Is Miscanthus, a Leading Bioenergy Crop, a Sink or Source of Carbon during the Establishment Year?

Bioenergy crops are important not only for contributing energy independence from fossil fuels but also for fossil fuel derived bioproducts. The IPCC reports pointed out that second-generation energy crops` (i.e., lignocellulosic crops), such as Miscanthus (Miscanthus ×giganteus) are a key strategy to achieve a 1.5 ℃ target that requires net global CO₂ emission to decrease to zero by 2050 (IPCC, 2018). Miscanthus is a leading bioenergy crop because of its productivity, high potential for high carbon uptake, and broad adaptation range, but as with many perennials, it requires time to achieve peak growth and carbon uptake. Most studies have found miscanthus to be a strong carbon sink, defined as a negative cumulative net ecosystem carbon exchange (cumulative_NEE ), once the stand is established (typically 3 years after planting). However the CO₂ exchanges during the establishment year varies significantly in the literature from a strong carbon source (up to 200 g C m^-2) with some reporting miscanthus started with a carbon sink (up to -58 g C m^-2) for the first year. These studies cover a wide range of locations therefore, many factors could be influencing the variability between reports, including the planting method and rate as well as soil and weather conditions. Therefore, understanding the differences in the first year of carbon flux strength, and its relationship with management, weather and soil factors is important to improve miscanthus carbon sequestration.

Here we present data from Iowa State University Sustainability Advanced Bioeconomy Research Farm (SABR) to compare carbon flux variations in miscanthus establishment year with literature. Miscanthus carbon fluxes have been monitored using eddy covariance techniques (EC) in the SABR site since June 2019. The preliminary flux comparisons showed that SABR Miscanthus's first-year establishment cumulative_NEE had almost six times higher carbon uptake at -647 g C m^-2 than the highest carbon uptake reported in the literature). An initial comparison within the studies suggests that among the differences in environmental parameters e.g., soil type, previous land use, and management practices, that the planting density showed the most significant variation in studies. Future work should work toward resolving the influence of planting density as a critical factor for driving a higher carbon sink strength for the first-year establishment of miscanthus. Resolving the role of this key management factor can lead to a standard protocol to achieve optimum parameters for establishing miscanthus at high carbon strength.