SOIL ORGANIC MATTER AND NUTRIENT CYCLING TO SUSTAIN AGRICULTURE IN THE SOUTHEASTERN USA
Location: Athens, Georgia
Title: Surface soil changes during 12 years of pasture management in the Southern Piedmont USA
Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 30, 2010
Publication Date: November 1, 2010
Citation: Franzluebbers, A.J., Stuedemann, J.A. 2010. Surface soil changes during 12 years of pasture management in the Southern Piedmont USA. Soil Science Society of America Journal. 74:2131-2141.
Interpretive Summary: Changes in surface-soil compaction and organic matter are of particular importance in determining ecosystem functioning in management systems with frequent traffic and plant removal, such as in grazed pastures. As well, sequestration of soil organic carbon and conservation of nitrogen are of keen scientific and political interests for developing management strategies to help combat climate change resulting from emission of common greenhouse gases like carbon dioxide and nitrous oxide. Scientists at the USDA-Agricultural Research Service in Watkinsville Georgia conducted a 12-year pasture experiment to investigate how compaction and soil organic matter would be affected by (1) inorganic and organic fertilization and (2) how forage was utilized. Whether nutrient source was inorganic, organic with broiler litter, or a combination of the two strategies made little difference on soil properties. The important fact was that nutrients were supplied to meet the demand by forage and subsequent animal intake. How forage was utilized had an enormous impact on the temporal development of soil properties. When forage was hayed continuously, surface residue was low, soil bulk density was high, and soil organic matter remained relatively unchanged. When forage was grazed by cattle, surface residue was low to moderate, soil bulk density was low to moderate, and soil organic matter was sequestered at high rates. We tested two grazing pressures and found that surface residue content declined, soil bulk density increased slightly, and soil organic matter was similar between low and high grazing pressure. When forage was unharvested (similar to a Conservation Reserve Program management scheme), surface residue was highest, soil bulk density was low (similar to low grazing pressure), and soil organic matter was intermediate between haying and grazing. Cattle grazing of mixed bermudagrass / tall fescue pastures can be considered a viable strategy to rehabilitate millions of acres of degraded cropland in the southeastern USA. Our data negate the perspective that only non-utilization of land will be the best strategy for rehabilitating degraded land.
Surface soil characteristics are of key importance in assessing the sustainability of agricultural management systems. 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) on soil organic matter during 12 years of pasture management on a Typic Kanhapludult in Georgia. Nutrient source had very little effect on bulk density and concentration and stocks of soil organic C (SOC) and total soil N (TSN). Forage utilization greatly affected all soil properties, more so nearest the surface than deeper in soil. For example at the end of 12 yr, SOC at a depth of 0-3 cm under low grazing pressure was 47.6 g/kg and under haying was 28.8 g/kg (P < 0.001), while at 3-6 cm it was 20.1 and 14.9 g/kg (P < 0.05), at 6-12 cm it was 12.7 and 11.0 g/kg (P > 0.05), and at 12-20 it was 8.0 and 6.3 g/kg (P > 0.05). Soil bulk density was inversely related to SOC across all samples. Total SOC (0-20 cm) plus surface residue C was 3.6 +/- 1.5 Mg C/ha greater in the zone nearest shade and water sources than farther away as a result of animal behavior and deposition of manure. Sequestration of TSN in the surface 6 cm averaged 8 kg N/ha/yr when hayed, 31 kg N/ha/yr when left unharvested, and 74 kg N/ha/yr when grazed by cattle with either low or high grazing pressure. These results indicate the large potential of well-managed grazing systems to improve the quality and functioning of soils in the southeastern USA.