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
Highlights
Particle size fractionation allows to distinguish young active from older intermediate and passive soil organic matter
The active soil organic matter pool with turnover rates < 10 years is best represented by the light fraction (< 1.6-2 g cm-3) obtained by density fractionation and by the soil microbial biomass
For N, the proportions were similar (Table 2). 89-92 % of Corg and 91-92 % of Nt were located in the particle size fractions smaller than 63 μm
Summary
Particle size fractionation allows to distinguish young active from older intermediate and passive soil organic matter. The organic matter in the sand-sized fraction consists mostly of plant residues, while the silt and clay fractions represent a sink for the products of the decomposition process, for material of microbial origin and for humified organic matter, which turns over more slowly (Magid et al, 1995). The active soil organic matter pool with turnover rates < 10 years is best represented by the light fraction (< 1.6-2 g cm-3) obtained by density fractionation and by the soil microbial biomass (von Lützow et al, 2007). The aim of this study was to find out in which fractions the Corg imported by compost fertilization accumulates
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