Changes in humic substances and phosphorus fractions during composting

The behavior of humic substances (HS) and phosphorus (P) fractions in livestock manure co-composted with rice straw was studied by chemical and spectroscopic analyses. Composting was accompanied by a decrease in the levels of carbon (C) and nitrogen (N) as well as HS and fulvic acid (FA) fractions. The elemental content (weight %) of humic acid (HA) extracted from the compost showed a high C content and low oxygen (O) content, possibly due to a low degree of oxidation. The N/C and H/C ratios of HA decreased as the compost matured, whereas the O/H and O/C increased. The decline in the H/C ratio from 1.24 at 0d to 1.0 at 195d indicated an increase in the aromatic structure of the HA. The absorbance curve for the HS showed a decrease in the slope with time up to 42d. The optical density of HS increased with time of composting with a peak at 42d. Also the A log K values of FA increased appreciably with time of composting whereas those of HS and HA were little changed. Water-soluble and HCl-soluble-P were the dominant fractions of P in the compost, and ranged between 18 and 39% and 18 and 36%, respectively. However, the former declined progressively with time of composting, while the latter increased, indicating transformation of the more vulnerable water soluble P to the more recalcitrant HCl-extractable P.