Cereal-legume mixtures increase net CO₂ uptake in a forage system of the Eastern Pyrenees

Abstract. Forage systems are the major land use, and provide essential resources for animal feeding. Assessing the influence of forage species on net ecosystem CO₂ exchange (NEE) is key to develop management strategies that can help to mitigate climate change, while optimizing productivity of these systems. However, little is known about the effect of forage species on CO₂ exchange fluxes and net biome production (NBP), considering: species ecophysiological responses; growth and fallow periods separately; and the management associated with the particular sown species. Our study assesses the influence of cereal monocultures vs. cereal legume mixtures on (1) ecosystem scale CO₂ fluxes, for the whole crop season and separately for the two periods of growth and fallow; (2) potential sensitivities of CO₂ exchange related to short-term variations in light, temperature and soil water content; and (3) NBP during the growth period; this being the first long term (seven years) ecosystem scale CO₂ fluxes dataset of an intensively managed forage system in the Pyrenees region. Our results provide strong evidence that cereal-legume mixtures lead to higher net CO₂ uptake than cereal monocultures, as a result of higher gross CO₂ uptake, while respiratory fluxes did not significantly increase. Also, management associated with cereal legume mixtures favoured vegetation voluntary regrowth during the fallow period, which was decisive for the cumulative net CO₂ uptake of the entire crop season. All cereal legume mixtures and some cereal monocultures had a negative NBP (net gain of C) during the growth period, indicating C input to the system, besides the yield. Overall, cereal legume mixtures enhanced net CO₂ sink capacity of the forage system, while ensuring productivity and forage quality.