Impact of long term compost amendments on soil fertility, soil organic matter distribution and nitrogen mineralization

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. 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 less protected SOM fractions. These results were combined with a 400 day long incubation experiment to investigate the influence of long term compost fertilization, causing an altered SOM distribution, on the soil respiration and nitrogen mineralization.