Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/4/2007
Publication Date: 6/23/2008
Citation: Skinner, R.H. 2008. High biomass removal limits carbon sequestration potential of mature temperate pastures. Journal of Environmental Quality. 37:1319-1326. Interpretive Summary: Proper management of agricultural lands has the potential to increase soil carbon sequestration and, thus, reduce the rate that carbon dioxide concentration is increasing in the atmosphere. Converting plowed, annual cropland to perennial pasture is one potential practice that could be beneficial in this regard. However, the ability to sequester carbon decreases when land has been in pasture for several years. This research examined how much carbon dioxide was being gained or lost over a four year period from two central Pennsylvania fields that had been managed as perennial pastures for at least 35 years. A complete carbon budget must include gains and losses from physiological processes such as photosynthesis and respiration, and from management processes such as removal of harvested forage and return of manure. The pastures examined in this study were small carbon dioxide sinks when only physiological processes were considered. However, when a complete carbon budget was calculated, they became net carbon dioxide sources to the atmosphere. Heavy utilization of the forage that was produced on the pastures prevented them from sequestering additional soil carbon. This suggests that mature pastures in the northeastern USA probably cannot help slow the increase in atmospheric carbon dioxide and may actually contribute to the increase.
Technical Abstract: Decades of plowing have depleted organic carbon stocks in many agricultural soils. Conversion of plowed fields to pasture has the potential to reverse this process, recapturing organic matter that was lost under more intensive cropping systems. Temperate pastures in the northeast USA are highly productive and could act as significant sinks for carbon dioxide. However, such pastures have relatively high biomass removal as hay or through consumption by grazing animals. In addition, the ability to sequester carbon dioxide decreases over time as previously depleted stocks are replenished and the soil returns to equilibrium conditions. Eddy covariance systems were used to quantify carbon dioxide fluxes over two fields in central Pennsylvania that have been managed as pastures for at least 35 years. Net ecosystem exchange (NEE) measurements averaged over eight site-years suggested that the pastures were acting as small net carbon dioxide sinks of 19 g C m-2 y-1 (positive values = uptake). However, when biomass removal and manure deposition were included to calculate net biome productivity (NBP), the pastures were a net source of -81 g C m-2 y-1 (negative values = loss to the atmosphere). Manure generated from the hay that was consumed off site averaged 18 g C m-2 yr-1. Returning that manure to the pastures would have only partially replenished the lost carbon, and the pastures would have remained net carbon sources. Heavy utilization of the biomass produced on these mature pastures prevented them from acting as carbon sinks.