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Report, 2013

Impact of biogas crop production on greenhouse gas emissions, soil organic matter and food crop production – A case study on farm level

Björnsson, Lovisa; Lantz, Mikael; Börjesson, Pål; Prade, Thomas; Svensson, Sven-Erik; Eriksson, Håkan


Soil degradation is a widespread problem: erosion, loss of soil organic matter and compaction are some of the degradation processes that are threatening soil fertility throughout the EU (Soilservice, 2012). Intensively cultivated clay soils have in Swedish studies been shown to give up to 20% de-creasing food crop harvest yields due to soil compaction and reduced soil organic matter content (Arvidsson & Håkansson, 1991). The remedy is a combination of improved farm machine technology and increased soil organic matter content. This can be achieved by;  replacing mineral fertilizers by manure or other biofertilizers containing organic matter, such as digestate (the effluent from biogas production), and  changing the crop rotation to include e.g. ley, green manuring crops and catch crops. In assessing the climate benefits of energy crops on arable land it is thus important to also consider the effects on the cultivation system and long term soil fertility. In an analysis of climate effects of changed agricultural practice and crop rotations, increased soil organic matter content will have a dual effect. The build-up of soil organic matter has been shown to be positively correlated to most soil ecosystem services (Soilservice, 2012). In addition, the long term carbon sequestration will reduce greenhouse gas (GHG) emissions (Anderson-Teixeira et al., 2009; Röing et al., 2005). In this project, a case where the biogas process potentially could contribute to more efficient land use by maximizing positive crop rotation effects and by supplying a biofertilizer on farm land, was investigated in a farm based case study. The purpose was to evaluate a scenario where a biogas plant has been integrated in an agricultural region with mainly stockless farming and intensively cultivated clay rich soils. Ley crops are introduced in crop production and used as biogas feedstock. The biogas plant also provides biofertilizer. The project also includes a systems analysis of the bio-gas plant as presently operated. The farm based case study also includes an analysis of possible scenarios for the farm scale system, and the impact on farm level of the introduction of biogas pro-duction and crops on arable land which act as biogas feedstock. The overall objective of the project was to analyse how the integrated production of food crops and energy crops for biogas production impacts the GHG emissions per land area, the soil organic matter and the total crop output. On the farm that is the model for this study, soil compaction on the medium to heavy clay soils is a prob-lem. The crop yields are 5–20% lower than average yields for the region. Aware of the problem, three years of meadow fescue for seed production has been integrated in the cereal based crop rotation on the most problematic soils. However, the market for grass seeds being limited, the eco-nomic possibilities for integrating ley crops in other parts of the crop rotation is limited in a region with little demand for cattle feed. The approach in the farm based case study was to integrate 1-2 years of ley crops in the crop rotation and to use this as feedstock for biogas production. The effects of this on GHG emissions, soil organic matter and food crop production was evaluated. The project contains of two parts. In the first part a life cycle assessment (LCA) was performed for the biogas plant at Söderåsens Bioenergi, that is presently in operation and that is located within the boundaries of the farm investigated in the farm based case study. Since the full results of this study have been scientifically published elsewhere (Lantz & Börjesson, 2013), the present report is a mere summary and the information relevant for the second part, the farm based case study, is sum-marized. In the second part of the project, the farm based case study was performed. The farm delivers al-ready today one biogas feedstock to the biogas plant, manure from pigs. Thus, the case study was split in two parts; in the first part the introduced change is that pig manure is used for biogas pro-duction instead of being used as biofertilizer directly. This reflects a change that has already occur-red. In the second part, ley is introduced in the present crop rotation, as described above, and used in addition to manure as biogas feedstock, a situation that can potentially occur in the future. This two stage approach allows separate assessment of the effects of introducing these two different bio-gas feedstocks on the farm based system. Data regarding agricultural aspects of the analysis were inventoried, e.g. crop rotations, harvest yields, soil properties, energy in- and output and emissions in biomass production. The functional unit was set to 1 hectare (ha) of arable land. The assessment included cultivation, harvest and storage of crops, manure storage, biogas production, upgrading and compressing, digestate storage and application and soil carbon changes. Data from the LCA of the biogas plant was used for the biogas production part of the farm based case study. The assess-ment applied a systems expansion approach, in accordance with the recommendation in the ISO standard of LCA (ISO, 2006). In the systems expansion, the total output of grains (wheat and oats) and oil seed (rape seed) is equivalent in the different scenarios. Thus, a reduced output of grains and oil seeds on a farm level, due to the introduction of ley crop cultivation, was compensated for by additional grain and oil seed production outside the farm. This additional cultivation was assum-ed to take place within the region on excess farmland, not leading to any indirect land use changes due to displacement effects. The output of upgraded biogas delivered to the natural gas grid, was assumed to replace fossil vehicle fuel. The reference scenarios in the farm based case study include the conventional handling of manure that took place before the biogas plant was established (Scenario A) or where biogas was produced only from manure (Scenario B). Scenario A is the reference system for Scenario B. For Scenarios C1-3, where ley crops are produced and used for biogas production, Scenario B is used as the refe-rence scenario. The climate benefit was shown to be high for all the investigated scenarios where ley is introduced in the crop rotation and used for biogas production. Introducing a change where 20-33% of the 650 ha farm is used for production of ley as a biogas crop, will give avoided GHG emissions of 1 240-1 500 kg CO2-eq per ha, yr. before systems expansion. In a systems expansion, the produc-tion of the biofuel (biogas) and the lost crop production is included in the analysis. Compensating the lost crop production decreases the climate benefit of the systems. Even so, the resulting reduc-tion in GHG emission is large. The resulting