BioGrassEnvironmental impact of

energy crops

This novel project investigated thesuitability of Irish grassland, under arange of management intensities, forbioenergy production. The factorsthat were studied included:

Greenhouse gas emissions forclimate change mitigation

Grass biomass yield forbioenergy feedstock

Digestion of grass for heat andelectricity generation

Land application of by-productsfor a closed-loop bioenergysystem

Experiments were conducted in threelocations on the island of Irelandduring 2008-2010.

Crossnacreevy, Belfast

Oakpark, Carlow

Johnstown Castle, Wexford

www.ucd.ie/bioenergy

Bioenergy: Energy generation from agricultural crops(bioenergy) can assist Ireland in reducing its depend-ency on non-renewable energy sources. Harvestedplant material can be anaerobically digested to pro-duce biogas, which can be used to generate heat andelectricity. Agricultural wastes (e.g. slurry, manure)are added to the anaerobic digester in a processcalled co-digestion. Nutrient-rich digestate, a by-product of digestion, can also be used as a fertiliser.

Climate Change: Agriculture contributes 29.1% ofIreland’s greenhouse gas (GHG) emissions, includingcarbon dioxide (CO

2), methane (CH

4) and nitrous ox-

ide (N2O). On the farm, emissions come primarily

from livestock, agricultural wastes, and soil manage-ment. While CO

2is more prominent in the atmos-

phere, N2O and CH

4have greater abilities to trap heat.

N2O is mostly emitted from soils due to fertiliser ap-

plication, whereas CH4

comes from fermentation inlivestock and manure. Any farm that minimizes emis-sion of these gases can contribute to climate changemitigation.

Biochar: Biochar is a soil amendment, similar to char-coal, produced from a broad variety of crops, includ-ing energy crops. CO

2from the atmosphere can be

retained in biochar and once applied to land, this CO2

remains in the ground. Studies show an increase incrop yield following biochar application.

Energy Crops: In a project survey, 172 Irish farmersfrom 25 counties identified their preferred bioenergycrop. 70% were interested in producing bioenergycrops, with Miscanthus, grass and willow being theirpreferred crops. However, conventional bioenergycrops (e.g. Miscanthus and willow) require majorland-use changes (e.g. ploughing, planting, and cropestablishment), which would dramatically change theIrish landscape. In contrast, grass can be used di-rectly for bioenergy production while maintainingecosystem services.

Project background

For more information visit theBioGrass website at

www.ucd.ie/bioenergy

Weather is important when applying fertiliser.Wet ground can lead to high N

2O emissions

due to denitrification.

Digestate has considerable potential as afertiliser for grassland due to its high nitro-gen content and low N

2O emissions.

Co-digestion of grass with slurry may miti-gate greenhouse gas emissions by capturingCH

4that would otherwise be lost during

slurry storage/land spreading.

Biochar improves soil fertility and can alsoreduce N

2O emissions from Irish soil.

Similar levels of biogas can be obtained fromvarious grass species and management in-tensities. Thus, less-intensively managedgrassland, such as former set-aside areas,can be used for biogas production

Grassland conversion to conventional energycrops requires ploughing and herbicide ap-plication, which can lead to increased CO

2

and N2O emissions from soil. Establishment

of these crops should take place as soon aspossible after ploughing and herbicide appli-cation should be minimised.

Recommendations &Contact information

Belfast

Carlow

Wexford W

B

C

W

C

B

110 kg N/ha/yr W

PloughingConventional energy crops require ploughing for es-tablishment and 3-5 years before they reach full po-tential. Ploughing Irish grassland led to CO

2emissions

because uptake of CO2

by plant photosynthesis couldnot take place until a new crop was planted.

Herbicide

Herbicide is typically applied for the first 3 years ofestablishment for conventional energy crops (e.g. wil-low & Miscanthus). N

2O emissions were found to in-

crease after the previous crop was sprayed off prior toploughing.

Biogas production

Current agricultural practices for slurry andgrassland management were compared to biogasproduction from cattle slurry and grass. Anaero-bic digestion for biogas production has the po-tential to reduce on-farm emissions of GHGs.However, adequate digestate storage capacityover summer may be a potential challenge. Di-gestate should be applied in early spring for op-timum efficiency as a fertiliser.

Biogas measurements were collected from sev-eral grass species grown under a variety of man-agement intensities. Approximately, 300 L ofmethane was produced per kg dry matter, irre-spective of species or fertiliser rate.

CELUP Oak Park

BioGrass: Environmental impact of energy cropsProject Outcomes

By-product application

DigestateThe fertiliser replacement value of digestatewas approximately 80% of mineral fertiliser(CAN/urea) for silage yield. Combined N

2O

and CH4

emissions from digestate were 75%lower than emissions from CAN, while emis-sions from urea and digestate were similar.

BiocharIn a laboratory experiment, addition of biocharled to reduced CO

2and N

2O emissions even

when earthworms were present in the soil.

C

Site at Crossnacreevy, Belfast

Site at Oak Park, Carlow

Site at Johnstown Castle, Wexford

C

B

W

C WB

Less N2O emissions occurred from grassland planted

with Cocksfoot variety Donata compared to the Per-ennial Ryegrass variety Portstewart when only asmall amount of fertiliser was applied.

Greenhouse gas emission

Wet soil conditions led to high emissions of N2O after

fertilising. While normally CH4

is changed into lessharmful CO

2in grassland soils, wet conditions can

actually lead to CH4

emissions.

Grassland managed with 425 kg N/ha/yr of mineralfertiliser led to N

2O emissions that were over 5 times

higher than with 110 kg N/ha/yr mineral fertiliser.

B

W

W

Anaerobicdigestion

Mineral fertilisationChoice of energy crop

Set-aside grassland

Intensive grassland

Conventionalenergy crop

Harvest

7.6 t/ha/yr W

13.6 t/ha/yr W

8.5 t/ha/yr B

12.0 t/ha/yr B

0 kg N/ha/yr W

105 kg N/ha/yr B

425 kg N/ha/yr B