Fourier Transform Infrared and Fluorescence Spectral Features of Organic Matter in Conventional and Organic Dairy Manure

Organic dairy production has exhibited potential for growth in the United States dairy sector. However, little information is available on whether there is any difference in manure composition and quality between organic (OD) and conventional (CD) dairy manure even though the composition and quality are important parameters with respect to availability, utilization, and cycling of manure nutrients and environmental impact evaluation. We comparatively characterized whole and water‐extracted materials of 15 OD and seven CD dairy manure samples by Fourier transform infrared (FT‐IR) and fluorescence spectroscopies. Fourier transform infrared features of manure organic matter varied mainly in the 1650 to 1550 cm⁻¹ range, reflecting the presence of different N compounds in these manure samples. Fluorescence data revealed five fluorophore components present in the water‐extracted organic matter from the manures. We found no clearly distinct value ranges in whole and water‐extractable organic matter between the two types of dairy manure with respect to C and N contents and FT‐IR and fluorescence spectral features. However, based on the average values, we observed general pattern differences on the effect of organic farming on the manure composition: OD contained less soluble C and N compounds on dry weight basis but more hydrophobic aliphatic groups in whole manure. The soluble organic matter in OD samples contained more stable humic‐ and lignin‐related components and less amino/protein N‐related components based on their spectroscopic features. These differences might be attributed to more forage feedstuffs in organic dairy farming management and more protein additives in conventional dairy feedstuffs. Information from this work may be useful in aiding organic dairy farmers in making manure management decisions.