The evolution of early soil microstructure is governed by organic matter and its impact on the weathering rates of host rock

Soil aggregation and the translocation of clay as well as organic matter are major processes of pedogenesis that manifest in the diagnostic soil horizons of mature soil. Yet, their onset might date to much earlier stages of soil development where host rock weathering is dominant and litter from pioneer vegetation is the only input of organic matter. To what extent aggregate formation is induced by early weathering and how clay transport facilitates aggregation is not yet comprehensively explored. Here, we present a time-lapse experiment on initial pedogenesis that reveals the formation of aggregates and clay translocation in response to irrigation with and without organic matter released from a litter layer. We show how organic matter increases total carbonate dissolution capacity with a characteristic surface morphology, but simultaneously slows down the dissolution rate. With the dissolution of carbonates, clay minerals of the host rock and iron from pyrite are released. Controlled by the presence of organic matter, both are either transported with the seepage water or form crusts and aggregates from clay minerals and freshly precipitated secondary iron oxides. The translocation and aggregation of organic matter and clay-sized minerals therefore shape soil structure already during initial pedogenesis and control the route in which soil development becomes apparent.