Can cup-plant (Silphium perfoliatum L.) as a perennial bioenergy crop surpass silage maize (Zea mays L.) for C sequestration?

Intensified silage maize (Zea mays L.) cultivation has led to several environmental problems such as high greenhouse gas emissions and groundwater pollution, thus further amplifying climate change. Therefore, alternative crops with high carbon (C) sequestration capacities are crucial for sustainable bioenergy production, particularly those that can cope with extreme climate events, such as drought stress. Therefore, this study aimed to evaluate the potential of the perennial cup-plant (Silphium perfoliatum L.) as a promising alternative for silage maize to increase soil C input and reduce C output (as CO₂) under optimum watering and drought conditions. For this purpose, a lysimeter experiment comparing these crops subjected to two watering regimes (well-watered and drought-stressed), was set up at the botanical garden of the University of Bayreuth, Germany. Soil respiration was correlated to soil moisture and temperature, biomass (aboveground and belowground), soil C and nitrogen (N), and dissolved organic C (DOC) and inorganic C in the leachates. Soil CO₂ efflux was measured using a Licor-6400XT gas exchange system (LI-COR, Lincoln, NE, USA)) fitted with a soil chamber over three consecutive years. The final cumulative CO₂ efflux was greater by 29% under drought-stressed cup-plant than in drought maize, which was explained by the greater belowground biomass production under cup-plant. Cup-plant increased root biomass by 143% (0.86±0.26 vs 2.08±0.33 kg m^-2) compared to silage maize, thus, implying a higher contribution of root respiration to the total soil respiration under cup-plant. Despite the soil respiration and DOC leaching being higher for cup-plant than maize, cup-plant increased soil C (by 4%) and N content (by 14%), after only two years of cultivation. Even though root respiration and enhanced microbial activity result in higher soil respiration and C mineralization, the continuous supply of fresh C via the root litter and rhizodeposits of the perennial cup-plant suggests long-term effective C farming and demonstrates the potential of the cup-plant as an ecological alternative to silage maize.

Keywords: Climate change; soil respiration; carbon sequestration; bioenergy crops; leachates