Cover crops are grown to keep the soil covered with plants after harvest of the main crop. In Norway, cover crops were introduced in the 1990’s to prevent loss of nitrogen and phosphorous from fields to waterways. Initially, grasses were predominantly used as cover crops. The cover crops contribute with several ecosystem services, and there are many species with different characteristics available. The area covered by cover crops is small but increasing. The use of cover crops in cereal production has been considered an applicable climate action but can currently not be posted in the greenhouse gas accounts for the agricultural sector in Norway because of insufficient documentation of its effect under our conditions.
The aim of the project Assessment of cover cropping as climate action in cereal production in Norway – CAPTURE, conducted in 2021–2025, has been to document the climate effect of using cover crops in cereal production in Norway, as well as to develop well-functioning cultivation strategies. The climate effect in this context is the ability of cover crops to sequester and store carbon in the soil, weighed against potential nitrous oxide emissions from the decomposition of plant residues. Key objectives have been to document the quantity and variability in cover crops' 1) biomass production, 2) contribution to carbon storage in soil, 3) nitrous oxide emissions, 4) ability to retain nitrate in the soil, and 5) how these factors come together in a synthesis of knowledge to assess the climate impact of cover crops under Norwegian conditions.
Data from real farming conditions and trials over two growing seasons showed that cover crops were well established in cereal systems in the Oslofjord region, Innlandet, and Trøndelag, with 820, 500, and 380 kg carbon per hectare in aboweground biomass late in autumn, respectively. In Trøndelag, spring-sown species produced a high amount of biomass, while establishment was poor for species sown at treshing. In Eastern Norway, there was little difference in biomass between annual cover crops sown around harvest and perennial cover crops sown in spring.
By using the ICBM-model, we estimated that repeated use of cover crops could, on average over 30 years, increase soil carbon storage by 270 ± 120 kg per hectare per year under Norwegian conditions. The amount varies with type of cover crop and location. As input, we used our own data on cover crop biomass measured on farms, combined with carbon concentration in various cover crops from the field experiments. The model was run with a parameter set based on long-term Swedish cover crop trials and climate data from the three main cereal producing regions.
By tracing the fate of carbon in shoots and roots of four selected cover crop species, we found that carbon allocated belowground by plants is more likely to be found in the soil's most stable carbon fraction (mineral-associated carbon) than carbon in shoots and leaves. On average, carbon allocated belowground by plants was 3.9 times more effectively converted into soil carbon than carbon in shoots and leaves.
Weekly measurements of nitrous oxide emissions in a two-year field experiment with cover crops at Ås showed that the choice of plant species, whether it survives winter, was crucial for whether the cover crop resulted in increased or reduced nitrous oxide emissions. High emissions were associated with the cold season. Compared to fields without cover crops, the use of ryegrass and a grass-clover-herb mixture resulted in lower nitrous oxide emissions, while oilseed radish led to higher emissions. Winter-hardy species also reduced the risk of nitrate runoff outside the growing season.
To calculate the total climate impact of cover crops, nitrous oxide emissions were converted to CO2 equivalents using the factor 1 kg nitrous oxide equals 273 kg CO2. Including estimated emissions from cover crop seed production, results from the Ås trial showed that ryegrass and a grass-clover-herb mixture reduced greenhouse gas emissions by 1100 kg CO2 equivalents per hectare per year, while oilseed radish increased emissions by 600 kg CO2 equivalents per hectare per year. In practice, there is significant variation in establishment and growth of cover crops from place to place and year to year. Additionally, there is considerable uncertainty related to calculations of both the leaf mass/root ratio of cover crops and the storage of fixed carbon. Therefore, our results should be interpreted qualitatively rather than quantitatively. This means that ryegrass-dominated cover crops are likely to reduce greenhouse gas emissions by approximately 1000 kg CO2 equivalents per hectare per year. Oilseed radish contributes to increased carbon storage, but this probably does not compensate for increased nitrous oxide emissions under our conditions.
Prosjektet har bidratt med ny kunnskap om hvordan ulike fangvekster påvirker karbonlagring, utslipp av lystgass og risiko for nitratavrenning under norske forhold. Prosjektet har også gitt økt kunnskap om hvilke fangvekster som er aktuelle i kornområdene i Trøndelag og Innlandet hvor vi har mindre erfaring med fangvekster enn rundt Oslofjorden. Resultatene er formidlet til bønder, korn- og klimarådgivere, andre aktører i kornbransjen, forvaltningen og øvrige myndigheter.
Resultatene gir et bedre grunnlag for beslutninger om bruk av fangvekster som klimatiltak i Norge. Det gjelder for inkludering av fangvekster i det nasjonale klimaregnskapet, i klimarådgiving til bønder gjennom Landbrukets klimakalkulator og for utformingen av insentiver for å implementere politisk ønskede miljø- og klimatiltak, og prosjektet vil slik bidra til en mer bærekraftig norsk matproduksjon.
Prosjektarbeidet har styrket faglige nettverk på tvers av organisasjoner og institusjoner, og har samlet sett bidratt til økt kompetanse blant deltakerne. Den internasjonale dimensjonen av prosjektet har gitt tilgang til et større spekter av ressurser og kompetanse, og har ført til utvidet forskningssamarbeid over landegrenser, i første omgang til det nordiske forskernettverket CoverCropsNordic.
Use of cover crops (CC) in cereal cropping has been proposed as a viable option to reduce greenhouse gas emissions in Norwegian agriculture, based on results of Swedish long-term experiments which showed appreciable net carbon (C) sequestration in soil. Few data from long-term experiments with CCs are available for Norway and sequestration of C cannot very easily be inferred from short term experiments. To parameterize models that can predict C sequestration by use of CCs under Norwegian conditions, an on-farm survey and a series of experiments will be initiated testing different cover crops in Norway’s main cereal regions for their yield potential with and without nitrogen fertilization. Detailed process studies with 13CO2 pulse labelling at one site (Ås) will be used to quantify short-term C sequestration in a SE Norwegian clay loam. The data will be used to parameterize the Introductory Carbon Balance Model (ICBM). The core experiment at Ås will be complemented by detailed nitrous oxide (N2O) emission measurements to quantify the trade-offs between C sequestration and increased N2O emissions, especially during the winter season. The effect of CCs on off-season nitrate leaching will be addressed by collecting leachate in Apelsvoll’s field lysimeter facility. There is little knowledge about how to optimize growth of CCs under Norwegian conditions for soil C capture without compromising grain yields and increasing N2O emissions. CAPTURE will test various management strategies regarding sowing time and fertilization for existing (rye grass) and CCs new to Norwegian cereal production (winter vetch, oilseed radish, phaselia, etc.) with emphasis on regional differences in day length and climatic conditions. The generated data will be used as a framework for model estimates of both C input and nitrogen emissions in different cover cropping systems, that can be used for developing guidelines for farmers and, ultimately, for national emission inventories.
