Organic matter mineralization in different soil aggregate fractions

The mineralization rate of the organic matter (OM) in the aggregate fractions of a gray forest soil separated by repeated sieving through sieves with different mesh sizes was assessed. The samples of the soil aggregate fractions were incubated for 141 days under constant temperature and moisture, and the C-CO2 emission rate was measured. The mineralizable OM pool in the aggregates of <0.25, 1–0.25, and 3–1 mm in size from the soil under a forest contained easily (C1, k 1 > 0.1 days−1), moderately (C2, k 2 > 0.01 days−1), and difficultly (C3, k 3 > 0.001 days−1) mineralized compounds; the C1 and C2 components were present in the coarser aggregates. In the arable soil, the C1, C2, and C3 OM components were separated in the aggregates of <0.25 and 1–0.25 mm; the C1 and C3 were separated in the aggregates of 3–1 and 5–3 mm; and the C1 and C2 were separated in the coarsest (10–5 mm) aggregates. The highest content of potentially mineralized OM (C0) occurred in the aggregates of 1–0.25 and 3–1 mm, but the size of the mineralizable OM pool was more dependent on the portion of the aggregate fraction in the soil than on the absolute C0 content in the fraction. It was shown that the decrease in the share of coarse structural aggregates is accompanied by a depletion of potentially mineralized OM in the arable soil, and the formation of coarse aggregates is an important condition of the soil carbon sequestration.