Size and density fractionation of soil organic matter and the physical capacity of soils to protect organic matter

Soil organic matter (SOM) has important chemical, physical and biological functions in the soil, It is difficult to predict the dynamics of SOM because it is very heterogeneous and because its behaviour is affected by soil texture. In this study we used a new size and density fractionation to isolate SOM fractions that differ in stability and we estimated the amount of SOM that can be preserved in different soils. An investigation was carried out into (1) how fast size and density fractions of soil organic matter respond to changes in C input, (2) whether the capacity of soils to preserve C by its association with clay and silt particles is limited and related to soil texture and (3) whether the long term dynamics of soil C can be described with a simple model that makes the assumption that the net rate of decomposition of soil C does not simply depend on soil texture, but on the degree to which the protective capacity of the soil is already occupied. Light and intermediate fractions of the macroorganic matter (>150 mu m) respond much faster to changes in C input than smaller size fractions, This shows that the light and intermediate macroorganic matter fractions can be used as early indicators of effects of soil management on changes in SOM. There was a close positive relationship between the proportion of particles <20 mu m in a soil and the amount of C associated with this fraction in the top 10 cm of grassland soils. Arable sandy soils, which contained less C than corresponding grassland soils, had the same amounts of C associated with the fraction <20 mu m, indicating that the amount of C that can become associated with this fraction had reached a maximum. The observed relationship: C in fraction <20 mu m (g/kg soil) = 6.9 + 0.29x % particles <20 mu m can be used as a first estimation for the capacity of a soil to preserve C. The amount of C in macroorganic matter is controlled by soil management, while the amount of C protected by clay and silt particles is controlled mainly by soil texture. The simulations of the changes in C in soil without input of C or with additions of lucerne or chaff were excellent in both sandy and clay soils. The build-up of C in soils receiving farmyard manure (FYM) was nor simulated so well. (C) 1997 Elsevier Science B.V.