This project will help us understand the environmental impacts of using retired agricultural lands to produce woody biofuels.

Research question

The deployment of willow and poplar biomass plantations on open lands in northern Michigan lands is gaining increased interest.  Willow and poplar plantations have the potential to greatly reduce the land base needed to produce woody biomass feedstock since their rates of production are 5-10 times greater than that of existing forestland. These open lands consist of a mixture of agricultural land in current crop production, abandoned agricultural land, and pasturelands, and make up a significant portion of the landscape the region.  Nevertheless, the potential environmental benefits and long-term sustainability of woody-biomass plantations remain uncertain.  Plantations have the potential to further reduce net greenhouse gas (GHG) emissions beyond simply providing a substitute for fossil fuels, if a portion of the CO₂ assimilated into belowground biomass and aboveground residues can be sequestered in long-term soil carbon (C) storage.  Conversely, soil C loss, emissions of greenhouse gases (CO₂, N₂O and CH₄) and leaching of nitrate to ground and surface waters all have the potential to undermine the environmental benefits of biomass energy production.  We are investigating the environmental impacts of converting open lands to willow and poplar biomass plantations across a range of sites in the Upper Peninsula and northern Lower Peninsula.

Approach

Our approach to this project will be to collect detailed data on baseline soil C stocks, soil fertility and GHG fluxes prior to establishment of plantations across a range of underlying soil conditions.  We will then monitor fluxes of GHGs from soils to the atmosphere, and fluxes of N to groundwater in willow plantations, poplar plantations and unconverted reference plots.  These data, together with soil C data and GHG fluxes, will be used to develop improved life cycle assessments for these systems that more accurately assess their true environmental footprint.  Key milestones for this project include: (1) collection of baseline condition data; (2) quantification of GHG emissions and nutrient leaching associated with plantation establishment; and (3) report/manuscript preparation.

Progress and Results

At the two Upper Peninsula sites established in 2008 and 2009 (WR and SK; Figure 1) we sampled CO₂, N₂O and CH₄ fluxes at least monthly from May through November of 2009.  Conversion of both these sites to woody biofuels crops resulted in rapid spikes in nitrification rates and soil NO₃^- pools which, in turn, drove 20- to 40-fold increases in soil N₂O emissions relative to undisturbed areas of the same sites (Figure 2). Soil CO₂ efflux also increased somewhat; however, large differences in the contribution of autotrophic respiration between controls and treated plots make it difficult to quantify soil C loss from CO₂ efflux measurements alone.  The increase in N₂O flux compared to the undisturbed controls resulted in a GHG “debt” equivalent to 5-10 Mg CO₂ ha^-1 from N₂O emissions in the first growing season alone.  This compares to reported rates of aboveground biomass sequestration in Upper Michigan plantations of 15 and 10 Mg-CO₂ ha^-1 y^-1 for poplar and willow, respectively, meaning that land conversion at these sites has incurred a minimum GHG debt equivalent to nearly 1 year of feedstock production from direct N₂O flux alone.

Attachments

Figure 1: Map of study site locations

Figure 2: Fluxes of (a) CO2, (b) N2O and (c) CH4 from the SK site for 2009.