Effect of perennial ryegrass cover crop on soil organic carbon stocks in southern Sweden

Soil organic carbon (SOC) is an important ecosystem property and a potential sink for atmospheric CO2. Many agricultural soils are depleted in SOC and thus have the need and potential to sequester carbon. Cover crops used to prevent nitrate leaching in agroecosystems might be an additional cost-effective and multi-beneficial carbon input, but little is known about their effect on SOC stocks. This study examined the effect of an ryegrass cover crop on SOC stocks in three Swedish long-term experiments (16–24 years) and compared it with that at a North American site (Sultan, Washington). Growth was temperature- and light-limited in Sweden and thus the ryegrass was undersown, while it was sown after harvest of the main crop at the Sultan site. In total, seven pairs of cover crop/no cover crop treatments were investigated. The introductory carbon balance model (ICBM) was used to calculate humification coefficients for ryegrass at each site as a measure of carbon sequestration efficiency. Mean aboveground biomass of ryegrass ranged from 550–1050 kg DM ha− 1 yr− 1 in the Swedish experiments and was 4650 kg DM ha− 1 yr− 1 at the Sultan site. Yield of the main crop was not significantly affected by the cover crop. Cover crop incorporation increased SOC stocks, with a significant mean carbon sequestration rate (0.32 ± 0.28 Mg C ha− 1 yr− 1) at the Sultan site and all Swedish sites except one. Mean humification coefficient of the ryegrass cover crop was 0.33 ± 0.27, which is comparable to that of highly efficient organic amendments such as farmyard manure and sewage sludge. This was attributed to high belowground productivity of ryegrass, although that was the most uncertain model input variable. A ryegrass cover crop is thus an effective, multi-beneficial measure to increase SOC stocks, even when undersown at northerly latitudes (55–58°N).