Conversion of open lands to short‐rotation woody biomass crops: site variability affects nitrogen cycling and N 2 O fluxes in the US Northern Lake States

Short‐rotation woody biomass crops ( SRWC ) have been proposed as a major feedstock source for bioenergy generation in the Northeastern US. To quantify the environmental effects and greenhouse gas ( GHG ) balance of crops including SRWC , investigators need spatially explicit data which encompass entire plantation cycles. A knowledge gap exists for the establishment period which makes current GHG calculations incomplete. In this study, we investigated the effects of converting pasture and hayfields to willow ( Salix spp.) and hybrid‐poplar ( Populus spp.) SRWC plantations on soil nitrogen (N) cycling, nitrous oxide (N 2 O) emissions, and nitrate ( NO 3 − ) leaching at six sites of varying soil and climate conditions across northern Michigan and Wisconsin, following these plantations from pre conversion through their first 2 years. All six sites responded to establishment with increased N 2 O emissions, available inorganic N, and, where it was measured, NO 3 − leaching; however, the magnitude of these impacts varied dramatically among sites. Soil NO 3 − levels varied threefold among sites, with peak extractable NO 3 − concentrations ranging from 15 to 49 g N kg −1 soil. Leaching losses were significant and persisted through the second year, with 44–112 kg N ha −1 leached in SRWC plots. N 2 O emissions in the first growing season varied 30‐fold among sites, from 0.5 to 17.0 Mg‐ CO 2 eq  ha −1 (carbon dioxide equivalents). N 2 O emissions over 2 years resulted in N 2 O emissions due to plantation establishment that ranged from 0.60 to 22.14 Mg‐ CO 2 eq  ha −1 above baseline control levels across sites. The large N losses we document herein demonstrate the importance of including direct effects of land conversion in life‐cycle analysis ( LCA ) studies of SRWC GHG balance. Our results also demonstrate the need for better estimation of spatial variability of N cycling processes to quantify the full environmental impacts of SRWC plantations.