Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/18/2007
Publication Date: 1/14/2008
Citation: Causarnao, H.J., Franzluebbers, A.J., Shaw, J.N., Reeves, D.W., Raper, R.L., Wood, C.W. 2008. Soil organic carbon fractions and aggregation in the Southern Piedmont and Coastal Plain. Soil Science Society of America Journal. 72:221-230. Interpretive Summary: A long history of tillage and soil erosion has depleted soil organic matter and subsequently degraded soil quality in the southeastern USA. Research has shown that conservation agriculture (including pasture) is able to restore soil organic matter levels, but there is a need for a broad scale evaluation to properly assess the impacts of changing agricultural practices. ARS scientists at the J. Phil Campbell Sr. Natural Resource Conservation Center, Watkinsville, GA and the Soil Dynamics Research Unit in Auburn, AL, cooperated with scientists from Auburn University and the University of Asuncion, Paraguay to conduct an on-farm survey of soil organic matter levels under long-term conventional tillage, conservation tillage, and pasture in 87 fields distributed within the Southern Piedmont and Coastal Plain Major Land Resource Areas of Alabama, Georgia, South Carolina, North Carolina and Virginia. Across locations, soil organic matter under pastures was 75% higher than under conventional tillage and 39% higher than under conservation tillage. Higher surface clay content and precipitation, and cooler temperature contributed to higher soil organic matter. Our results complement experiment-station data, and the information can be used by USDA-NRCS, extension agents and producers to promote pasture-based systems and conservation tillage on 202 million acres in the southeastern USA. Restoration of higher soil organic matter throughout the region will result in improved soil quality, plant productivity and the potential for mitigating global warming.
Technical Abstract: Quantifying the impact of long-term agricultural land use on soil organic C (SOC) is important to farmers and environmental policy makers but few studies have characterized land use and management effects on SOC levels across wide-scale physiographic regions. We measured the total stock of SOC at a depth of 0-20 cm (TOC) and its stratification ratio under conventional tillage (CvT), conservation tillage (CsT), and pasture in 87 fields in the Southern Piedmont and Coastal Plain Major Land Resource Areas. Across locations, TOC followed the order: pasture (38.9 Mg ha-1) > CsT (27.9 Mg ha-1) > CvT (22.2 Mg ha-1). Variation in TOC was explained by management (41.6%), surface horizon clay content (5.2%), mean annual temperature (1.0%), and mean annual precipitation (0.1%). Higher surface clay content and precipitation, and cooler temperature contributed to higher TOC. Management affected TOC primarily at the soil surface (0-5 cm). All SOC fractions (i.e., TOC, particulate organic C, soil microbial biomass C, and potential C mineralization) were strongly correlated across a diversity of soils and management systems (r from 0.85 to 0.96). Our results agree with a threshold value of 2 for stratification ratio of SOC fractions to distinguish previously degraded soils with improved soil quality from degraded soils. This on-farm survey of TOC complements experimental data and shows that sod-based systems and conservation tillage will lead to significant SOC sequestration throughout the region, resulting in improved soil quality, plant productivity and the potential for mitigating global warming.