Organic matter governs weathering rates and microstructure evolution during early pedogenesis

Soil aggregation and the translocation of clay and organic matter are significant pedogenic processes that manifest in diagnostic horizons in mature soil. Yet, their onset might date to much earlier stages of soil devel-opment where host rock weathering is dominant and litter from pioneer vegetation is the only input of organic matter. We present a time-lapse experimental-pedogenesis study on early host rock weathering that shows the formation of aggregates and clay translocation in response to irrigation with and without organic matter released from a litter layer. The presence of organic matter increases total carbonate dissolution capacity and results in a characteristic surface morphology, while simultaneously slowing 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. Our study shows that the interplay of dissolution, neoformation of secondary minerals, translocation, and aggregation of organic matter and clay-sized minerals shape soil structure evolution during early pedogenesis in carbonate host rocks.