Skinner, Howard (USDA-ARS, University Park, PA USDA-ARS, Pasture Systems & Watershed Manageme, University Park, PA, 16802; Phone: 814-863-8758; Fax: 814-863-0935; Email: howard.skinner@ars.usda.gov)

Management Effects on Carbon Dioxide Exchange over Northeastern Pastures

H. Skinner *

Land use in the Northeastern U.S. is diverse, with more than half the land area in forest, one-fourth used for agriculture, and about 14% in urban and other land uses. Grasslands occupy 3.5 million ha in this region and are becoming more important as components of low cost, sustainable, and environmentally sound forage-livestock production systems. Most available data on grassland carbon sequestration are from Great Plains and Western rangelands. Because of greater rainfall and a longer growing season, grasslands in the Northeastern U.S. have a greater capacity for plant biomass production than do rangelands. However, these same conditions also favor increased plant and soil respiration. The net effect of the balance between greater productivity and respiratory loss on carbon storage is not known. Net carbon storage also depends on biomass removal and utilization. In animal production systems, biomass is removed from pastures either by grazing or mechanical harvest. Because a direct relationship exists between carbon storage and the amount of plant residues returned to the soil, the method and timing of biomass removal can significantly affect soil carbon pools. Eddy covariance systems monitored carbon dioxide fluxes over two pastures in Central Pennsylvania for two years. The first pasture was planted to mixed cool-season grasses in 1982 (Grass pasture) while the second was planted to alfalfa in 1995 (Alfalfa pasture) but by the time of the experiment also contained a significant cool-season grass component. Pastures were cut for hay or grazed 3 to 4 times per year. Flux measurements indicated that both pastures were significant carbon dioxide sinks in 2003, with the Alfalfa pasture taking up 959 and the Grass pasture 730 g CO₂ m^-2. However, in 2004, the Alfalfa pasture was a much smaller sink, accumulating only 166 g CO₂ m^-2. The Grass pasture became a source in 2004, losing 421 g CO₂ m^-2 to the atmosphere. When the biomass removed by grazing and haying was taken into consideration, both pastures remained sinks in 2003 with an average accumulation of 500 g CO₂ m^-2, but both pastures were sources in 2004, losing 315 g CO₂ m^-2 to the atmosphere. The summer of 2004 was cooler than 2003 while the rest of the year was generally warmer. Both years were relatively wet. Differences in environmental conditions were not sufficient to explain the large differences in fluxes between the two years. The timing of harvest events appeared to have the greatest impact on net carbon sequestration. Spring harvests that were timed to maximize forage quality and animal production had a negative effect on the carbon storage potential of the pastures. Harvesting right before the critical fall regrowth period also negatively affected carbon sequestration. Haying removed more biomass from the pastures than did grazing and, thus, had a more negative effect of carbon sequestration. Management practices that maximize animal productivity may not be the best for maximizing carbon accumulation.