Soil wettability, aggregate stability, and the decomposition of soil organic matter

In well-structured topsoils, part of the soil organic matter (SOM) is located in the interior of the soil aggregates. Because of its location, this part of the SOM is little accessible to micro-organisms, and consequently not readily mineralised. Additionally, the physico-chemical conditions on the aggregate surfaces, being the main habitat of the organisms, control the accessibility, and hence, the rate of mineralisation. We hypothesise that hydrophobic conditions on aggregate surfaces reduce the rate of mineralisation of inside SOM, and simultaneously enhance the aggregate stability. The objectives of this study were therefore to study the significance of soil wettability with respect to both SOM mineralisation and aggregate stability. We used soil material from a loess-derived Gleyic Luvisol, either used as cropland or as grassland. Wettability was measured in terms of both the advancing soil-water contact angle and the solid surface free energy. Aggregate stability was assessed by immersion of aggregates in water-ethanol mixtures of varying surface tension. The impact of aggregation on SOM mineralisation was determined by respiration experiments that measured the CO2-release both from the aggregates and from the corresponding homogenised soil material. It was found that the contact angle of the soil samples ranged from 17° to 79°, and the solid surface free energy from 34 to 68 mJ m−2. Aggregates showed increasing stability with decreasing surface tension of the testing liquid. With increasing contact angle, the initial aggregate breakdown was decreased, which we attribute to the wettability-dependence of the liquid adsorption rates of the aggregates. Soil respiration measurements showed that microbial SOM decomposition was affected by the aggregation status, i.e., the homogenised samples released significantly more CO2 than the aggregates. We conclude that even subcritical soil water repellency (with contact angles <90°) can have a significant impact on the protection of SOM against microbial decomposition.