Forage vs. Grain Legumes: Contrasting Effects on Soil Organic Carbon Stocks–Evidence From 30 European Field Experiments

Sustainable land management can play an important role in climate change mitigation by reducing soil organic carbon (SOC) losses or even by sequestering C in soils. This can be achieved through practices that increase C inputs to the soil and/or improve the quality of these inputs, thereby facilitating the removal of atmospheric carbon dioxide (CO₂) and storing it in the soil as SOC. In this study, we investigated the potential of an increased share of legumes in crop rotations to enhance SOC accrual—defined as the increase in SOC stocks at a given land unit compared to the baseline scenario—using data from 30 mid-term (MTEs, 5–20 years) and long-term (LTEs, 20+ years) field experiments across Europe. Our findings indicate that increasing the proportion of forage legumes in rotations (based on 21 experiments and 39 paired comparisons) led to SOC accrual of up to 13.25 Mg ha⁻¹ (0.44 Mg ha⁻¹ year⁻¹), while grain legumes (based on nine experiments and 28 paired comparisons) resulted in a decrease in SOC stocks of up to 14.37 Mg ha⁻¹ (−0.48 Mg ha⁻¹ year⁻¹) compared to the reference treatment. For forage legumes, the largest SOC gains were achieved at sites with the smallest reference SOC stocks and greater share of forage legumes in the rotation. Our observations suggested that the duration of crop growth of the forage legumes (annual vs. perennial) did not exert a significant impact on SOC stock increase, while pedoclimatic zone did. Positive effects on SOC stocks were more pronounced in the Atlantic climatic zone in contrast to the Mediterranean climatic zone. For grain legumes, larger SOC losses were observed with a greater share of grain legumes in the rotation. Overall, integrating forage legumes in cropping systems can enhance their sustainability and present a viable option for climate change mitigation. Finally, we present a regression equation to derive emission factors (EFs) for estimating SOC changes due to the increase of the share of forage legumes in a rotation, and another due to the increase of the share of grain legumes in the rotation. The first can be used to support the assessment of management impacts for the purpose of rewarding carbon farming and the estimation of a national-scale SOC accrual potential, while the second can be used for estimating national-scale SOC losses.