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High Biomass Removal Limits Carbon Sequestration Potential of Mature Temperate Pastures

USDA-ARS Pasture Systems and Watershed Management Research Unit, Building 3702 Curtin Road, University Park, PA 16802. Mention of a specific brand name is for identification purposes only and does not constitute endorsement by the USDA-ARS at the exclusion of other appropriate sources

^* Corresponding author (howard.skinner{at}ars.usda.gov).

Received for publication May 24, 2007. Decades of plowing have depleted organic C 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 C sinks. However, such pastures have relatively high biomass removal as hay or through consumption by grazing animals. In addition, the ability to sequester C decreases over time as previously depleted stocks are replenished and the soil returns to equilibrium conditions. The objective of this research was to use eddy covariance systems to quantify CO₂ fluxes over two fields in central Pennsylvania that had been managed as pastures for at least 35 yr. Net ecosystem exchange measurements averaged over 8 site-years suggested that the pastures were acting as small net C sinks of 19 g C m^–2 yr^–1 (positive values indicate uptake). However, when biomass removal and manure deposition were included to calculate net biome productivity, the pastures were a net source of –81 g C m^–2 yr^–1 (negative values indicate 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 C, and the pastures would have remained net C sources. Heavy use of the biomass produced on these mature pastures prevented them from acting as C sinks.

Abbreviations: NEE, net ecosystem exchange • NBP, net biome productivity