Decisions made during the cultivation and harvest of grasslands, such as selection of plant species for establishment and time of harvest, affect net greenhouse gas (GHG) emissions during the biofeedstock production. Both parameters can affect the dry weight fractions of cellulose, hemicellulose, lignin, and protein in the feedstock that is converted to ethanol. Grasslands planted with diverse mixtures of species or tall C4 grasses were compared for cellulose and hemicellulose yield and the resulting ethanol yield was included in the LCA. The most common times for harvest of grasslands are in the summer (mid-August) or during dormancy (October through April). Since the concentration of N is twice as high in C4 grass biomass harvested in the summer as during the dormant season, twice the amount of N is needed to maintain productivity. We used the biogeochemical model DAYCENT to quantify the changes in soil N2O emissions resulting from management changes associated with harvest season and potential ethanol yield improvements from a range of prospective plant species mixtures for biofuel production grown on conservation grasslands.
Results/Conclusions
Due to increases in cellulose/hemicellulolse composition, there was potential for a 24% increase in ethanol yield per unit biomass as the cover of tall C4 prairie grasses increased from 2 to 82%. We found that C4 grass biomass harvested during the dormant period has higher cellulose and hemicellulose composition, while C4 grass biomass harvested in the summer has a higher protein composition in the biofeedstock. This seasonal difference in biomass composition influences the optimal harvest time depending upon whether a protein-rich feed or a sugar-rich feed is sought for biorefinery output. The life cycle implications of harvest time are that GHGs from increased N input were 70% higher and from soil N2O emissions were 80% higher for grasslands harvesting in the summer compared with the dormant season. Managing grasslands for multiple functions such as increase diversity of vegetative structure and cover and consequently wildlife habitat benefits, production of protein feed, increased continuity of feedstock supply to biorefinery, and reduce size of farm equipment requirements may provide a basis for multi-season grassland harvest on farms even though life cycle GHGs may be better for a single harvest season.
