Carbon and energy balances for cellulosic biofuel crops converted from CRP lands

Background/Question/Methods

Cellulosic biofuels produced on lands not used for food production have the potential to avoid competition for food and associated indirect land use costs. Understanding the carbon and energy balance implications for different cellulosic production systems is important for the development of decision making tools and policies. Here we present carbon and energy balances of alternative agricultural management and land-use decisions. We use 20 years of data from KBS LTER experiments, together with results from a set of Conservation Reserve Program (CRP) fields recently converted to biofuel production, to produce farm level CO₂ and energy balances for different management practices. Measurements include fluxes of N₂O and CH₄, soil organic carbon change, agricultural yields, and agricultural inputs (e.g. fertilization and farm fuel use). We additionally use measurements of field-level CO₂ exchange to estimate real-time carbon costs of converting CRP grasslands back to agricultural production.

Results/Conclusions

Our results indicate that management decisions such as tillage and plant types have a great influence on the net carbon and energy balances and benefits of cellulosic biofuels production. Specifically, we show that cellulosic biofuels produced from an early successional, minimally managed system have a net C sequestration (i.e., negative balance) of -464±31 gCO₂e m^-2 yr^-1 vs. -230±92 gCO₂e m^-2 yr^-1 for more productive and management intensive alfalfa, and vs. 10±153 gCO₂e m^-2 for poplar. The reference agricultural system (a conventionally tilled corn-soybean-wheat rotation) has net sequestration of -41±33 g CO₂e m^-2 yr^-1, despite greater fuel yields. We show that cellulosic biofuels have an energy efficiency similar to grain production systems if the latter systems are not used for livestock feed.

Additionally, we show that converting marginal farmland now in CRP or successional vegetation into cellulosic biofuel production – keeping it out of grain production – has large environmental benefits. C sequestration due to conversion of non-productive lands to CRP grasslands and harvest of cellulosic biomass for renewable fuel production have a negative greenhouse gas impact of  ~ -150 g CO₂e per MJ of energy produced as compared to conversion of CRP grasslands to grain based biofuel systems, which will effectively emit ~1720 g CO₂e MJ^-1 for about a century.