Marin Palmer1, David Rothstein2, Jodi Forrester1, David Mladenoff1 and Anthony D'Amato3, (1)Forest and Wildlife Ecology, University of Wisconsin, Madison, WI (2)Forestry, Michigan State University, East Lansing, MI (3)Forest Resources, University of Minnesota, St. Paul, MN
Short-rotation woody biofuels crops (SRWC) have been proposed as one of the major feedstocks for bioenergy generation in the Midwest region. While the general consensus is that these crops have a favorable energy and greenhouse gas (GHG) balance compared to fossil fuel alternatives, the environmental impacts of widespread land conversion and the magnitude of potential GHG benefits are highly uncertain. In our study, we are investigating the effects of converting open lands to willow and hybrid-poplar plantations on soil N cycling, N2O emissions, and NO3- leaching at six sites of varying soil and climate conditions across northern Michigan and Wisconsin. At two of these sites we are developing detailed budgets of net ecosystem production and GHG balances.
All six sites responded to establishment with increased N2O emissions, available inorganic N, and NO3- leaching; however, the magnitude of these impacts varied dramatically among sites. The range of N2O emissions in the first growing season was 30-fold among sites, from 0.50 to 17 Mg-CO2eq ha-1. These large N-losses have implications for long-term site fertility, eutrophication, and greenhouse gas emissions. Including these N impacts in LCA studies and better estimating the site-to-site variability through process modeling will be essential in estimating the full environmental impacts of SRWC plantations. At the two intensively-studied sites we found that SRWC plantations were a net source of GHG to the atmosphere over the first two years of production. Across the two sites and treatments the net global warming potential of SRWC ranged from 9 to 24 Mg CO2 equivalents per hectare for the first two years of plantation growth. Thus we conclude that plantation establishment has incurred a substantial GHG debt. The time required to pay back this debt will depend on the rate at which these plantations can transition from GHG source to GHG sink, as well as the maximum rates of biomass production that are eventually reached.