Characterisation of the organic matter pool in manures

In this research, different types of animal manure were evaluated with respect to organic matter (OM), total organic carbon (Cot), total N (Nt), Cot/Nt ratio, water-soluble organic carbon (Cw), organic N (Norg), carbohydrates, Cw/Norg ratio, humic acid-like carbon (Cha), fulvic acid-like carbon (Cfa), humification index ((Cha/Cot) × 100) (HI) and the Cha/Cfa and NH4+-N/NO3−-N ratios. In comparison with the limits set by the Spanish legislation for organic fertilisers, most of the manures had high OM contents, moderate Norg concentrations (except in the case of the chicken and pig manures where this parameter was high) and Cot/Nt ratios above the value stated in the legislation. The study of the different fractions of organic matter showed that the horse, pig and rabbit manures had the greatest content of Cot. However, the fraction of easily-biodegradable organic compounds (Cw) was significantly higher in the horse, goat and chicken manures. The study also showed that, in most cases, the percentage of fulvic acid-like C was greater than that of the humic acid-like C, indicating that the organic matter of these wastes is not completely humified. Values of HI ((Cha/Cot) × 100) and Cha/Cfa ratio in the studied manures were not significantly different. Regarding the parameters related to the organic matter stability such as Cw, carbohydrates and the Cot/Nt, Cw/Norg and NH4+-N/NO3−-N ratios, it has been determined that the organic matter of these materials was not completely stabilised. The heterogeneity in OM composition of the studied manures did not allow the formulation of simple equations for evaluation of the composition of these wastes from easily-determined parameters.