The first replicated productivity trials of the C4 perennial grass Miscanthus 9 giganteus in the United States showed this emerging ligno-cellulosic bioenergy feedstock to provide remarkably high annual yields. This covered the 5 years after planting, leaving it uncertain if this high productivity could be maintained in the absence of N fertilization. An expected, but until now unsubstantiated, benefit of both species was investment in roots and perennating rhizomes. This study examines for years 5-7 yields, biomass, C and N in shoots, roots, and rhi-zomes. The mean peak shoot biomass for M. 9 giganteus in years 5-7 was 46.5 t ha À1 in October, declining to 38.1 t ha À1 on completion of senescence and at harvest in December, and 20.7 t ha À1 declining to 11.3 t ha À1 for Panicum virgatum. There was no evidence of decline in annual yield with age. Mean rhizome biomass was significantly higher in M. 9 giganteus at 21.5 t ha À1 compared to 7.2 t ha À1 for P. virgatum, whereas root biomass was similar at 5.6-5.9 t ha À1. M. 9 giganteus shoots contained 339 kg ha À1 N in August, declining to 193 kg ha À1 in December, compared to 168 and 58 kg ha À1 for P. virgatum. The results suggest substantial remobilization of N to roots and rhizomes, yet still a substantial loss with December harvests. The shoot and rhizome biomass increase of 33.6 t ha À1 during the 2-month period between June and August for M. 9 giganteus corresponds to a solar energy conversion of 4.4% of solar energy into biomass, one of the highest recorded and confirming the remarkable productivity potential of this plant.

For the C4 perennial grasses, Miscanthus 9 giganteus and Panicum virgatum (switchgrass) to be successful for bioenergy production they must maintain high yields over the long term. Previous studies under the less conducive climate for productivity in N.W. Europe found little or no yield decline in M. 9 giganteus in the long term. This study provides the first analysis of whether yield decline occurs in M. 9 giganteus under United States. Midwest conditions in side-by-side trials with P. virgatum over 8-10 years at seven locations across Illinois. The effect of stand age was determined by using a linear regression model that included effects of weather. Miscanthus 9 giganteus produced yields more than twice that of P. virgatum averaging 23.4 AE 1.2 Mg ha À1 yr À1 and 10.0 AE 0.9 Mg ha À1 yr À1 , respectively, averaged over 8-10 years. Relationships of yield with precipitation and growing degree days were established and used to estimate yields corrected for the stochastic effects of weather. Across all locations and in both species, yield initially increased until it reached a maximum during the fifth growing season and then declined to a stable, but lower level in the eighth. This pattern was more pronounced in M. 9 giganteus. The mean yields observed over this longer term period of 8-10 years were lower than the yields of the first 5 years. However, this decline was proportionately greater in M. 9 giganteus than in P. virgatum, suggesting a stronger effect of stand age on M. 9 giganteus. Based on the average yield over the period of this study, meeting the United States Renewable Fuel Standard mandate of 60 billion liters of cellulosic ethanol by 2022, would require 6.8 Mha of M. 9 giganteus or 15.8 Mha of P. virgatum. These appear manageable numbers for the United States, given the 16.0 Mha in the farmland Conservation Reserve Program in addition to another 13.0 Mha abandoned from agriculture in the last decade.

No abstract

No abstract