Mellangrödor som verktyg för att minska jordbrukets klimatpåverkan – en jämförande livscykelanalys av olika hanteringsåtgärder

The agricultural sector is today an important contributor to anthropogenic greenhouse gas emissions. In order to improve its greenhouse gas balance, it has been proposed to implement measures to increase soil carbon concentrations. Intermediate crops are currently grown in Sweden for a variety of reasons, such as to control weeds, prevent nitrogen leakage and promote biodiversity, but also to increase soil carbon content. However, if the biomass of catch crops is left in the field over winter, there may be a risk that N2O emissions will increase, which could counteract the climate mitigation effect of increased soil organic carbon. Harvesting certain types of catch crops could potentially reduce N2O emissions during the winter season, while the removed above-ground biomass could be used as a resource, for example as feedstock for biogas production.The aim of this study was to investigate the climate impact of different management measures in intermediate crop cultivation, using oilseed radish as a model crop. The quantification of the potential climate impact was carried out from a life cycle perspective, focusing on the soil carbon effect and N2O emissions. Furthermore, the influence of the time of establishment and harvest on the results was investigated. Two different scenarios 1) harvesting the above-ground biomass and 2) harvesting both the above-ground and below-ground biomass, were compared with a Reference scenario where the biomass was left and incorporated into the soil without being harvested. The harvested biomass in the alternative scenarios was assumed to be used as a substrate for biogas production, where the biogas was upgraded to biomethane and replaced diesel as a vehicle fuel.The results showed that the potential soil carbon effect was dependent on the times of establishment and harvest, where an early establishment and late harvest generally resulted in the largest carbon sequestration. The highest soil carbon effect was calculated for the Reference scenario, where the intermediate crop was not harvested. In this scenario, the maximum carbon sequestration was equivalent to 1.7 tonnes of CO2 per ha per year. In the two alternative scenarios, where the intermediate crop was harvested, the carbon sequestration from the plant residues was significantly lower. However, this was partly compensated by the recycling of the digestate as organic fertilizer in the same field. This entailed carbon sequestration corresponding to between 77- 89% and 74-81% of the Reference scenario in Scenario 1 and Scenario 2, respectively. Nitrous oxide emissions from the intermediate crop cultivation were calculated using the IPCC “Tier I” method. The results showed that the climate impact of the calculated N2O emissions was slightly lower in the alternative scenarios compared to the Reference scenario. However, results from previous studies indicate that the N2O emission could have been even lower if the timing of the nitrogen application had been accounted for in the calculation. Overall, the climate impact from land-use was estimated to be higher in the alternative scenarios compared to the Reference scenario, mainly due to a lower soil carbon effect. However, the production of biomethane in the alternative scenarios resulted in the substitution of diesel, leading to an overall negative greenhouse gas balance for both Scenario 1 and Scenario 2. A larger negative greenhouse gas balance was modelled for Scenario 2 as more diesel was calculated to be substituted in this scenario. For an early establishment and late harvest of the intermediate crop, the greenhouse gas savings were calculated to be 1.9 for Scenario 1 and 2.3 tonnes CO2-eq per ha per year for Scenario 2, compared to the Reference scenario. This means that there is a relatively large potential to reduce the climate impact of agriculture by harvesting catch crops.