-m of soil SOC stocks averaged 72.1 ± 6.6 Mg C ha^-1 and SOC_R averaged 25.8 ± 3.2 Mg C ha^-1. Depending on the site, the ratio of SOC_R:SOC ranged from 0.25 to 0.50 in the upper 1-m of soil. On clayey soils the ratio of SOC_R:SOC increased with soil depth. One site containing 33% clay at 50 to 100 cm depth had a SOC_R:SOC ratio of 0.68. The radiocarbon age of SOC_R increased with soil depth, ranging from approximately 2,000 years before present (YBP) at 0 to 10 cm to over 5,500 YBP at 50 to 100 cm depth. Across all sites, SOC_R makes up a considerable portion of SOC. What isn’t clear is the proportion of SOC_R that is of pyrogenic origin (black carbon), versus SOC_R that is stabilized by association with the mineral phase. Ongoing analysis with 13C nuclear magnetic resonance spectroscopy will provide data on the degree of aromaticity of the SOC_R and some indication of the nature of its biochemical stability.