Resilience and microstructural resistance of Archaeological Dark Earths with different soil organic carbon contents in Western Amazonia, Brazil

Abstract

Archaeological Dark Earths (ADE) are anthropogenic soils with high fertility and excellent physical conditions due to high soil organic carbon (SOC) content. However, climate change, land use and soil management can increase SOC mineralization, resulting in microstructure damage of these soils. To verify the effect of SOC loss, we collected deformed samples from the surface horizon and simulated the reduction of C with the application of 0.2, 0.4 and 0.6 ml of hydrogen peroxide 35% per gram of soil, resulting in three treatments of different oxidation levels and untreated soil. Both original and oxidized soil were submitted to an amplitude sweep test with controlled strain and a thixotropy test, in a compact modular rheometer. To characterize the effect of soil properties on rheology and resilience of ADEs, we performed a correlation analysis with physico-chemical properties from untreated soil. Higher clay and organic matter contents increased the microstructure elasticity of ADEs. The increase in base saturation, mainly due to the high Ca+2 content, also favors elasticity. The soil’s resilience is a result of the joint effect of particle size distribution, base saturation and SOC content. The microstructure recovers fast, regardless of the disturbance intensity. The SOC loss affected the microstructure differently in each ADE. These differences are not dependent on the amount of SOC lost and mostly labile SOC (as removed by low oxidation intensity) was responsible for soil strength.