Forbs enhance productivity of unfertilised grass-clover leys and support low-carbon bioenergy

Wen-Feng Cong,Jingying Jing,Karen Søegaard,Jim Rasmussen,Jørgen Eriksen

doi:10.1038/s41598-017-01632-4

Abstract

Intensively managed grasslands are dominated by highly productive grass-clover mixtures. Increasing crop diversity by inclusion of competitive forbs may enhance biomass production and sustainable biofuel production. Here we examined if one or all of three forbs (chicory, Cichorium intybus L.; caraway, Carum carvi L.; plantain, Plantago lanceolata L.) included in ryegrass-red clover mixtures enhanced above- and below-ground productivity, and assessed their biofuel potentials, based on a three-year experiment with and without fertilisation as cattle slurry. We determined herbage yield, standing root biomass, and estimated methane energy output and greenhouse gas (GHG) emissions per energy unit using life cycle assessment. Results showed that plantain-containing grass-clover mixtures significantly increased herbage yield, while chicory- or caraway-containing mixtures maintained similar yields to the grass-clover mixture. Standing root biomass of the grass-clover mixture was enhanced by inclusion of caraway and plantain, with that of plantain further enhanced by fertilisation. The highest methane energy output was achieved in plantain-containing grass-clover mixtures. All unfertilised mixtures achieved the 60% reduction in GHG emissions compared to fossil fuel, whereas all fertilised mixtures did not meet the 60% reduction target. These findings suggest that including competitive forbs such as plantain in grass-clover mixtures enhances productivity, supporting low-carbon footprint bioenergy production.

Highlights

Increasing global population poses huge challenges in sustainable production of food, feed and fuel, reducing anthropogenic impacts on climate, environment and biodiversity[1]
It is yet to be elucidated if the inclusion of forbs leads to greater above- and below-ground primary production, and greater bioenergy production, soil C sequestration and lower greenhouse gas (GHG) emissions
The most common way of using grassland biomass for bioenergy production in Europe is to convert harvested biomass into methane through anaerobic digestion[20]. Management practices such as fertilisation generally increase biomass production in managed grasslands and enhance methane energy output[21, 22], whereas fertilisation requires a higher energy input and contributes to higher GHG emissions related to, e.g., nitrous oxide (N2O) emissions derived from production and field application of N fertiliser[23]

Summary

Introduction

Increasing global population poses huge challenges in sustainable production of food, feed and fuel, reducing anthropogenic impacts on climate, environment and biodiversity[1]. Agronomists have recently identified three productive deep-rooting perennial forbs: chicory (Cichorium intybus L.), caraway (Carum carvi L.) and plantain (Plantago lanceolata L.)[12,13,14,15] These deep-rooting forbs are known to complementarily utilize resources (e.g. light and N) in shallow-rooted grass-clover mixtures[16], holding the potential to further enhance the productivity of intensively managed grasslands. That most developed countries are witnessing increasing areas with surplus agricultural grassland (e.g. predicted to be 13–22% of the permanent grassland in Europe by 2020), because of improved animal performance[20] These surplus agriculturally managed grasslands are generally more fertile than marginal land, representing a substantial bioenergy production potential. We hypothesized that: (1) inclusion of competitive forbs in the grass-clover mixture would improve productivity and bioenergy production, and (2) that unfertilised mixtures would have greater potential for GHG mitigation than fertilised mixtures due to field N2O emissions from fertilisation

Methods

Results

Conclusion