net avoided GHG emissions after systems expansion are 2.2 to 3.2 t CO2-eq per ha, yr. on average for this 650 ha farm. The emission reductions are also calculated as avoided emission per GJ fuel produced and utilized for replacing fossil vehicle fuels to enable comparison with GHG emissions for other vehicle fuels and amount to -88 to -107 kg CO2-eq per GJ fuel used. The reference GHG emission for fossil fuels in the renewable energy directive is 84 kg CO2-eq/GJ. The effect of introducing ley for biogas production at a farm and using it as biogas energy crop will thus give a biofuel with an emission reduction of 106-128% when replacing fossil fuels. This can be compared to the emission reduction of 90% presented in part one of this project (Lantz & Börjesson, 2013) when biogas produced from a mix of manure and industrial residues replaces fossil fuels, or the reduction of 159% when only manure is used for biogas production in scenario B in the farm based case study. Emission reductions above 100% in-dicate that the production itself, not only the utilization of the biofuel to replace fossil fuels, gives avoided emissions. When using only manure for biogas production, the avoided emissions of methane and N2O during storage and after soil application when the manure is handled as digestate are the main causes for the avoided GHG emissions from biogas production. When introducing ley in a cereal based crop rotation, the main cause of the avoided emissions from production is the soil carbon build up, both from the crop residues from ley in the crop rotation, and from the carbon-rich digestate that is recirculated at the farm.
The climate benefit for scenarios with ley production is to a large extent the effect replacing fossil fuels with the biogas produced, 480-870 l petrol/ha, yr. or 15-24 TJ/yr. over the whole 650 ha farm. Equally important is the effect on increased soil organic matter content on farm level. Apart from the role as a carbon sink and the impact on GHG emissions, the increase in soil carbon levels is im-portant for long term soil fertility and productivity on this type of compacted clay soils. In the ley scenarios, the soil carbon content increases steadily from 2% today to 3% within 20–30 years to reach a steady state level of 4–5 %. Here, the possibility of using ley for biogas production opens up for a possibility of integrating ley in the crop rotation in cereal intensive areas even if there is no demand for cattle feed. Still, land would be taken out of food production. The impact of increased soil organic matter on soil fertility and the potential of increasing yields could partly compensate for this. A yield increase of 10% would partly counteract the loss of grain and oil seed in the scena-rios with ley in the crop rotations. In this study, however, the loss in food crop production is from a climate perspective compensated for by adding GHG emissions for crop from the additional grain and oil seed cultivation outside the farm to fulfil an unchanged total output of food products. One important aspect in all ley scenarios is that the ley is undersown the year before the main harvest (called year 0), making ley biomass harvest possible in autumn year 0. This gives good land use efficiency, using the benefit of harvesting that extra biomass. The economic feasibility of this small harvest remains to be evaluated. The sensitivity analysis shows that the ley based systems are sensi-tive to the chosen data for calculation of amounts of crop residues. However, the IPCC data set, which gives very high straw amounts for cereals, is not valid for the actual conditions in the Nordic countries, and the calculation method developed within the project is considered to give a better estimation of actual conditions. The ley scenarios are also sensitive to calculations assuming high methane leakage from the digestate storage. The emissions evaluated in the sensitivity assessment are however high considering that the digestate is used as fertilizer in the period 1 April-15 May, and the share of the annual digestate production that is stored during the warm part of the year, May-October, is less than 1/3 of the annual production. An aspect like this, the time for spreading, is important to consider when a digestate containing high amounts of organic matter, like in this case from ley crops with relatively low biodegradability, is produced.
The soil carbon contribution is important to consider in all systems where biomass is removed from farm land. Part of the evaluation was also to assess the effect of using manure for biogas produc-tion, and then recycling the residue, the digestate, to the farm as biofertilizer. The climate benefit for this manure based biogas production is very good, but it has been argued that the impact on soil organic matter could be negative since much of the easy degradable carbon is removed as biogas when digesting manure. However, the negative effect on soil carbon was in this study shown to be small; the impact on soil carbon change in the long perspective is negligible. The main positive impact of biogas production from manure is when the biofuels replaces fossil transportation fuels, but the impact on reduced biogenic emissions of N2O and methane is also important. In the study it has also been shown which features of the investigated scenarios that are most im-portant for good GHG efficiency. At the same time, effects on soil organic matter content and food crop yields have been presented. The outcome is important in fulfilling the future criteria in sus-tainability certification of biofuel systems, such as avoiding indirect land use changes and maximi-zing GHG performance (Ahlgren & Börjesson, 2011). Both outcomes are equally important for improved understanding of scenarios involving soil fertility challenges and land use competition between food and energy crop production. Neither biogas production from manure nor from ley crops shows good profitability from a biogas plant perspective at present biofuels prices (Lantz et al., 2013), which might hinder the introduction of such systems in spite of the positive impact on GHG emissions. The intention of the researchers behind the present study is to follow up with a study encompassing several Swedish regions and in-cluding economic evaluations of ley as biogas feedstock, including aspects that are important from the farm perspective as effects of preceding crop and soil carbon contribution.

Published in

f3 report
2013, number: 2013:27
Publisher: f3 The Swedish Knowledge Centre for Renewable Transportation Fuels

Authors' information

Björnsson, Lovisa
Lund University
Lantz, Mikael
Lund University
Börjesson, Pål
Lund University
Swedish University of Agricultural Sciences, Department of Biosystems and Technology
Swedish University of Agricultural Sciences, Department of Biosystems and Technology
Eriksson, Håkan

Sustainable Development Goals

SDG7 Affordable and clean energy
SDG13 Climate action
SDG12 Responsible consumption and production

UKÄ Subject classification

Agricultural Science

URI (permanent link to this page)