The production of cellulosic feedstocks for second generation biorefineries offers the promise of real CO2 stabilization in addition to co-benefits related to nitrogen and phosphorus conservation, reduced soil erosion, carbon sequestration, attenuation of nitrous oxide fluxes, and biodiversity benefits. Perennial crops grown for aboveground biomass require less fertilizer, have the ability to trap nitrate and phosphorus that would otherwise be transported to groundwater and streams, and accumulate carbon in both soil organic matter and roots. And if grown in mixed-species assemblages, additionally provide biodiversity services.
This may or may not be a realistic scenario with desirable outcomes. Maximizing production of crop and forest biomass to support a cellulosic bioenergy program will require the expansion and intensification of agricultural and silvicultural practices. Intensification will almost certainly alter biogeochemical processes that will be a major aspect of these systems’ long-term sustainability. But whether intensification of one part of the landscape will help to mitigate biogeochemical excess elsewhere – including the production of indirect carbon debt – will depend on a detailed understanding of the components of the biogeochemical response and the factors that affect their complex interactions.
Results/Conclusions
Creatively designed biofuel production systems can provide net greenhouse gas mitigation at both field and landscape scales. Net mitigation can be achieved through greater soil carbon retention; lower fuel, lime, and nitrogen fertilizer use; lower N2O emissions; and a positive energy yield. Mitigation potentials will vary among production systems and with time. Soil carbon sequestration, for example, will be greatest in fertilized woody perennial crops and in native grasslands but will diminish with ecosystem age as soil carbon pools equilibrate. N2O mitigation, on the other hand, will be greatest in perennial systems with low N inputs but will not diminish with age and, with no risk of future release, will have more long-term impact. Sustainability (maximum net mitigation relative to energy yield) will likely be best achieved by some combination of perennial vegetation, low synthetic-N use, and high aboveground net primary production (ANPP) on marginal lands with depleted soil carbon stores planted to diverse species.
