Hierarchical drivers of aggregation in a constructed Technosol

Abstract

The relation between organisms and parent materials is essential to optimise their effect on soil structure, especially when humans benefit from this relation to create a new engineering soil (Technosols) created for different ecosystem services. Constructed Technosols represent good models to test the relative 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 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 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 total percentage of variance of organic carbon distribution in aggregates explained by organisms was similar to that explained by GWC, 28% and 22%, respectively, while their interaction accounted for 26%. Organisms had the dominant (70%) effect on structural stability compared to GWC (2%). The structural characteristics depend more on biota than on the proportion of organic matter. The effect of earthworms and plants in combination was complex: whereas plants had a dominant effect on the distribution of the size of aggregates by disrupting earthworm casts, 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 the indirect effects of the organic matter: in this case, increasing compost proportion has a negligible effect on aggregation in the absence of plants or earthworms.