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.

We would like to thank F. Gouriveau, E. Ceschia and J. Elbers for their critical contribution to the installation of the eddy covariance tower and to data analysis, and D. Estany and H. Sarri for field assistance. The flux tower was installed during the FLUXPYR project (EFA34/08, INTERREG IV-A POCTEFA, financed by EU-ERDF, Generalitat de Catalunya and Conseil Régional Midi-Pyrénées). The following additional projects also contributed with funding to this work: CAPACITI (FP7/2007-2013 grant agreement n° 275855), AGEC 2012 (Generalitat de Catalunya), CAPAS (Spanish Science Foundation, CGL2010-22378-C03-01), BIOGEI (Spanish Science Foundation, CGL2013-49142-C2-1-R, supported by a FPI fellowship for Mercedes Ibañez, BES-2014-069243) and IMAGINE (Spanish Science Foundation, CGL2017-85490-R). We would like to acknowledge the Forest Science and Technology Centre of Catalonia (CTFC) for support with study site maintenance.