The Effect of Long Term Annual compost application on the Distribution and Stability of Soil Organic Matter

Soil organic matter is a major carbon pool and can play a significant role in carbon mitigation measures. It is also a crucial factor for several soil physical properties and a major nutrient source for crops. To obtain an understanding of the changes that occur in the soil following long term annual compost application, the Soil Service of Belgium started a long term field trial in Boutersem, Belgium in 1997. Here 12 different treatments (fallow, unfertilized, mineral fertilized and 9 compost treatments differing in intensity from 15 to 45 t/ha and in frequency from annual to tri-annual) were implemented in 4 repetitions. All compost amended treatments substituted (part) of the mineral nutrient requirements of the crop and had a positive influence on soil chemical and physical parameters. The continued application of compost also has important effects on the amount, distribution and stability of the soil organic matter. To quantify this, soil samples from 5 treatments were divided into 7 fractions differing in physical and biochemical protection levels of the associated SOM. Carbon, nitrogen and their respective stable isotope concentrations were measured in all fractions. Not only did the total amount of carbon in the amended soils increase significantly over the course of the experiment, it also increased specifically in the free and occluded micro-aggregate fractions. A recently developed model based on the C/N and ?15N ratio of the SOM was used to determine its relative age and stability in all of the isolated SOM fractions.