Influence of Calcium Carbonate and Charcoal Applications on Organic Matter Storage in Silt-Sized Aggregates Formed during a Microcosm Experiment

Silt-sized aggregates (2-53 mu m) can store a high percentage of organic matter (OM) in agricultural soils. This study aimed to determine whether additions of charcoal and CaCO3 may enhance the retention of organic C (OC) and total N (N-t) in silt-sized aggregates. We used artificial soil mixtures without a silt component (89% sand, 10% clay, 1% OM) to emulate sandy soils with little natural structure. Charcoal and/or CaCO3 were added, and the resulting mixtures were incubated for 16 wk in the dark. The newly formed silt-sized fraction was separated and analyzed for OC and N-t concentrations and characterized using FTIR and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS). Compared to the control treatment, CaCO3 addition had no positive effects on C and N retention in the silt-sized fraction (17-20 g kg(-1) OC, 0.15-0.17 g kg(-1) N-t) whereas the silt-sized fraction from treatments with charcoal additions showed significantly higher OC and N-t concentrations (50-56 g kg(-1) OC, 0.31-0.85 g kg(-1) N-t). Silt-sized fractions from the charcoal treatments also showed a significant increase in the proportion of C=O groups. These initial results justify more detailed investigations into the improvement of the structure and nutrient retention of sandy soils by charcoal and CaCO3 applications.