Changes in organic matter composition during poultry manure composting: A new perspective on compost maturity using DAX resin fractionation and spectroscopic analysis

Ensuring consistent quality and effective utilization of livestock manure compost requires an understanding of organic matter (OM) transformation and the development of reliable maturity indicators. However, in-depth investigations into the fully matured phase of the compost are limited. We investigated the 100-day composting process of poultry manure by analyzing its physicochemical properties, germination index (GI), dissolved organic matter (DOM) spectral indicators (humification index [HIX], fluorescence regional integration [FRI], and absorbance values ratio), and DOM fractions using liquid chromatography-organic carbon detection and DAX resin fractionation to track molecular structural changes. During the initial stabilization phase (days 0–10, GI < 90 %), mineralization (CO₂, NH₃) and hydrophilic (Hi) component degradation led to the formation of high-molecular-weight hydrophobic acids (HoA). In the cooling phase (days 10–30, GI 90–120 %), a temperature drop (55 °C to 34 °C) facilitated polymeric organic transformation into compounds with higher HIX values. During the matured phase (days 31–100, GI 120–160 %), HoA significantly decreased, whereas low-molecular-weight hydrophobic neutral (HoN) components rapidly increased. During the fully matured phase (days 51–100, GI > 140 %), HoN stabilized and spectral indices remained consistent, indicating the completion of OM transformation. The DOM composition of the poultry manure compost followed a sequential transition from Hi to HoA to HoN. The fluorescence characteristics of HoN suggested that FRI Region III could serve as a predictive maturity indicator. These findings enhance our understanding of compost OM transformation mechanisms and contribute to the development of a rapid and convenient maturity assessment method.