CAPTURE - Assessment of cover cropping as climate action in cereal production in Norway

Cover crops are plants grown to ensure that soil remains covered after the main field crops have been harvested. Such a practise has been proposed as a measure to reduce greenhouse gas emissions from grain production. Today, this practice is not included in our national emission accounting because of a lack of documentation under our conditions.

The aim of the CAPTURE project, carried out from 2021 to 2025, was to document the climate impact of cover crops on grain fields in Norway, as well as to develop effective cultivation strategies. In this context, climate impact refers to the ability of these crops to capture and store carbon in the soil, weighed against potential nitrous oxide emissions from the decomposition of plant residues in the soil.

We found that in cereal cultivation, the use of cover crops is a highly effective measure for binding additional atmospheric carbon, and we estimated with the ICBM-model that such practice most probably increases the soil organic matter content. However, cover crops can both increase and decrease nitrous oxide emissions from the soil-plant system outside the growing season, depending on the type of cover crop used. Therefore, the choice of cover crop species is crucial to determining whether the overall climate effect is positive or negative.

Our results show that cover crops establish and grow well on grain fields in Norway's primary cereal-producing regions: the Oslofjord area, Innlandet, and Trøndelag. On actual farms, we found on average 820, 500, and 380 kg of carbon per hectare in leaf biomass samples late in the fall in these three regions, respectively. In the Oslofjord area and Innlandet, annual cover crops sown at harvest captured as much atmospheric carbon as perennial cover crops sown as an understory in spring. In Trøndelag, spring-sown species yielded well, while summer-sown species had poor establishment. Biomass variation from field to field was smaller for spring-sown than for summer-sown crops.

Retaining straw is another climate measure in cereal farming. We found that such practice reduces cover crop establishment by about 15 %.

Field trials in Ås (Oslofjord), Apelsvoll (Innlandet), and Tuv (Trøndelag) were used to test traditional cover crop types (grasses and clover) and newer species and varieties (oilseed radish, vetch, and phacelia). Spring-sown cover crops competed to some extend with the cereal crop, but the yield reduction (0–5 %) was not significant. As a representative of spring-sown species, ryegrass yielded 410, 280, and 600 kg of carbon per hectare in leaf biomass in autumn at Ås, Apelsvoll, and Tuv, respectively. As a representative of summer-sown species, oilseed radish yielded 270, 200, and 120 kg of carbon at the same locations. For the spring-sown cover crops, 13 % of carbon was lost from the leaf biomass over winter at Apelsvoll and 32 % at Tuv. Nitrogen losses were higher: 32 % and 57 %, respectively. The summer-sown species were annuals that completely died off over the winter. We found no correlation between nitrogen losses from above-ground plant material during winter and the amount of inorganic nitrogen in the soil (0–40 cm) in spring.

A significant portion of the carbon captured through photosynthesis is allocated to the roots, with some of it leaking into the soil. We measured the shoot-to-root carbon ratio for some common cover crops and combined our results with other Scandinavian data. For carbon, we concluded that ryegrass and clover have a shoot-to-root ratio of about 2 (excluding root exudates) down to 30 cm depth. For oilseed radish and vetch, the ratio was closer to 4. To track the fate of carbon in shoots and roots, we used the ¹³C carbon isotope in a field trial at Ås. We found that carbon allocated below ground was more likely to be incorporated into the most stable soil carbon fraction (mineral-associated carbon) than carbon from shoots and leaves. On average, carbon from below-ground sources was 2–3 times more stable than above-ground carbon.

We used the ICBM model, also used in our national greenhouse gas accounting, to estimate how much of the carbon fixed yearly by cover crops that is stored in the soil over a 30-year period. The model was run with a parameter set based on long-term Swedish cover crop trials and with climate data from our three cereal-producing regions. As input, we used our own carbon data measured on actual farms. We found that using cover crops could on average increase soil carbon stocks by 270 ± 120 kg per hectare per year under Norwegian conditions. Depending on species and location, this number ranged from 60 kg of soil carbon per hectare per year for a summer-sown mix in Trøndelag to 480 kg per hectare per year for ryegrass in the Oslofjord region.

We also used the ICBM model to weigh the climate benefit of carbon storage against the downside of nitrous oxide emissions. For this we used data from our experiment at Ås where both carbon capture (aboveground carbon) and nitrous oxide emissions were measured on plots with spring- or summer-sown cover crops. Some plots received fertilization (25 kg nitrogen per hectare) after harvesting. Nitrous oxide emissions due to cover crop use and/or fertilization were calculated by subtracting emissions measured on plots without cover crops. In the experiment, carbon sequestration was somewhat lower than expected under practical conditions, but for unfertilized ryegrass, we calculated carbon storage of 200–250 kg per hectare in 2021 and 80–180 kg in 2022. For oilseed radish, the corresponding numbers were approximately 150 and 50 kg per hectare. Fertilization to ensure good growth did not increase carbon capture in this trial. Nitrous oxide emissions were low during the growing season and high during the cold period, especially in spring. The choice of plant species was decisive in whether the use of cover crops increased or decreased nitrous oxide emissions. Compared to the control without cover crops, ryegrass use resulted in 2,9 kg per hectare lower nitrous oxide emissions, while oilseed radish increased emissions by 2,3 kg per hectare. When calculating the total climate effect of cover crops, we converted nitrous oxide emissions into CO2 equivalents using the factor that 1 kg of nitrous oxide equals 273 kg CO2-equivalents. We also included other emissions related to the practice, such as seed production and transport—totalling 400 kg CO2-equivalents per hectare per year. The results from the Ås experiment showed that ryegrasses (perennial and Italian) and a grass-clover-herb mix 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 depending on location and year. Additionally, there is substantial uncertainty in estimating both the shoot/root ratio of cover crops and the storage of the fixed carbon. More data showing the relationship between aboveground biomass and root/shoot ratio and rhizodeposition is important. Therefore, our results should be interpreted qualitatively rather than quantitatively. This means that a ryegrass-dominated cover crop likely reduces greenhouse gas emissions by around 1000 kg CO2-equivalents per hectare per year. Oilseed radish contributes to increased carbon storage, but under Norwegian conditions, this likely does not offset the increased nitrous oxide emissions.