Significant difference in Miscanthus species root carbon exudation rate

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, the aim of this study was to quantify root C exudation rates and track fine root growth.

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

The exudation rate was significantly lower for M. sacchariflorus compared with M. sinensis and M. × giganteus (0.0 versus 0.6 g C g-1 root dry mass year-1). Coupled with this, M. sacchariflorus had greater above-ground biomass, a smaller increase in root mass and a higher root C concentration. Rapid root growth was observed, especially for M. × giganteus, for which root length density (0-30 cm depth) was higher compared with both M. sacchariflorus and M. sinensis in the earlier growth weeks.

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-1 soil 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.