Understanding the effects of organic versus conventional farming on soil organic carbon characteristics – a chronosequence study

• We tested the effect of organic vs. conventional farming on SOC in 2 chronosequences. • Organic farming increased, or tended to do so, total and labile SOC content in clay. • These effects depended on variation in soil properties, particularly Fe oxide content. • In sand, organic farming did not affect the SOC content or quality. Organic farming aims at producing high quality, nutritious food while sustaining the health of soils and ecosystems, for which it relies on ecological processes. The amount and quality of soil organic carbon (SOC) influence many soil ecological processes that underlie ecosystem services. However, the effect of organic farming on the amount and especially the quality of SOC is not yet clear. We therefore investigated the long-term influence of organic arable farming on SOC characteristics in topsoil (0–15 cm). We sampled a chronosequence of certified organically managed commercial farms (0–34 years of organic farming) on clay-dominated soils, paired with nearby conventional counterparts. A similar chronosequence (0–69 years of organic farming) was established on sand-dominated soils. Alongside soil samples, we collected basic information on soil management such as crop rotation and fertilization. Total SOC content and soil properties that influence SOC were measured (i.e., pH, content of silt, clay, iron oxides and aluminum oxides). Four different techniques were used to characterize SOC quality, i.e., permanganate oxidizable carbon (POXC), Rock-Eval thermal analysis, size fractionation into particulate (POM) and mineral-associated (MAOM) organic matter, and elemental C:N analysis. In the clay chronosequence, particularly for soils with a low Fe oxide content, we found that organic farming can increase the total SOC content and labile organic carbon fractions over time, thus resulting in a lower overall SOC stability. We also found indications that the effects of management on SOC quality depend on the total SOC content. In the sand chronosequence, the duration of organic farming did not affect the content nor quality of SOC, potentially because management practices influencing SOC cycling were not distinctive for organic vs. conventional farming in the sand but were in the clay chronosequence. Despite our strict farm selection criteria, large variations in environmental soil conditions, farm-specific practices, and land use history challenged the identification of individual processes that drive the observed changes in SOC dynamics in response to organic farming. This variation also highlighted the importance of intrinsic soil properties for SOC dynamics, and the need to develop soil-specific farm management recommendations. Despite this complexity, this study has clearly revealed the potential of organic farming to change SOC cycling towards a higher and overall more bioavailable SOC content, thus strengthening associated ecosystem services.