Refining the Size Fractionation of Soil Organic Matter – Application to Agricultural Soils in Central Europe

Ultrasonic dispersion (sonication) is essential in the physical fractionation of soil organic matter (SOM). High sonication power, necessary to achieve complete soil dispersion, may cause fragmentation of particulate organic matter (POM), especially of the coarser organic fragments. It has been stressed that the coarser organic fragments should be spared from sonication. In this paper, we propose a method for the size fractionation of SOM, designed to avoid this problem. The method involves sieving twice the soil sample (20 g, air-dried), (a) after agitation in water, and (b) after sonication in water (100 W, 10 minutes), applied only to fractions <200 μm. Five fractions are isolated: (i) COF: coarse organic fragments, 2000–200 μm; (ii) POM I: particulate organic matter 200–50 μm; (iii) POM II, particulate organic matter 50–20 μm; (iv) MAOM I: mineral-associated organic matter <20 μm, released before sonication; and (v) MAOM II: mineral-associated organic matter <20 μm, released after sonication. The method is applied to three soils from experiments about replacement of C3 by C4 crops, which allows the renewal of the obtained fractions to be compared. Consistently, the percentage of new organic material (C4-derived) in the fractions follows the order COF >POM I >POM II >MAOM II >MAOM I, although the difference between the two MAOM fractions (I and II) is very small. The method successfully isolates carbon fractions with different turnover which, therefore, represent functionally different fractions. The method is useful as a routine protocol for SOM fractionation.