The incubation history of soil samples strongly affects the occlusion of particulate organic matter

Abstract. Soil structure is a key proxy for carbon and nutrient storage, stable pore space, erosion stability and rootability. It is often quantified based on the degree of aggregation or the mechanical stability of soil aggregates. This work compares two methods representing basic principles of aggregate measurement. Undisturbed soil samples of loamy sand, clayey silt and silty loam were analyzed by ultrasonication/density fractionation (USD) to quantify different soil organic carbon (SOC) pools and by wet-sieving to measure the amount of water stable aggregates (%WSA). The measurements were carried out on field-fresh soil as well as samples that were air-dried, reset to field capacity (pF 1.8) by capillary action and incubated for 0, 1 and 4 weeks. Our results show, that the strength of POM occlusion sharply decreases after rewetting and is not fully recovered within the following four weeks. In contrast, the amount of water stable aggregates (%WSA) remains largely stable except in clayey silt. In consequence, field-fresh measurements are highly recommended to avoid overestimation of free and weakly bound SOM fractions or erosion stability.