Carbon storage potential increases with increasing ratio of C₄ to C₃ grass cover and soil productivity in restored tallgrass prairies

Long-term soil carbon (C) storage is essential for reducing CO₂ in the atmosphere. Converting unproductive and environmentally sensitive agricultural lands to grasslands for bioenergy production may enhance C storage. However, a better understanding of the interacting effects of grass functional composition (i.e., relative abundance of C₄ and C₃ grass cover) and soil productivity on C storage will help guide sustainable grassland management. Our objective was to examine the relationship between grass functional composition and potential C storage and how it varies with potential soil productivity. We estimated C inputs from above-and belowground net primary productivity (ANPP and BNPP), and heterotrophic respiration (R H) to calculate net ecosystem production (NEP), a measure of potential soil C storage, in grassland plots of relatively highand low-productivity soils spanning a gradient in the ratio of C₄ to C₃ grass cover (C₄: C₃). NEP increased with increasing C₄: C₃, but only in potentially productive soils. The positive relationship likely stemmed from increased ANPP, rather than BNPP, which was possibly related to efficient resource-use and physiological/anatomical advantages of C₄ plants. R H was negatively correlated with C₄: C₃, possibly because of changes in microclimate or plant–microbe interactions. It is possible that in potentially productive soils, C storage can be enhanced by favoring C₄ over C₃ grasses through increased ANPP and BNPP and reduced R H. Results also suggest that potential C storage gains from C₄ productivity would not be undermined by a corresponding increase in R H.