The role of SOM and CaCO3 on soil aggregate development in reclaimed soils

Summary

Soil organic matter (SOM) and extracellular polymeric substances (EPS) from biological processesare considered to be major contributors in aggregate formation. But there is limited knowledge onsoil structural formation after reclamation – the step when SOM content is low and soil propertiesare mostly controlled by the parent material. In our study we used a chronosequence approach in the reclaimed open-cast mining area near Cologne, Germany to elucidate the development of soilstructure and soil organic matter during initial soil formation in a loess material. We selected sixplots with different ages of agricultural management after reclamation (0, 1, 3, 6, 12, and 24 yearsafter first seeding). In each reclaimed field 12 spatially independent locations were sampled with stainless steel cylinders (100 cm3) at two depths in the topsoil (1-5 cm and 16-20 cm). Sampleswere wet sieved into four aggregate size classes of <63 μm, 63-200 μm, 200-630 μm and 630-2000μm. Each aggregate size class was characterized by organic carbon (OC), total nitrogen (TN) andCaCO3 concentration. The chemical composition of the SOM of selected samples was characterized using solid-state 13C NMR spectroscopy. Wet sieving into aggregate size classes showed different trends along the chronosequence. Contradicting relation between CaCO3 and OC contribution to aggregate size classes display two different mechanisms on soil aggregate formation in young loess derived soils. CaCO3 influence daggregation predominantly in finer aggregate size classes, where the highest concentration and contribution was measured. SOM, on the other hand, played an important role on formation oflarge macro-aggregates after organic manure application in year 4. Furthermore, the loss of totalOC after year 12 was connected with the loss of OC contributing to the largest aggregate size class. Our findings reveal that SOM and CaCO3 role on stabilizing aggregates is not equally distributedand is aggregate size class dependent.