Influence of Input Materials and Composting Operation on Humification of Organic Matter

Composts play an important role in the context of resource recovery and soil improvement by increasing soil organic matter. This study focuses on compost quality in terms of humic acid (HA) content which due to its favorable properties provides benefits for soils and plants. A series of Austrian and foreign composts (174 samples) were investigated to find out the level of HAs built up during composting. HA contents from 5.4% ODM to 46.8% ODM were determined. The influence of input materials and process operation on HA formation was demonstrated by monitoring several composting processes. Fourier Transform Infrared (FT-IR) spectroscopy and thermal analysis such as thermogravimetry (TG) and differential scanning calorimetry (DSC) were applied as analytical tools for process and product control. Well-balanced mixtures of biowaste comprising kitchen-, marketand yard waste were found to provide the best conditions for HA formation. In these biowastes not only suitable molecules for humification were available, but microbial activity was also maintained for a longer period due to the diverse degradability of chemical compounds. Moderate aeration favored humification whereas too much air supply promoted mineralization. HAs undergo maturation during composting which was revealed by changes of spectral and thermal patterns and the increase of enthalpy. HAs are a sink for carbon and nitrogen. The nitrogen content in HAs was in the range from 5.11 to 8.91% of HA dry matter. The contribution to the total nitrogen in the composts was found to be 15 to 61%. HA molecules contain 48 to 54% carbon. HAs play an important role in carbon sequestration and will be considered as a key factor for carbon credits in the future. _____________________________________________________________________________________________________________