Role of CaCO3 and Charcoal Application on Organic Matter Retention in Silt-sized Aggregates

The effectiveness of charcoal and calcium carbonate (CaCO₃) applications to improve soil conditions has been well documented. However, their influence on the formation of silt-sized aggregates and the amount and protection of associated organic matter (OM) against microbial decomposition under differing soil mineralogical and microbiological conditions are still unknown. For sustainable management of agricultural soils, silt-sized aggregates (2-50 μm) are of particularly large importance because they store up to 60% of soil organic carbon and with mean residence times between 70 and 400 years. The objectives of this study are i) to analyze the ability of soil amendments (CaCO₃, charcoal and their combined application) to increase the amount of silt-sized aggregates and associated organic matter, ii) vary soil mineral conditions to establish relevant boundary conditions for amendment-induced aggregation process, iii) to determine how amendment-induced changes in formation of silt-sized aggregates relate to microbial decomposition of OM. We set up artificial high reactive (clay: 40%, sand: 57%, SOM: 3%) and low reactive soils (clay: 10%, sand: 89%, SOM: 1%) and mixed them with charcoal (1%) and/or CaCO₃ (0.2%). The samples were adjusted to a water potential of 0.3 bar using a nutrient solution and sub samples were incubated with microbial innoculum. After four months, silt-sized aggregates are separated by a combination of wet-sieving and sedimentation. We hypothesize that the relative increase in amount of silt-sized aggregates and associated OM is larger for less reactive soils than for high reactive soils because of a relative larger increase in binding agents by addition of charcoal and/or CaCO₃ in less reactive soils. The effect of charcoal and/or CaCO₃ application on the amount of silt-sized aggregates and associated OM is expected to increases with an increase in microbial activity. Between different treatments, we expect the incubated 'charcoal+CaCO₃' combination to have the largest effect on silt-size scale aggregation processes because the amount of microbial derived cementing agents, charcoal derived functional groups containing OM, and Ca²⁺ ions are enhanced at the same time.