Conversion of open lands to short‐rotation woody biomass crops: site variability affects nitrogen cycling and N2O fluxes in the US Northern Lake States

摘要

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 (Nsub2/subO) emissions, and nitrate (NOsub3/subsup−/sup) 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 Nsub2/subO emissions, available inorganic N, and, where it was measured, NOsub3/subsup−/sup leaching; however, the magnitude of these impacts varied dramatically among sites. Soil NOsub3/subsup−/sup levels varied threefold among sites, with peak extractable NOsub3/subsup−/sup concentrations ranging from 15 to 49 g N kgsup−1/sup soil. Leaching losses were significant and persisted through the second year, with 44–112 kg N hasup−1/sup leached in SRWC plots. Nsub2/subO emissions in the first growing season varied 30-fold among sites, from 0.5 to 17.0 Mg-COsub2/subsubeq/sub hasup−1/sup (carbon dioxide equivalents). Nsub2/subO emissions over 2 years resulted in Nsub2/subO emissions due to plantation establishment that ranged from 0.60 to 22.14 Mg-COsub2/subsubeq/sub hasup−1/sup 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.