Submitted to: Agriculture Ecosystems and the Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 23, 2008
Publication Date: January 5, 2009
Citation: Franzluebbers, A.J., Stuedemann, J.A. 2008. soil-profile organic carbon and total nitrogen during 12 years of pasture management in the Southern Piedmont USA. Agriculture Ecosystems and the Environment. 129:28-36. Interpretive Summary: Organic carbon and nitrogen in soil are important components of maintaining soil fertility. Accumulation of organic carbon in soil is also a mechanism that helps to reduce carbon dioxide concentration in the atmosphere. Various land management systems have the potential to sequester carbon from the atmosphere through photosynthesis and subsequent storage of organic matter in soil. By providing more carbon input from photosynthesis than carbon output from decomposition, soils can sequester carbon and help mitigate the greenhouse effect. Researchers at the USDA-Agricultural Research Service in Watkinsville Georgia measured soil organic carbon and nitrogen during a 12-year period of different pasture management systems and found that sequestration of carbon and nitrogen occurred in the surface foot of soil, but not deeper. Farmers, scientists, and environmental specialists can benefit from this information to guide more effective sampling strategies and to acquire better estimates of soil carbon sequestration with cattle grazing systems. Significant sequestration of carbon in soils under pasture is possible, given that pastures occupy more than 100 million acres in the United States alone. Policy makers need this information to design effective strategies to mitigate rising carbon dioxide concentration in the atmosphere.
Technical Abstract: Soil organic C (SOC) and total soil N (TSN) sequestration estimates are needed to improve our understanding of management influences on soil fertility and terrestrial C cycling related to greenhouse gas emission. We evaluated the factorial combination of nutrient source (inorganic, mixed inorganic and organic, and organic as broiler litter) and forage utilization (unharvested, low and high cattle grazing pressure, and hayed monthly) on soil-profile distribution (0-150 cm) of SOC and TSN during 12 yr of pasture management on a Typic Kanhapludult in Georgia. Nutrient source had very few significant effects on SOC and TSN in the soil profile, despite addition of 73.6 Mg/ha (dry weight) of broiler litter during 12 yr of treatment. At the end of 12 yr, contents of SOC and TSN at a depth of 0-90 cm under haying were only 82 +/-5% of those under grazed management. Within grazed pastures, contents of SOC and TSN at a depth of 0-90 cm were greatest within 5 m of shade and water sources and only 83 +/- 7% of maximum at a distance of 30 m and 92 +/- 14% of maximum at a distance of 80 m, suggesting significant lateral redistribution of organic matter within pastures due to animal behavior. Linear change in SOC (0-90 cm) followed the order: low grazing pressure (1.17 Mg C/ha/yr) > unharvested (0.64 Mg C/ha/yr) = high grazing pressure (0.51 Mg C/ha/yr) > hayed (-0.22 Mg C/ha/yr). Linear change in TSN (0-90 cm) followed a similar order: low grazing pressure (278 kg N/ha/yr) = high grazing pressure (246 kg N/ha/yr) = unharvested (224 kg N/ha/yr) > hayed (163 kg N/ha/yr). Significant decline in SOC with time occurred in most management systems from 60-150 cm (-0.49 +/- 0.11 Mg C/ha/yr), which negated a moderate portion of that sequestered within 0-60 cm (0.74 +/- 0.60 Mg C/ha/yr). However, TSN increased with time at 0-60 cm (177 +/- 51 kg N/ha/yr) and at 60-150 cm (119 +/- 17 kg N/ha/yr). This study demonstrated that surface accumulation of SOC and TSN occurred, but that increased variability and loss of SOC with depth reduced the significance of surface effects.