Initial effects of long-term pastureland conversion to hybrid poplar and willow bioenergy systems on N leaching and greenhouse gas emissions

Nikiema P. and D. Rothstein

Presented at the GLBRC Sustainability Retreat (2010-02-10 to 2010-02-12 )

We investigated the effects of converting pastureland into woody bioenergy plantations on soil greenhouse gas fluxes and nitrate leaching. Soil CO2 emissions did not differ among treatments, suggesting that increases in heterotrophic respiration in cultivated plots were matched by greater root respiration in the pasture controls. Methane emissions increased significantly in the poplar and willow plots; however, flux rates were very low overall. In contrast, pasture conversion resulted in 39- and 29-fold increases in N2O flux in the poplar and willow plots, with rates as high as 250 ng N2O-N cm-2 h-1. Similarly, cultivation resulted in marked increases in NO3- concentrations of soil leachate which were negligible under the controls (0.1 ppm), but regularly exceeded 5 ppm in the plantations. Stimulation of nitrification following cultivation appears to be the driving factor behind increased N2O and NO3- fluxes. An initial estimate of cumulative emissions suggests that pastureland conversion resulted in a greenhouse gas “debt” equivalent to 6.2±0.7 and 5.0±1.3 Mg-CO2 ha-1 for poplar and willow plots, respectively, from N2O emissions in the first growing season alone. This compares to reported rates of aboveground C sequestration in Upper Michigan plantations of 15 and 10 Mg-CO2ha-1y-1 for poplar and willow, respectively.

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