Meta-analysis on soil organic carbon and cereal production in organic farming systems across Europe

Organic farming may improve agroecosystems’ resilience against external stressors, favour below-ground biodiversity, soil health, and increase soil water holding capacity. At the same time, organic farming systems are repeatedly reported to have lower average crop yields than conventional systems. To date, global meta-analyses on organic farming systems include a diverse range of crops, but none of them specifically focus on arable systems with cereal-based rotations. Further, they are not representative for specific European agro-environmental zones and often show weaknesses in the applied meta-analytical methodology.

This meta-analysis aimed at quantitatively summarizing existing knowledge and outcomes on soil organic carbon (SOC) in the topsoil (0-20/30 cm) and cereal production (i.e., yields and yield stability) in organic farming systems compared to conventional farming systems across Europe.

The database consisted of 43 independent field studies on SOC and 50 field studies on cereal yields across 16 European countries, covering nine European agro-environmental zones. Cereal-based rotations were cultivated organically and conventionally on mineral soils, up to several decades. Yields for winter rye, winter and spring wheat, spring barley and spring oats were annually measured. SOC was measured as stock or concentration at the end of the experiments. Organic farming systems relied either on animal-based or plant-based fertilizers, or on both sources of nitrogen input. Conventional systems received solely mineral fertilization in most experiments. For both farming systems conventional tillage was applied without irrigation. The meta-analysis was conducted by using Meta Win 2.0 and IBM SPSS Statistics 29. As an index of effect size, we used ln (R), i.e., relative SOC, yield or temporal yield variation.  All studies were weighted by inverse variance.

The overall effect of organic farming was a 5% increase of topsoil SOC (95% CI: 1% – 9%, n=43) compared to conventional systems. Pedoclimatic factors, such as mean annual precipitation and clay content had a profound impact on SOC response under organic farming (p=0.014). With increasing annual precipitation and clay content, SOC response to organic farming was increasing, and reached 20% in areas with clayey soils and annual precipitation of 700 mm. In addition, the response of SOC to organic farming showed some positive trend with increasing soil pH (p=0.059).

Overall, cereal yield in organic farming was about 30% lower compared to conventional farming systems. However, yield performance of organic systems varied statistically significantly across farming types (p=0.021): a 20% yield gap was observed in organic systems using animal-based fertilizers, while a 35% yield gap was shown in organic systems using only legumes or mixed green manure. Moreover, the yield gap decreased with increasing average annual temperature (p=0.002). Overall, the temporal yield variation of organic farming systems was about 50% larger than in conventional systems, which was not related to any pedoclimatic factors studied.

In conclusion, organic farming systems had a positive impact on SOC in the topsoil, compared to conventional systems. The magnitude of this influence mainly depended on pedoclimatic characteristics in Europe. In terms of cereal production, organic farming had lower yields and yield stability compared to conventional farming.