Interactive effects of compost, plants and earthworms on the aggregations of constructed Technosols

- In a laboratory we studied organisms and parent materials effects on soil structure.
- Organisms had a greater effect on soil aggregation and structure stability.
- Both factors have similar roles in organic carbon distribution in aggregation.
- Organisms can enhance pedogenetic processes for functional Technosols.

Aggregation is an important physical process to study during the early formation of Technosols. It is known to be influenced both by the organic matter content and soil biota. Constructed Technosols represent good models to test the importance of these factors since their composition can be easily manipulated by mixing different proportions of parent materials and introducing soil organisms. In this study, we performed a 5 month mesocosm experiment, using excavated deep horizons of soils (EDH) as mineral material mixed with green waste compost (GWC) at six different proportions (from 0 to 50%) in the presence or absence of plants and/or earthworms. After 21 weeks of incubation, aggregation was characterized by: 1) determining the size fraction and morphology, 2) measuring the distribution of organic carbon (OC) in each fraction and 3) testing the aggregate stability. Results showed that organisms accounted for 50% of soil aggregation variance while GWC was responsible for only 5% of the variance. The percentage of total variance of OC distribution in aggregates explained by organisms, GWC, and the interaction of the two was similar (28%, 22% and 26%, respectively). The effect of GWC on structural stability was negligible (2%) compared to that of organisms (70%). The effect of earthworms combination with plants was complex: plants had a dominant effect on the distribution of the size of aggregates by disrupting earthworm casts, but earthworms had a dominant effect over plants for aggregate stability under fast wetting only when the percentage of compost was low. This study underlines the importance of considering the interaction of the organic matter and soil biota: in this case, increasing compost proportion in a Technosol has significant effects on aggregation only in the presence of plants or earthworms.