Seasonal dynamics of above‐ and below‐ground biomass and nitrogen partitioning in M iscanthus × giganteus and P anicum virgatum across three growing seasons

The first replicated productivity trials of the C 4 perennial grass M iscanthus  ×  giganteus in the U nited S tates 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 rhizomes. The mean peak shoot biomass for M . ×  giganteus in years 5–7 was 46.5 t ha −1 in O ctober, declining to 38.1 t ha −1 on completion of senescence and at harvest in D ecember, and 20.7 t ha −1 declining to 11.3 t ha −1 for P anicum virgatum . There was no evidence of decline in annual yield with age. Mean rhizome biomass was significantly higher in M . ×  giganteus at 21.5 t ha −1 compared to 7.2 t ha −1 for P . virgatum , whereas root biomass was similar at 5.6–5.9 t ha −1 . M . ×  giganteus shoots contained 339 kg ha −1   N in A ugust, declining to 193 kg ha −1 in D ecember, compared to 168 and 58 kg ha −1 for P . virgatum . The results suggest substantial remobilization of N to roots and rhizomes, yet still a substantial loss with D ecember harvests. The shoot and rhizome biomass increase of 33.6 t ha −1 during the 2‐month period between J une and A ugust for M . ×  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.