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

Thursday, 18 December 2014
John R Butnor1, Kurt H Johnsen2, Jason A Jackson2, Peter H Anderson2, Lisa J Samuelson3 and Klaus Lorenz4, (1)USDA Forest Service, Southern Research Station, Burlington, VT, United States, (2)USDA Forest Service Southern Research Station, Research Triangle Park, NC, United States, (3)Auburn University, School of Forestry and Wildlife Sciences, Auburn, AL, United States, (4)Ohio State University Main Campus, Columbus, OH, United States
This study aimed to quantify the vertical distribution of soil organic carbon (SOC) and its biochemically resistant fraction (SOCR; 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 SOCR 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 SOCR 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 SOCR. In the upper 1-m of soil SOC stocks averaged 72.1 ± 6.6 Mg C ha-1 and SOCR averaged 25.8 ± 3.2 Mg C ha-1. Depending on the site, the ratio of SOCR:SOC ranged from 0.25 to 0.50 in the upper 1-m of soil. On clayey soils the ratio of SOCR:SOC increased with soil depth. One site containing 33% clay at 50 to 100 cm depth had a SOCR:SOC ratio of 0.68. The radiocarbon age of SOCR 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, SOCR makes up a considerable portion of SOC. What isn’t clear is the proportion of SOCR that is of pyrogenic origin (black carbon), versus SOCR 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 SOCR and some indication of the nature of its biochemical stability.