Carbon storage in soil size-density fractions after 20 years of compost fertilization

Abstract

Fractionation by particle size provides a rough differentiation between young active, and older intermediate and passive soil organic matter. Soil samples from three treatments of a 20 years` fertilization experiment, C2 which had been fertilized with 10 t ha^-1 (wet wt.) compost per year on average, N2 with mineral N fertilizer at 32 kg N ha^-1 year^-1, and the unfertilized control (O) were subjected to particle size fractionation and to density fractionation. After low-energy sonication the samples were separated into the size fractions coarse sand (200-200 µm), fine sand (200-63 µm) silt (63-2 µm) and clay (2-0,1 µm). Density fractionation using Na-polytungstate with 1.8 g cm^-3 density was applied to separate particulate organic matter (POM) from the sand-sized fraction. Compost fertilization resulted in an increase in C_org in all size and density fractions. In total, the C_org content was 10 % higher with compost fertilization than in the unfertilized control. Approximately 40 % of the additional soil carbon was located in the POM, 56 % in the silt-sized fraction and 3 % in the clay-sized fraction. With mineral N fertilization the sum of C_org contents of all fractions was about the same as without fertilization, with an increase of POM-C_org and a decrease of C_org in the silt and clay-sized fraction.

Keywords: fractionation, particulate organic matter, POM, soil organic matter