Implications of Observed and Simulated Soil Carbon Sequestration for Management Options in Corn-based Rotations.

Managing cropping systems to sequester soil organic C (SOC) improves soil health and resilience to changing climate. Perennial crops, no-till planting, manure, and cover crops can add SOC; however, their impacts have not been well documented in the northeastern United States. Our objectives were (i) to monitor SOC from a bioenergy cropping study in Pennsylvania that included a corn ( L.)-soybean [ (L.) Merr.]-alfalfa ( L.) rotation, switchgrass ( L.), and reed canarygrass ( L.); (ii) to use the CQESTR model to predict SOC sequestration in the bioenergy crops (with and without projected climate change); and (iii) to use CQESTR to simulate influence of tillage, manure, cover cropping, and corn stover removal in typical dairy forage (silage corn-alfalfa) or grain corn-soybean rotations. Over 8 yr, measured SOC increased 0.4, 1.1, and 0.8 Mg C ha yr in the bioenergy rotation, reed canarygrass, and switchgrass, respectively. Simulated and measured data were significantly correlated ( < 0.001) at all depths. Predicted sequestration (8-14 Mg C ha over 40 yr) in dairy forage rotations was much larger than with corn-soybean rotations (-4.0-0.6 Mg C ha over 40 yr), due to multiple years of perennial alfalfa. No-till increased sequestration in the simulated dairy forage rotation and prevented a net loss of C in corn-soybean rotations. Simulations indicated limited impact of cover crops and manure on long-term SOC sequestration. The low solids content of liquid dairy manure is the likely reason for the less-than-expected impact of manure. Overall, simulations suggest that inclusion of alfalfa provides the greatest potential for SOC sequestration with a typical Pennsylvania crop rotation.

High biomass removal limits carbon sequestration potential of mature temperate pastures.

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 CO(2) 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.