Detecting the Disintegration: Insights into soil structure decay following OC depletion

Aggregate forming and stabilizing processes have been intensively studied as they are closely linked to organic carbon (OC) sequestration. However, soils are no static systems and consequently, their structure is subject to constant breakup and turnover processes.

In order to study soil structure turnover with respect to the loss of OC, we designed an incubation experiment with soil microcosms, allowing OC-loss by leaching and microbial respiration, while preventing any mechanical disturbance.

We incubated intact soil cores of an arable Luvisol derived from Loess-deposits in south-east Germany for 300 days at constant water-tension and 25 °C to promote microbial activity. During the incubation, CO₂-release and OC leaching from the microcosms were monitored. A subset of microcosms was sampled each month to assess the effect of progressing OC depletion on the size distribution, OC content and stability of the aggregates.

The incubation led to a reduction of the initial OC (11.2 mg g^-1) by 2.2 mg per g soil and a more narrow C:N ratio, which corresponded to a reduced OC coverage of the mineral surfaces (1.7 m² g^-1 to 0.9 m² g^-1, as determined by N₂-BET). Despite the OC reduction, the aggregate size distribution (as determined both by wet- and dry-sieving) did not change significantly, although there was a trend towards a reduced aggregate mean weight diameter (higher reduction after wet-sieving). The aggregates’ mechanical stability (as determined by dry-crushing), even slightly increased with a lower OC-content in the bulk soil.

Those observations highlight that OC depletion, without additional mechanical influence, does not immediately lead to the decay of soil structure. This suggests the existence of OC-storage sites that are not prone to OC-loss by leaching or microbial degradation. In contrast, the sites of initial OC-loss might not contribute to the structural stability of a soil.