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

Soil structure is a key proxy for carbon and nutrient storage, stable pore space, 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 soils at field capacity (pF 1.8) as well as samples that were air-dried; reset to pF 1.8 by capillary action; and incubated for 0, 1 and 4 weeks. Our results show that the strength of particulate organic matter (POM) occlusion sharply decreases after rewetting, indicated by the reduction in the more strongly bound occluded carbon fraction. The respective amounts decreased by −4.5 wt % for loamy sand, −6.8 wt % for clayey silt and −16.3 wt % for silty loam, and the field fresh values are not fully recovered within the following 4 weeks. In contrast, the amount of water-stable aggregates (%WSA) remains largely stable except in clayey silt, which shows an increase by +5.9 wt % directly after rewetting. In consequence, field-fresh measurements are highly recommended to avoid overestimation of free and weakly bound soil organic matter (SOM) fractions or the degree of aggregation.