Understanding the Effects of Organic Versus Conventional Farming on Soil Organic Carbon Characteristics– a Chronosequence Study

Abstract

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 the amount and the quality of SOC in topsoil.On two contrasting soil types (i.e., clay vs. sand), we sampled a chronosequence of certified organically managed commercial farms (0 to 34 and 69 years of organic farming, respectively) that we paired with nearby conventional counterparts on the same soil. We collected soil samples and basic information on soil management such as crop rotation and fertilization. Total SOC content and soil properties that influence SOC were measured, while four different techniques (POX-C oxidation, Rock-Eval thermal analysis, POM-MAOM fractionation, elemental C:N analysis) were used to characterize SOC quality.In clay soils, particularly those with a low Fe oxide content, we confirmed our hypothesis 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 are co-dependent on the total SOC content. The duration of organic farming was not found to affect the content or quality of SOC in sandy soils.Despite our strict farm selection criteria, large variations in environmental soil conditions, farm-specific practices, and land use history complicated identification of individual processes that drive the observed changes in SOC dynamics in response to organic farming. This variation also highlighted the importance of soil intrinsic 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.

Keywords: organic agriculture, soil organic matter, soil carbon, regenerative agriculture, soil management, confounding factors