Significant difference in Miscanthus species root carbon exudation rate

Background and Aims: The cultivation of Miscanthus, a giant perennial grass and promising biomass crop, is expected to increase globally in response to climate mitigation policies and sustainable agriculture goals. Little is known about root carbon (C) exudation and fine root architecture, or how this might differ between Miscanthus species. To understand the functional biology of three diverse Miscanthus species, and to evaluate impacts on soil C cycling, this study aims to quantify root C exudation rates and track fine root growth.

Methods: We use a controlled environment with plants grown in rhizotron boxes (28 litre) to quantify living root C exudation rates and fine root growth of M. sacchariflorus, M. sinensis and M × giganteus. Weekly non-destructive images of visible roots were analysed for root length density (RLD) and root diameter during the growing season. Above and below ground biomass, and C and nitrogen content, were also recorded immediately after exudate sampling.

Key Results: The exudation rate was significantly lower for M. sacchariflorus compared to M. sinensis and M × giganteus (0.0 vs 0.6 g C g-1 root Md y-1). Coupled with this M. sacchariflorus had greater above ground biomass, a lower increase in root mass, and a larger root C concentration. Rapid root growth was observed, especially for M × giganteus where RLD (0-30 cm depth) was higher compared to both M. sacchariflorus and M. sinensis in the earlier growth weeks.

Conclusions: The results reveal a possible fundamental difference in nutrient resource acquisition and allocation between M. sacchariflorus versus M. sinensis and M × giganteus. We estimate that Miscanthus root C exudation could add up to 2 g C kg soil-1 month-1 (during the peak growing season), a considerable influx of new labile C. This unique insight into differences in Miscanthus exudation indicates the potential for targeting Miscanthus breeding for enhanced soil C sequestration.