Fertilizer Management Controls the Long-Term Carbon Intensity of Bioenergy Feedstocks.

Second generation biofuels are expected to promote transportation fuel sustainability and climate change mitigation by reducing fossil fuel dependence and decreasing greenhouse gas (GHG) emissions. Dedicated perennial energy crops grown on marginally-productive croplands can provide a sustainable supply of ligno-cellulosic feedstock while enhancing ecosystem services, but compete with the expansion of annual row-crops into marginal lands. The lack of long-term soil GHG emissions and soil organic carbon (SOC) stock measurements pose a major constraint to validating modeled GHG outcomes and associated errors for both annual and perennial bioenergy feedstock production systems. Here we show that long-term (16 yr) agronomic GHG mitigation on marginal land is greater in dedicated perennial energy crop systems compared to annual row-crop systems when nitrogen (N) fertilizer rate is optimized to match bioenergy crop type. We found that converting a conventionally-managed cropping system to bioenergy crops under conservation management led to similar long-term SOC gains in both annual and perennial agroecosystems. When soil nitrous oxide (N₂O) emissions and GHG emissions from N fertilizer manufacturing were accounted for, however, SOC gains were offset partially or completely for higher N fertilizer rates. Our results from this unique long-term study empirically demonstrate the key role of management in determining the carbon intensity of both annual and perennial bioenergy production systems on marginally-productive lands.